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The recently constructed vacuum ultraviolet (VUV) free electron laser (FEL) at the Dalian Coherent Light Source (DCLS) is yielding a wealth of new and exquisitely detailed information about the photofragmentation dynamics of many small gas-phase molecules. This Review focuses particular attention on five triatomic molecules—H2O, H2S, CO2, OCS and CS2. Each shows excitation wavelength-dependent dissociation dynamics, yielding photofragments that populate a range of electronic and (in the case of diatomic fragments) vibrational and rotational quantum states, which can be characterized by different translational spectroscopy methods. The photodissociation of an isolated molecule from a well-defined initial quantum state provides a lens through which one can investigate how and why chemical reactions occur, and provides numerous opportunities for fruitful, synergistic collaborations with high-level ab initio quantum chemists. The chosen molecules, their photofragments and the subsequent chemical reaction networks to which they can contribute are all crucial in planetary atmospheres and in interstellar and circumstellar environments. The aims of this Review are 3-fold: to highlight new photochemical insights enabled by the VUV-FEL at the DCLS, notably the recently recognized central atom elimination process that is shown to contribute in all of these triatomic molecules; to highlight some of the potential implications of this rich photochemistry to our understanding of interstellar chemistry and molecular evolution within the universe; and to highlight other and future research directions in areas related to chemical reaction dynamics and astrochemistry that will be enabled by increased access to VUV-FEL sources. [ABSTRACT FROM AUTHOR]

The spatial and temporal evolution of the absolute electron densities and temperatures in plasmas formed by nanosecond pulsed laser ablation of silicon in vacuum at two wavelengths (1064 and 532 nm), at similar irradiances, have been explored by complementary simulation (using combined hydrodynamic and adiabatic models) and experiment. Modelling the laser-target and laser-plume interactions with the POLLUX code reveals the evolving composition and dynamics of the laser induced plasma (LIP) during the incident laser pulse: 532 nm irradiation causes more ablation, but the LIP formed by 1064 nm excitation has a higher average charge state and expands faster. The experimental data, from the analysis of Stark broadened line shapes of SiIII and SiIV cations in time-gated, position- and wavelength-resolved images of the plume emission, allow characterisation of the plume dynamics at later times. These dynamics are compared with predictions from two forms of adiabatic expansion model. Both take as input parameters the plume properties returned by the POLLUX simulations for the end of the laser pulse, but differ according to whether the initial plasma is assumed isothermal or isentropic. The study illustrates the important λ-dependences of the target absorption coefficient (in establishing the ablated material density) and of electron–ion inverse bremsstrahlung absorption (in coupling laser radiation into the emergent plasma); the extents to which these interactions, the relative ablation yields, and the plume expansion dynamics depend on λ; and the importance of identifying appropriate initial conditions for adiabatic expansion modelling of LIP in vacuum. [ABSTRACT FROM AUTHOR]

Determining the product velocities offers one of the most direct and penetrating experimental probes of the dynamics of gas phase molecular photodissociation and bimolecular collision processes and provides an obvious point of contact with theoretical molecular dynamics simulations, potential energy surfaces, and non-adiabatic couplings between such surfaces. This perspective traces the development of the H Rydberg atom translational spectroscopy technique from a serendipitous first encounter through to the present, highlights the advances that make it the method of choice for studying many benchmark photofragmentation and photoinduced collision processes that yield H (or D) atoms amongst the products, and anticipates some future opportunities afforded by the technique. [ABSTRACT FROM AUTHOR]

Non-adiabatic couplings between Born–Oppenheimer (BO)-derived potential energy surfaces are now recognized as pivotal in describing the non-radiative decay of electronically excited molecules following photon absorption. This opinion piece illustrates how non-BO effects provide photostability to many biomolecules when exposed to ultraviolet radiation, yet in many other cases are key to facilitating 'reactive' outcomes like isomerization and bond fission. The examples are presented in order of decreasing molecular complexity, spanning studies of organic sunscreen molecules in solution, through two families of heteroatom containing aromatic molecules and culminating with studies of isolated gas phase H2O molecules that afford some of the most detailed insights yet available into the cascade of non-adiabatic couplings that enable the evolution from photoexcited molecule to eventual products. This article is part of the theme issue 'Chemistry without the Born–Oppenheimer approximation'. [ABSTRACT FROM AUTHOR]

Nitrogen is ubiquitous in both natural and laboratory-grown diamond, but the number and nature of the nitrogen-containing defects can have a profound effect on the diamond material and its properties. An ever-growing fraction of the supply of diamond appearing on the world market is now lab-grown. Here, we survey recent progress in two complementary diamond synthesis methodshigh pressure high temperature (HPHT) growth and chemical vapor deposition (CVD), how each is allowing ever more precise control of nitrogen incorporation in the resulting diamond, and how the diamond produced by either method can be further processed (e.g., by implantation or annealing) to achieve a particular outcome or property. The burgeoning availability of diamond samples grown under well-defined conditions has also enabled huge advances in the characterization and understanding of nitrogen-containing defects in diamondalone and in association with vacancies, hydrogen, and transition metal atoms. Among these, the negatively charged nitrogen-vacancy (NV-) defect in diamond is attracting particular current interest in account of the many new and exciting opportunities it offers for, for example, quantum technologies, nanoscale magnetometry, and biosensing. [ABSTRACT FROM AUTHOR]

Spatially resolved optical emission spectroscopy (OES) has been used to investigate the gas phase chemistry and composition in a microwave activated CH4/Ar/H2 plasma operating at moderate power densities (∼30 W cm-3) and pressures (≤175 Torr) during chemical vapor deposition of polycrystalline diamond. Several tracer species are monitored in order to gain information about the plasma. Relative concentrations of ground state H (n=1) atoms have been determined by actinometry, and the validity of this method have been demonstrated for the present experimental conditions. Electronically excited H (n=3 and 4) atoms, Ar (4p) atoms, and C2 and CH radicals have been studied also, by monitoring their emissions as functions of process parameters (Ar and CH4 flow rates, input power, and pressure) and of distance above the substrate. These various species exhibit distinctive behaviors, reflecting their different formation mechanisms. Relative trends identified by OES are found to be in very good agreement with those revealed by complementary absolute absorption measurements (using cavity ring down spectroscopy) and with the results of complementary two-dimensional modeling of the plasma chemistry prevailing within this reactor. [ABSTRACT FROM AUTHOR]

Microwave (MW) plasma-enhanced chemical vapor deposition (PECVD) reactors are widely used for growing diamond films with grain sizes spanning the range from nanometers through microns to millimeters. This paper presents a detailed description of a two-dimensional model of the plasma-chemical activation, transport, and deposition processes occurring in MW activated H/C/Ar mixtures, focusing particularly on the following base conditions: 4.4%CH4/7%Ar/balance H2, pressure p=150 Torr, and input power P=1.5 kW. The model results are verified and compared with a range of complementary experimental data in the companion papers. These comparators include measured (by cavity ring down spectroscopy) C2(a), CH(X), and H(n=2) column densities and C2(a) rotational temperatures, and infrared (quantum cascade laser) measurements of C2H2 and CH4 column densities under a wide range of process conditions. The model allows identification of spatially distinct regions within the reactor that support net CH4→C2H2 and C2H2→CH4 conversions, and provide a detailed mechanistic picture of the plasma-chemical transformations occurring both in the hot plasma and in the outer regions. Semianalytical expressions for estimating relative concentrations of the various C1Hx species under typical MW PECVD conditions are presented, which support the consensus view regarding the dominant role of CH3 radicals in diamond growth under such conditions. [ABSTRACT FROM AUTHOR]

Comparative studies of the pulsed laser ablation of graphite in 20 mTorr of N2 using both 15 ns and 450 fs pulses at a wavelength of 248 nm are reported. Emissions from the resulting ablation plumes, and from collisions with ablated material and the background N2 gas molecules, have been investigated by wavelength-, space-, and time-resolved optical emission spectroscopy (OES), and the observations correlated with the results of the analyses of films formed when such material is incident on a silicon substrate. Wavelength-dispersed spectra of the plume arising in nanosecond ablation reveal C I, C II, and C2 emissions—concentrated close to the target—and, at greater distances, strong CN and weak N2+ emissions. N2+(B–X) emission dominates in the case of femtosecond ablation. Time-gated imaging studies have allowed estimation of propagation velocities for these various emissions. Possible production routes for secondary emitters such as CN and N2+ are discussed, and arguments presented to show that measurements of the apparent propagation “velocities” of such emissions are unlikely to provide meaningful measures of the velocities (or energies) with which these carriers impact on a substrate surface. Laser Raman spectroscopy confirms nitrogen incorporation within the films grown by both nanosecond and femtosecond ablations; the former films are deduced to be both thicker and to have higher N content—findings that accord with the OES analyses. [ABSTRACT FROM AUTHOR]

The H(D) Rydberg atom photofragment translational spectroscopy technique has been applied to a further detailed investigation of the photodissociation dynamics of NH3 and ND3 molecules following excitation to the lowest two (v2=0 and 1) vibrational levels of the first excited (A 1A2″) singlet electronic state. Analysis of the respective total kinetic energy release spectra, recorded at a number of scattering angles Θ [where Θ is the angle between the ε vector of the photolysis photon and the time-of-flight (TOF) axis], enables quantification of a striking, quantum state dependent, μ-v correlation in the NH2(ND2) products. The spatial distribution of the total flux of H(D) atom photofragments is rather isotropic (βlab∼0). However, more careful analysis of the way in which the TOF spectra of the H(D) atom photofragments vary with Θ reveals that each H+NH2(D+ND2) product channel has a different ‘‘partial’’ anisotropy parameter, βlab(v2,N), associated with it: The H(D) atom ejected by those molecules that dissociate to yield NH2(ND2) fragments with little rotational excitation largely appear in the plane of the excited molecule (i.e., perpendicular to the transition moment and the C3 axis of the parent, with β tending towards -1). Conversely, the H(D) atoms formed in association with the most highly rotationally excited partner NH2(ND2) fragments tend to recoil almost parallel to this C3 axis (i.e., β→+2). Such behavior is rationalized in the context of the known potential energy surfaces of the A and X states of ammonia using a classical, energy and angular momentum conserving impact parameter model in which we assume that all of the product angular momentum is established at the ‘‘point’’ of the conical intersection in the H–NH2(D–ND2) dissociation coordinate. We conclude by reemphasizing the level of care needed in interpreting... [ABSTRACT FROM AUTHOR]

The technique of H Rydberg atom photofragment translational spectroscopy has been applied to a high resolution study of the primary photochemistry of methanethiol (CH3SH) following excitation at a wide range of wavelengths in the near ultraviolet. In accord with previous studies of this molecule, excitation within its first (1 1A‘-X 1A’) absorption continuum is shown to result in S–H bond fission. Spectral analysis yields a refined value for the bond dissociation energy: D00(CH3S–H)=30 250±100 cm-1. The resulting CH3S(X) fragments are deduced to carry only modest vibrational excitation, distributed specifically in the ν3 (C–S stretching) mode and in one other mode having a wave number of ∼1040 cm-1. We associate this latter mode with bending of the CH3 moiety in the plane containing the C and S nuclei and the lobe of the unpaired electron which was originally involved in the S–H bond. Decreasing the excitation wavelength (while remaining within the first absorption continuum) results in an increase in both the vibrational and rotational excitation of the CH3S(X) fragments, but a decrease in the relative yield of the upper (2E1/2) spin–orbit component. Excitation at still shorter excitation wavelengths accesses the second (2 1A‘-X 1A’) absorption band of CH3SH. The CH3S fragments resulting from S–H bond fission at these excitation wavelengths are observed to carry very much higher levels of vibrational excitation in the above two modes. The observation of H atoms attributable to secondary photolysis of SH(X) fragments indicates increased competition from the alternative C–S bond fission channel at these shorter excitation wavelengths. Additional peaks in the H atom time-of-flight spectrum, most clearly evident following excitation at wavelengths in the range 213–220 nm, are interpretable in terms of secondary photolysis of the primary CH3S(X) fragments yielding... [ABSTRACT FROM AUTHOR]

Five new triplet excited states of the ND radical (three in the case of NH) in the wave-number range 85 000–91 000 cm-1 have been identified through analysis of the two-photon resonance enhancements they provide to the wavelength-resolved multiphoton ionization spectrum of X 3Σ- state NH(ND) radicals. The lowest energy of these, the B 3Π state, is found to be a ‘‘regular’’ Rydberg state which, on the basis of its observed quantum defect and its deduced rotational and spin–orbit coupling constant, is surmised to be the 3Π state derived from a 3pσ electron built on the 2Π ground-state ion core. Perturbations are evident in the B 3Π–X 3Σ- origin bands of both NH and ND. In the case of ND the perturbing state provides its own resonance enhancements, the analysis of which enables its definitive identification as the C 3Σ- state. The very small spin–orbit splitting found for the D 3Π state is taken to indicate that (at least in the Franck–Condon region) its wave function is dominated by the configuration involving one 3pπ Rydberg electron and a 4Σ- ion core. To still higher wave number we identify two more 3Σ- excited states, the upper of which (the F 3Σ- state) has a very small rotational constant which we take to imply that it has substantial valence character. Further indications that the F 3Σ- (and C 3Σ-) states possess significant valence character is provided by the observation that both parent and daughter (N+) ions contribute to the overall ion yield when the multiphoton ionization proceeds via these two states. Daughter-ion formation is considered to occur via an overall four-photon excitation process in which the coherent two-photon excitation to the 3Σ- state of interest is followed by a one-photon excitation to a ‘‘superexcited’’ state of the neutral. This is then presumed to absorb a... [ABSTRACT FROM AUTHOR]

The rovibrational state distribution of the nascent NH2(A 2A1) fragments generated by 193.3 nm photodissociation of a room temperature sample of NH3 is determined through an analysis of a major portion (6000–13 000 cm-1) of the NH2(A 2A1→X 2B1) near infrared emission spectrum obtained by time-resolved Fourier transform infrared emission spectroscopy. The NH2(A) fragments are observed to be formed predominantly in their zero-point vibrational level, with substantial rotational excitation about their a-inertial axis up to the limit of the available energy, ∼3150 cm-1, but with little excitation about the other axes. The pattern of this energy disposal is discussed within the framework of existing knowledge regarding the form of the NH3 A state potential energy surface on which the dissociation occurs. The essential features are entirely consistent with a direct carry over, into the fragment, of the out-of-plane bending vibrational motion introduced in the parent molecule by the photoexcitation process. [ABSTRACT FROM AUTHOR]

Diamond synthesis by chemical vapour deposition (CVD) from carbon-containing gas mixtures has by now long been an industrial reality, but commercial interest and investment into the technology has grown dramatically in the last several years. This Feature Article surveys recent advances in our understanding of the gas-phase chemistry of microwave-activated methane/hydrogen plasmas used for diamond CVD, including that of added boron-, nitrogen- and oxygen-containing dopant species. We conclude by considering some of the remaining challenges in this important area of contemporary materials science. [ABSTRACT FROM AUTHOR]

Exciting a molecule with an ultraviolet photon often leads to bond fission, but the final outcome of the bond cleavage is typically both molecule tnd phase dependent. The photodissociation of an isolated gas-phase molecule can be viewed as a closed system: Energy and momentum are conserved, and the fragmentation is irreversible. The same is not true in a solutionphase photodissociation process. Solvent interactions may dissipate some of the photoexcitation energy prior to bond fission and will dissipate any excess energy partitioned into the dissociation products. Products that have no analog in the corresponding gas-phase study may arise by, for example, geminate recombination. Here, we illustrate the extent to which dynamical insights from gas-phase studies can inform our understanding of the corresponding solution-phase photochemistry and how. in the specific case of photoinduced ring-opening reactions, solution-phase studies can in some cases reveal dynamical insights more clearly than die corresponding gas-phase study. [ABSTRACT FROM AUTHOR]

We report the first spectroscopic characterization of any triplet Rydberg state of the imidogen radical. The B 3Π state of NH and ND is identified via the two photon resonance enhancement it provides in the MPI spectrum of ground (X 3Σ-) state NH(ND) radicals, and shown to be well described (at least near its equilibrium geometry) as a 3p Rydberg state built on the ground (X 2Π) state ion core. We consider also the origin of an overlapping 3Σ-X 3Σ- two photon resonance apparent only in the spectrum of the ND radical. [ABSTRACT FROM AUTHOR]

The dissociation of diatomic molecules (H2, N2, O2) on the surface of a metal hot filament (HF) has been studied theoretically, within the framework of a unified two-step reaction mechanism, and via joint experimental-modelling studies of the gas temperature and selected species concentration distributions in the vicinity of the HF. This approach allows description of the variations in atomic concentrations with distance (d, measured from the HF) as functions of HF temperature and gas pressure. Complications associated with the extension of such analyses to interpretations of prior experimental measurements for larger, heteronuclear molecules (CH4, nitrogen oxides) are discussed. The present analysis also provides a rationale for the d-dependent non-equilibrium [N2(v = 1)]/[N2(v = 0)] vibrational population ratios measured in tungsten HF-activated N2 gas samples. [ABSTRACT FROM AUTHOR]

High-resolution time-of-flight measurements of H atom products from photolysis of phenol, 4-methylphenol, 4-fluorophenol, and thiophenol, at many UV wavelengths (λphot), have allowed systematic study of the influence of ring substituents and the heteroatom on the fragmentation dynamics. All dissociate by X—H (X = 0, 5) bond fission after excitation at their respective S1(¹ππ*)-S0 origins and at all shorter wavelengths. The achieved kinetic energy resolution reveals population of selected vibrational levels of the various phenoxyl and thiophenoxyl coproducts, providing uniquely detailed insights into the fragmentation dynamics. Dissociation in all cases is deduced to involve nuclear motion on the ¹πσ* potential energy surface (PES). The route to accessing this PES, and the subsequent dynamics, is seen to be very sensitive to λphot and substitution of the heteroatom. In the case of the phenols, dissociation after excitation at long λphot is rationalized in terms of radiationless transfer from S1 to S0 levels carrying sufficient O—H stretch vibrational energy to allow coupling via the conical intersection between the S0 and ¹πσ* PESs at longer O—H bond lengths. In contrast, H + C6H5O(X²B1) products formed after excitation at short λphot exhibit anisotropic recoil-velocity distributions, consistent with prompt dissociation induced by coupling between the photoprepared ¹ππ* excited state and the ¹πσ* PES. The fragmentation dynamics of thiophenol at all λphot matches the latter behavior more closely, reflecting the different relative dispositions of the ¹ππ* and ¹πσ* PESs. Additional insights are provided by the observed branching into the ground (X²B1) and first excited (²B2) states of the resulting C6H5S radicals. [ABSTRACT FROM AUTHOR]

This study of the photodissociation of dimethyl sulfide at λ = 227.5 nm demonstrates the opportunities (and some of the challenges) of product detection using vacuum ultraviolet photoionization combined with recently developed multi-mass imaging methods. The capability of imaging different charged products simultaneously allows determination of the primary fragmentation dynamics through, for example, product fragment momentum and angular distribution matching and reveals potential complications from dissociative ionization, product alignment-dependent photoionization probabilities, and the effects of space charging. [ABSTRACT FROM AUTHOR]

The near ultraviolet photolysis of jet-cooled acetylene molecules has been further investigated using the technique of H atom photofragment translational spectroscopy. Analysis of the rotational structure evident in the total fragment kinetic energy spectrum yields a precise value for the C–H bond dissociation energy: D0(HCC–H)=46 074±8 cm-1 (551.2±0.1kJ mol-1). [ABSTRACT FROM AUTHOR]

When phenol is photoexcited to its S1 (11ππ*) state at wavelengths in the range 257.403 ≤ λphot ≤ 275.133 nm the O-H bond dissociates to yield an H atom and a phenoxyl co-product, with the available energy shared between translation and well characterised product vibration. It is accepted that dissociation is enabled by transfer to an S2 (11πσ*) state, for which the potential energy surface (PES) is repulsive in the O-H stretch coordinate, RO-H. This S2 PES is cut by the S1 PES near RO-H = 1.2 Å and by the S0 ground state PES near RO-H = 2.1 Å, to give two conical intersections (CIs). These have each been invoked-both in theoretical studies and in the interpretation of experimental vibrational activity-but with considerable controversy. This paper revisits the dynamic mechanisms that underlie the photodissociation of phenol and substituted phenols in the light of symmetry restrictions arising from torsional tunnelling degeneracy, which has been neglected hitherto. This places tighter symmetry constraints on the dynamics around the two CIs. The non-rigid molecular symmetry group G4 necessitates vibronic interactions by a2 modes to enable coupling at the inner, higher energy (S1/S2) CI, or by b1 modes at the outer, lower energy (S2/S0) CI. The experimental data following excitation through many vibronic levels of the S1 state of phenol and substituted phenols demonstrate the effective role of the ν16a (a2) ring torsional mode in enabling O-H bond fission. This requires tunnelling under the S1/S2 CI, with a hindering barrier of ∼5000 cm-1 and with the associated geometric phase effect. Quantum dynamic calculations using new ab initio PESs provide quantitative justification for this conclusion. The fates of other excited S1 modes are also rationalised, revealing both spectator modes and intramolecular vibrational redistribution between modes. A common feature in many cases is the observation of an extended, odd-number only, progression in product mode ν16a (i.e., the parent mode which enables S1/S2 tunnelling), which we explain as a Franck-Condon consequence of a major change in the active vibration frequency. These comprehensive results serve to confirm the hypothesis that O-H fission following excitation to the S1 state involves tunnelling under the S1/S2 CI-in accord with conclusions reached from a recent correlation of the excited state lifetimes of phenol (and many substituted phenols) with the corresponding vertical energy gaps between their S1 and S2 PESs. [ABSTRACT FROM AUTHOR]

A comprehensive study of microwave (MW) activated CH4/H2/Ar plasmas used for diamond chemical vapor deposition is reported, focusing particularly on the effects of gross variations in the H2/Ar ratio in the input gas mixture (from H2/Ar mole fraction ratios of > 10:1, through to ∼1:99). Absolute column densities of C2(a) and CH(X) radicals and of H(n = 2) atoms have been determined by cavity ringdown spectroscopy, as functions of height (z) above a substrate and of process conditions (CH4, H2, and Ar input mole fractions, total pressure, p, and input microwave power, P). Optical emission spectroscopy has also been used to explore the relative densities of electronically excited H atoms, and CH, C2, and C3 radicals, as functions of these same process conditions. These experimental data are complemented by extensive 2D (r, z) modeling of the plasma chemistry, which provides a quantitative rationale for all of the experimental observations. Progressive replacement of H2 by Ar (at constant p and P) leads to an expanded plasma volume. Under H2-rich conditions, > 90% of the input MW power is absorbed through rovibrational excitation of H2. Reducing the H2 content (as in an Ar-rich plasma) leads to a reduction in the absorbed power density; the plasma necessarily expands in order to accommodate a given input power. The average power density in an Ar-rich plasma is much lower than that in an H2-rich plasma operating at the same p and P. Progressive replacement of H2 by Ar is shown also to result in an increased electron temperature, an increased [H]/[H2] number density ratio, but little change in the maximum gas temperature in the plasma core (which is consistently ∼3000 K). Given the increased [H]/[H2] ratio, the fast H-shifting (CyHx + H ↔ CyHx-1 + H2; y = 1-3) reactions ensure that the core of Ar-rich plasma contains much higher relative abundances of 'product' species like C atoms, and C2, and C3 radicals. The effects of Ar dilution on the absorbed power dissipation pathways and the various species concentrations just above the growing diamond film are also investigated and discussed. [ABSTRACT FROM AUTHOR]

Resolved sets of photoproducts arising from the photodissociation of axial and equatorial conformers of 3-pyrroline have been observed using H(Rydberg) atom photofragment translational spectroscopy following excitation in the wavelength range of 250-213 nm. 3-pyrroline (alternatively 2,5-dihydropyrrole) is a five membered partially saturated heterocycle in which the bonding around the N atom is pyramidal (sp3 hybridized) and the N-H bond can lie either axial or equatorial to the ring. Careful analysis of total kinetic energy release data derived from H atom time-of-flight measurements reveals excitation of the 3-pyrrolinyl cofragment consistent with N-H bond fission in both the axial and equatorial conformers. This allows determination of the energy difference between the ground state conformers to be 340±50 cm-1 and the N-H bond strength for axial and equatorial conformers as 31 610±50 and 31 270±50 cm-1, respectively. [ABSTRACT FROM AUTHOR]

This article reports a comprehensive study of the mechanisms of H atom loss in aniline (C6H5NH2) following ultraviolet excitation, using H (Rydberg) atom photofragment translational spectroscopy. N–H bond fission via the low lying 1πσ* electronic state of aniline is experimentally demonstrated. The 1πσ* potential energy surface (PES) of this prototypical aromatic amine is essentially repulsive along the N–H stretch coordinate, but possesses a shallow potential well in the vertical Franck–Condon region, supporting quasibound vibrational levels. Photoexcitation at wavelengths (λphot) in the range 293.859 nm≥λphot≥193.3 nm yields H atom loss via a range of mechanisms. With λphot resonant with the 11ππ*←S0 origin (293.859 nm), H atom loss proceeds via, predominantly, multiphoton excitation processes, resonantly enhanced at the one photon energy by the first 1ππ* excited state (the 11ππ* state). Direct excitation to the first few quasibound vibrational levels of the 1πσ* state (at wavelengths in the range 269.513 nm≥λphot≥260 nm) induces N–H bond fission via H atom tunneling through an exit barrier into the repulsive region of the 1πσ* PES, forming anilino (C6H5NH) radical products in their ground electronic state, and with very limited vibrational excitation; the photo-prepared vibrational mode in the 1πσ* state generally evolves adiabatically into the corresponding mode of the anilino radical upon dissociation. However, as the excitation wavelength is reduced (λphot<260 nm), N–H bond fission yields fragments with substantially greater vibrational excitation, rationalized in terms of direct excitation to 11ππ* levels, followed by coupling to the 1πσ* PES via a 11ππ*/1πσ* conical intersection. Changes in product kinetic energy disposal once λphot approaches ∼230 nm likely indicate that the photodissociation pathways of aniline proceed via direct excitation to the (higher) 21ππ* state. Analysis of the anilino fragment vibrational energy disposal—and thus the concomitant dynamics of 1πσ* state mediated photodissociation—provides a particularly interesting study of competing σ*←π and π*←π absorption processes and develops our appreciation of the photochemistry of aromatic amines. It also allows revealing comparisons with simple amines (such as ammonia and methylamine) as well as the isoelectronic species, phenol. This study yields a value for the N–H bond strength in aniline, D0(H-anilino)=31630±40 cm-1. [ABSTRACT FROM AUTHOR]

CH4 and C2H2 molecules (and their interconversion) in hydrocarbon/rare gas/H2 gas mixtures in a microwave reactor used for plasma enhanced diamond chemical vapor deposition (CVD) have been investigated by line-of-sight infrared absorption spectroscopy in the wavenumber range of 1276.5-1273.1 cm-1 using a quantum cascade laser spectrometer. Parameters explored include process conditions [pressure, input power, source hydrocarbon, rare gas (Ar or Ne), input gas mixing ratio], height (z) above the substrate, and time (t) after addition of hydrocarbon to a pre-existing Ar/H2 plasma. The line integrated absorptions so obtained have been converted to species number densities by reference to the companion two-dimensional (r,z) modeling of the CVD reactor described in Mankelevich et al. [J. Appl. Phys. 104, 113304 (2008)]. The gas temperature distribution within the reactor ensures that the measured absorptions are dominated by CH4 and C2H2 molecules in the cool periphery of the reactor. Nonetheless, the measurements prove to be of enormous value in testing, tensioning, and confirming the model predictions. Under standard process conditions, the study confirms that all hydrocarbon source gases investigated (methane, acetylene, ethane, propyne, propane, and butane) are converted into a mixture dominated by CH4 and C2H2. The interconversion between these two species is highly dependent on the local gas temperature and the H atom number density, and thus on position within the reactor. CH4→C2H2 conversion occurs most efficiently in an annular shell around the central plasma (characterized by 1400

This article reports systematic measurements of the power utilization by Ta (and Re) hot filaments (HFs) operating in a poor vacuum, in pure He, N2, and H2, and in CH4/H2 gas mixtures of relevance to diamond growth by HF chemical vapor deposition, as functions of filament temperature Tfil (in the range of 1800–2700 K) and gas pressure p (in the range of 10-2–100 Torr). In the cases of H2 and the CH4/H2 gas mixtures, the power consumption studies are complemented by in situ measurements of the relative H atom densities [H] near the HF—which are seen to maximize at p∼10–20 Torr and thereafter to remain constant or, at the highest Tfil, to decline at higher p. These (and many previous) findings are rationalized by a companion theoretical analysis, which reduces the complex array of chemisorption and desorption processes that must contribute to the HF-surface mediated dissociation of H2 to a two-step mechanism involving H atom formation by dissociative adsorption at bare (S*) sites and by desorption at hydrogenated (SH) sites on the HF surface. [ABSTRACT FROM AUTHOR]

Absolute column densities of C2(a) and CH radicals and H(n=2) atoms have been measured in a diamond growing microwave reactor operating with hydrocarbon/Ar/H2 gas mixtures as functions of height (z) above the substrate surface and process conditions. The monitored species are each localized in the hot plasma region, where Tgas∼3000 K, and their respective column densities are each reproduced, quantitatively, by two-dimensional (r,z) modeling of the plasma chemistry. The H(n=2) distribution is seen to peak nearer the substrate, reflecting its sensitivity both to thermal chemistry (which drives the formation of ground state H atoms) and the distributions of electron density (ne) and temperature (Te). All three column densities are found to be sensitively dependent on the C/H ratio in the process gas mixture but insensitive to the particular choice of hydrocarbon (CH4 and C2H2). The excellent agreement between measured and predicted column densities for all three probed species, under all process conditions investigated, encourages confidence in the predicted number densities of other of the more abundant radical species adjacent to the growing diamond surface which, in turn, reinforces the view that CH3 radicals are the dominant growth species in microwave activated hydrocarbon/Ar/H2 gas mixtures used in the chemical vapor deposition of microcrystalline and single crystal diamond samples. [ABSTRACT FROM AUTHOR]

This work describes the design and validation of an instrument to measure the kinetic energies of ions ejected by the pulsed laser ablation (PLA) of a solid target. Mass spectra show that the PLA of Ni, Al, and ZnO targets, in vacuum, using the second harmonic of a Nd:YAG laser (532 nm, pulse duration ∼10 ns) generates abundant Xn+ ions (n≤3 for Ni, ≤2 for Al, ≤3 and ≤2 for Zn and O respectively from ZnO). Ions are selected by their mass/charge (m/z) ratio prior to the determination of their times of flight. PLA of Ni has been studied in most detail. The mean velocities of ablated Nin+ ions are shown to follow the trend v(Ni3+)>v(Ni2+)>v(Ni+). Data from Ni2+ and Ni3+ are fitted to shifted Maxwellian functions and agree well with a model which assumes both thermal and Coulombic contributions to ion velocities. The dependence of ion velocities on laser pulse energy (and fluence) is investigated, and the high energy data are shown to be consistent with an effective accelerating voltage of ∼90 V within the plume. The distribution of velocities associated with Ni3+ indicates a population at cooler temperature than Ni2+. [ABSTRACT FROM AUTHOR]

Ion imaging methods have been used to study the dynamics of H2(D2) molecular elimination from H2S+(D2S+) cations following photoexcitation to the à 2A1 state in the wavelength range 300<λ<360 nm. Ground (X 2B1) state parent ions were formed by multiphoton ionization of a jet cooled H2S(D2S) sample, resonance enhanced at the two photon energy by the v=0 level of the 1A2(...2b114pb21) Rydberg state. This Rydberg excited state predissociates sufficiently slowly that the 2+1 resonance enhanced multiphoton ionization (REMPI) spectrum shows resolved rovibronic structure, thereby allowing full quantum state selectivity at this intermediate stage of the cation preparation process. Analysis of the S+ ion images following one photon excitation of the resulting H2S+(D2S+) cations shows that these fragments are formed in their ground 4S state, and that the H2(D2) cofragments are formed predominantly (if not exclusively) in rotational states with either odd or even J rotational quantum number—depending on the chosen REMPI preparation wavelength. This striking specificity for forming ortho- or para-H2(D2) products can be traced back to the state selectivity introduced in the REMPI preparation step. In the case of H2S, therefore, the nuclear spin symmetry of the two equivalent H nuclei in ortho-H2S (and H2S+) carries through into ortho-H2 products, and para-H2S molecules map into para-H2 fragments, surviving photoionization of the Rydberg state, photolysis of the resulting parent cation, and two subsequent radiationless transitions during the evolution from the photoexcited (à 2A1 state) cation through to S(4S)+H2 products. We identify two distinct fragmentation pathways. One, which we term route I, involves nonadiabatic (Renner-Teller) coupling to the X state at near linear configurations and subsequent (spin-orbit induced) coupling to the repulsive 4A2 potential energy surface (PES) at smaller bond angles. This process operates throughout the photolysis wavelength range investigated and yields rotationally “cool” and vibrationally “cold” H2 products. The second (route II) shows a long wavelength threshold λ∼335 nm, and gradually becomes dominant as the photolysis wavelength is reduced. Route II dissociation involves vibronically facilitated nonadiabatic transfer from the à to the B 2B2 state, followed by spin-orbit induced transfer to the 4A2 PES; the route II fragmentation dynamics results in H2 products carrying higher levels of rotational and vibrational excitation. [ABSTRACT FROM AUTHOR]

Pulsed laser ablation of LiF was studied using both nanosecond (ns) and femtosecond (fs) pulses at 248 nm. Optical emission from electronically excited Li and F atoms in the plume of ejected material was investigated by wavelength, time and spatially resolved imaging methods. Careful analysis of images of species selected optical emission yielded estimates of the mean velocities of the Li+ ions arising in both excitation schemes (∼11 and ∼13 km/s, respectively), and highlighted the dramatic effects of radiation trapping, most notably by the reabsorption of Li(2p→2s) emission by ground state Li atoms in the ns ablation studies. Plumes formed by fs excitation are found to contain a higher fraction of energetic/electrically excited components, including excited F atoms and ions, indicative of an explosive boiling mechanism, whereas the ablation plume resulting from ns ablation is deduced to arise primarily from thermal evaporation of the transiently heated target surface. The amount of target material removed per shot is significantly less in the case of fs excitation. The density (and size) of unwanted droplets in films grown by fs ablation is much smaller than in the case of ns ablation, especially on substrates mounted in an off-axis ablation geometry, implying that hydrodynamic sputtering is much reduced by the use of short pulses and that fs ablation must be the preferred route to forming very thin LiF coatings. [ABSTRACT FROM AUTHOR]

The ablation plumes arising after irradiation of graphite, silicon, copper and aluminum with a pulsed nanosecond ArF (λ=193 nm) laser at fluences between 2 and 20 J cm[sup -2] in vacuum are studied and compared. The neutral and ionic components in the ablation plume have been measured via quadrupole mass spectrometry and ion probes, respectively. Additional information about the degree of ionization and the velocities of singly and multiply charged ions in the plume have been deduced via optical emission spectrometry, and the electron velocity distributions have been measured with Langmuir probes. Probing the plasma properties with this range of techniques is shown to provide a rather detailed picture of the ablation characteristics. The velocity distributions of the neutral atoms are comparatively narrow (∼1 km s[sup -1] full width at half maximum) and peaked at a center of mass velocity of ∼3–4 km s[sup -1]. Their general form is reminiscent of those of species expanding supersonically from a pulsed nozzle. The electron and ion velocity distributions are much broader, and centered at much higher velocities (and kinetic energies). The relative ion yield, and the overall degree of ionization, both increase with increasing fluence and scale inversely with the ionization potentials of the respective target materials. Both charged components are found to be accelerating at short distances from the target. Such effects have been rationalized by assuming that incident laser radiation ionizes (by multiphoton ionization) neutral species ablated from the target surface, and that these ions and electrons then act as “seeds” for subsequent plume heating, ionization and plasma formation by inverse bremsstrahlung. This absorption due to inverse bremsstrahlung ensures the incident laser intensity is highest at the outer edge of the expanding plume. The outer region thus receives preferential excitation and heating—traditionally pictured in terms of the so-called two electron temperature model. Some of the resulting “hot” electrons escape from this coronal region, leading to an overall charge imbalance within the plasma, which manifests itself as an acceleration (driven by Coulombic interactions) of the charged components within the plume. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The boundaries of the diamond deposition region in the C-H-O (Bachmann) atomic phase composition diagram have been reproduced successfully for 38 different C, H, and O containing gas mixtures using the CHEMKIN computer package, together with just two criteria—a minimum mole fraction of methyl radicals [CH[sub 3]] and a limiting value of the [H]/[C[sub 2]H[sub 2]] ratio. The diamond growth/no-growth boundary coincides with the line along which the input mole fractions of C and O are equal. For every gas mixture studied, no-growth regions are found to coincide with a negligible (<10[sup -10]) mole fraction of CH[sub 3] radicals, while for gas mixtures lying within the diamond growth region the CH[sub 3] mole fraction is ∼10[sup -7]. Each no-growth→diamond growth boundary is seen to be accompanied by a 2-3 order of magnitude step in CH[sub 3] mole fraction. The boundary between diamond and nondiamond growth is less clearly defined, but can be reproduced by assuming a critical, temperature dependent [H]/[C[sub 2]H[sub 2]] ratio (0.2, in the case that T[sub gas]=2000 K) that reflects the crucial role of H atoms in the etching of nondiamond phases. The analysis allows prediction of the composition process window for good quality diamond growth for all stable input gas mixtures considered in this study. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The technique of H(D) Rydberg atom photofragment translational spectroscopy has been used to investigate the photodissociation dynamics of the mixed isotopomers NH2D and NHD2 following the excitation to the v2′=0 and 1 levels of their lowest lying A 1B1 (C2v) excited electronic states. Peaks in the resulting total kinetic energy release (TKER) spectra are assigned to levels of the NH2, NHD, or ND2 fragments with a wide range of quantum numbers Ka for rotation about their a inertial axes, and with N=Ka, N=Ka+1, or N=Ka+2 as appropriate. These data provide the first measurements of high rotational levels for the ground electronic state of the NHD radical. The least squares fitting of all these spectra, and those resulting from NH3 and ND3, to the best calculated NH2, NHD, and/or ND2 rotational term values provides accurate estimations of the respective N–H and N–D bond dissociation energies D00 across the whole series. These values are D00(H–NH2)=37 115±20 cm-1 (4.602±0.002 eV); D00(H–NHD)=37 240±50 cm-1; D00(H–ND2)=37 300±30 cm-1; D00(D–NHD)=37 880±60 cm-1; and D00(D–ND2)=38 010±20 cm-1. The differences between these values are fully consistent with differences in zero-point energies and lead to a mean value of De=40 510±25 cm-1. Dissociation of NH2D or NHD2 through their (A-X) 210 bands to give an NHD product leads to TKER spectra with a much higher statistical character than those leading to an NH2 or ND2 product, and to those obtained following excitation through the 000 bands. This is rationalized in a semiquantitative manner in terms of a varying contribution to the dissociation rate of the parent molecules from internal conversion (IC) to high levels of their respective ground states. Nuclear permutation symmetry appears to play an important role both for the IC rates and for the subsequent branching between product channels. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The technique of H(D) atom photofragment translational spectroscopy has been applied to the photodissociation of HCN(DCN) at 121.6 nm. Analyses of the H(D) atom time-of-flight spectra reveal the partner CN fragment to be formed predominantly in its A 2Π excited electronic state. Branching into the H/D+CN(B 2∑+) product channel accounts for a few percent of the total fragment yield, but we discern no evidence for any contribution from the product channel yielding H/D atoms in conjunction with ground state CN(X 2∑+) fragments. The majority of the CN (A) fragments are formed in their v’=0 level but with a markedly bimodal rotational state population distribution. This bimodality has been rationalized in the light of the available information regarding the form of the potential energy surface of the excited 1Π state of HCN(DCN) populated following photoexcitation at 121.6 nm. [ABSTRACT FROM AUTHOR]

The article presents the authors' view on study investigating the scrambling of hydrogen and deuterium (H/D) atoms in acetaldehyde at the energetic threshold. It notes that the study indicates possible occurrence of significant photo-initiated isomerization dynamics in cases where barriers to various isomerization steps lie at energies lower than the bond dissociation limit. It also describes the photo-excitation wavelengths used in the study as characteristic of those from solar spectrum reaching the troposphere. Furthermore, it highlights the implications that the findings from the study may have for atmospheric chemistry models.

Coulomb explosion velocity-map imaging is a new and potentially universal probe for gas-phase chemical dynamics studies, capable of yielding direct information on (time-evolving) molecular structure. The approach relies on a detailed understanding of the mapping between the initial atomic positions within the molecular structure of interest and the final velocities of the fragments formed via Coulomb explosion. Comprehensive on-the-fly ab initio trajectory studies of the Coulomb explosion dynamics are presented for two prototypical small molecules, formyl chloride and cis-1,2-dichloroethene, in order to explore conditions under which reliable structural information can be extracted from fragment velocity-map images. It is shown that for low parent ion charge states, the mapping from initial atomic positions to final fragment velocities is complex and very sensitive to the parent ion charge state as well as many other experimental and simulation parameters. For high-charge states, however, the mapping is much more straightforward and dominated by Coulombic interactions (moderated, if appropriate, by the requirements of overall spin conservation). This study proposes minimum requirements for the high-charge regime, highlights the need to work in this regime in order to obtain robust structural information from fragment velocity-map images, and suggests how quantitative structural information may be extracted from experimental data. [ABSTRACT FROM AUTHOR]

The full text of this article is available in the PDF provided. [ABSTRACT FROM AUTHOR]

We report a computational study of excited state intramolecular proton transfer in a series of related and progressively more complex organic chromophores ranging from 2-(2′-hydroxyphenyl)-benzoxazole (HBO) through to the 5-(benzo[d]oxazol-2-yl)-2-(4-((bis(pyridin-2-ylmethyl)amino)methyl)benzo[d]oxazol-2-yl)-4 hydroxyphenolate (HDBO′) anion. The latter chelates group 12 metal cations (X = Zn2+, Cd2+ and Hg2+), and can serve as a fluorescence-based sensor for such metals. Initial π* ←π excitation of the ground (S0) state enol-tautomer induces charge separation in the first excited singlet (S1) state and drives the subsequent proton transfer (i.e. enol→keto tautomerism). The keto-tautomer constitutes a local minimum on the S1 PES, and is responsible for highly Stokes shifted fluorescent emission; S1(enol) → S0 fluorescence is proposed to account for the shorter wavelength emission from the X–HDBO′ complexes. Derivatives of HDBO′ that should retain the favourable visible absorption and heavily Stokes shifted emission properties but, additionally, offer higher fluorescence quantum yields (i.e. enhanced metal sensing capability) are proposed. [ABSTRACT FROM AUTHOR]

Interest in Criegee intermediates (CIs), often termed carbonyl oxides, and their role in tropospheric chemistry has grown massively since the demonstration of laboratory‐based routes to their formation and characterization in the gas phase. This article reviews current knowledge regarding the electronic spectroscopy of atmospherically relevant CIs like CH2OO, CH3CHOO, (CH3)2COO and larger CIs like methyl vinyl ketone oxide and methacrolein oxide that are formed in the ozonolysis of isoprene, and of selected conjugated carbene‐derived CIs of interest in the synthetic chemistry community. Of the aforementioned atmospherically relevant CIs, all except CH2OO and (CH3)2COO exist in different conformers which, under tropospheric conditions, can display strikingly different thermal loss rates via unimolecular and bimolecular processes. Calculated photolysis rates based on their absorption properties suggest that solar photolysis will rarely be a significant contributor to the total loss rate for any CI under tropospheric conditions. Nonetheless, there is ever‐growing interest in the absorption cross sections and primary photochemistry of CIs following excitation to the strongly absorbing 1ππ* state, and how this varies with CI, with conformer and with excitation wavelength. The later part of this review surveys the photochemical data reported to date, including a range of studies that demonstrate prompt photo‐induced fission of the terminal O–O bond, and speculates about possible alternate decay processes that could occur following non‐adiabatic coupling to, and dissociation from, highly internally excited levels of the electronic ground state of a CI. [ABSTRACT FROM AUTHOR]

The isomerization dynamics of α-pyrone dissolved in CH3CN have been probed by femtosecond 267 nm pump/broadband infrared (IR) probe spectroscopy. A novel experimental setup allowed the populations of the parent molecule and ring-opened photoproducts to be monitored over pump/probe time delays ranging between 2 ps and 100 μs within a single experiment, and at 5 different temperatures between 0 and 40 °C. The photochemically prepared α-pyrone(S1) molecules decay rapidly (<10 ps) through internal conversion to the S0 potential energy surface, with an initial quantum yield for parent molecule re-formation of ∼60%. Probing the antisymmetric ketene stretch region (2100-2150 cm-1) confirms the presence of at least two ring-opened photoproducts, which are assumed to have an E-configuration with respect to the central CC double bond. These ketenes are observed to undergo two distinct, thermally driven, isomerization processes which occur on the nanosecond and microsecond time scales, respectively. The former reaction is ascribed to thermalization of the initially prepared E-isomer populations, while the slower (microsecond) process involves rotation around the central CC double bond leading to formation of Z-isomers. Subsequent rapid Z → Z isomerizations (occurring on a nanosecond time scale) result in ring-closure and a second, longer time recovery of parent molecule population. By determining rates as a function of the sample temperature, barrier heights of 0.23(3) eV and 0.43(2) eV are obtained for the E → E and E → Z transformations, respectively. [ABSTRACT FROM AUTHOR]

Oxybenzone(OB) and ferulic acid (FA) both find use in commercialsunscreens; caffeic acid (CA) differs from FA by virtue of an −OHgroup in place of a −OCH3group on the aromaticring. We report the results of ab initiocalculationsdesigned to explore the excited state nonradiative relaxation pathwaysthat provide photostability to these molecules and the photoprotectionthey offer toward UV-A and UV-B radiation. In the case of OB, internalconversion (IC) is deduced to occur on ultrafast time scales, viaa barrierless electron-driven H atom transfer pathway from the S1(11nπ*) state to a conical intersection (CI)with the ground (S0) state potential energy surface (PES).The situation with respect to CA and FA is somewhat less clear-cut,with low energy CIs identified by linking excited states to the S0state following photoexcitation and subsequent evolutionalong (i) a ring centered out-of-plane deformation coordinate, (ii)the E/Zisomerism coordinate and,in the case of CA, (iii) an O–H stretch coordinate. Analogywith catechol suggests that the last of these processes (if active)would lead to radical formation (and thus potential phototoxicity),encouraging a suggestion that FA might be superior to CA as a sunscreeningredient. [ABSTRACT FROM AUTHOR]

Extracting meaningful kinetic traces from time-resolved absorption spectra is a non-trivial task, particularly for solution phase spectra where solvent interactions can substantially broaden and shift the transition frequencies. Typically, each spectrum is composed of signal from a number of molecular species (e.g., excited states, intermediate complexes, product species) with overlapping spectral features. Additionally, the profiles of these spectral features may evolve in time (i.e., signal nonlinearity), further complicating the decomposition process. Here, we present a new program for decomposing mixed transient spectra into their individual component spectra and extracting the corresponding kinetic traces: KOALA (Kinetics Observed After Light Absorption). The software combines spectral target analysis with brute-force linear least squares fitting, which is computationally efficient because of the small nonlinear parameter space of most spectral features. Within, we demonstrate the application of KOALA to two sets of experimental transient absorption spectra with multiple mixed spectral components. Although designed for decomposing solution-phase transient absorption data, KOALA may in principle be applied to any time-evolving spectra with multiple components. [ABSTRACT FROM AUTHOR]

The dynamics of N-methylpyrrole following excitation at wavelengths in the range 241.5-217.0 nm were studied using a combination of time-resolved photoelectron spectroscopy (TRPES), ab initio quantum dynamics calculations using the multi-layer multi-configurational time-dependent Hartree method, as well as high-level photoionization cross section calculations. Excitation at 241.5 and 236.2 nm results in population of the A2(πσ*) state, in agreement with previous studies. Excitation at 217.0 nm prepares the previously neglected B1(π3py) Rydberg state, followed by prompt internal conversion to the A2(πσ*) state. In contrast with the photoinduced dynamics of pyrrole, the lifetime of the wavepacket in the A2(πσ*) state was found to vary with excitation wavelength, decreasing by one order of magnitude upon tuning from 241.5 nm to 236.2 nm and by more than three orders of magnitude when excited at 217.0 nm. The order of magnitude difference in lifetimes measured at the longer excitation wavelengths is attributed to vibrational excitation in the A2(πσ*) state, facilitating wavepacket motion around the potential barrier in the N-CH3 dissociation coordinate. [ABSTRACT FROM AUTHOR]

Velocity map imaging methods, with a new and improved ion optics design, have been used to explore the near ultraviolet photodissociation dynamics of gas phase 2-bromo- and 2-iodothiophene molecules. In both cases, the ground (X) and spin-orbit excited (X*) (where X = Br, I) atom products formed at the longest excitation wavelengths are found to recoil with fast, anisotropic velocity distributions, consistent with prompt C-X bond fission following excitation via a transition whose dipole moment is aligned parallel to the breaking bond. Upon tuning to shorter wavelengths, this fast component fades and is progressively replaced by a slower, isotropic recoil distribution. Complementary electronic structure calculations provide a plausible explanation for this switch in fragmentation behaviour--namely, the opening of a rival C-S bond extension pathway to a region of conical intersection with the ground state potential energy surface. The resulting ground state molecules are formed with more than sufficient internal energy to sample the configuration space associated with several parent isomers and to dissociate to yield X atom products in tandem with both cyclic and ring-opened partner fragments. [ABSTRACT FROM AUTHOR]

The dynamics of pyrrole excited at wavelengths in the range 242-217 nm are studied using a combination of time-resolved photoelectron spectroscopy and wavepacket propagations performed using the multi-configurational time-dependent Hartree method. Excitation close to the origin of pyrrole's electronic spectrum, at 242 and 236 nm, is found to result in an ultrafast decay of the system from the ionization window on a single timescale of less than 20 fs. This behaviour is explained fully by assuming the system to be excited to the A2(πσ*) state, in accord with previous experimental and theoretical studies. Excitation at shorter wavelengths has previously been assumed to result predominantly in population of the bright A1(ππ*) and B2(ππ*) states. We here present time-resolved photoelectron spectra at a pump wavelength of 217 nm alongside detailed quantum dynamics calculations that, together with a recent reinterpretation of pyrrole's electronic spectrum [S. P. Neville and G. A. Worth, J. Chem. Phys. 140, 034317 (2014)], suggest that population of the B1(πσ*) state (hitherto assumed to be optically dark) may occur directly when pyrrole is excited at energies in the near UV part of its electronic spectrum. The B1(πσ*) state is found to decay on a timescale of less than 20 fs by both N-H dissociation and internal conversion to the A2(πσ*) state. [ABSTRACT FROM AUTHOR]

The ultraviolet photodissociation of the prototypical organometallic half-sandwich compound dicarbonylcyclopentadienyliodoiron(II) [η5-CpFe(CO)2I] has been studied in the gas phase across the wavelength range 260 ≤ λ ≤ 310 nm using multi-mass velocity-map ion imaging with photoproducts detected using both resonance enhanced multiphoton and vacuum ultraviolet (λ = 118.2 nm) single photon ionisation methods. Ion images recorded for the atomic iodine and the cyclopentadienyl photoproducts reveal fast, anisotropic components to their recoil velocity distributions. The experimental work is supported by multi-reference (spin–orbit averaged) electronic structure calculations that suffice to illustrate the high electronic state density in such transition metal complexes and provide insights into the rival fragmentation dynamics. The ground state parent molecule has singlet spin multiplicity, but the product energy disposal measured following Fe–Cp bond fission shows the involvement of nominally spin-forbidden transitions. The Fe–I and Fe–Cp bond fissions should both be viewed as homolytic and occurring on excited state potentials that are dissociative in the relevant ligand elimination coordinate. [ABSTRACT FROM AUTHOR]

The UV photochemistry of various fluorinated iodobenzenes (4-fluoro-, 2,4-difluoro-, 3,5-difluoro-, and perfluoro-iodobenzene) has been investigated at many wavelengths by velocity map imaging, time-resolved near infrared absorption spectroscopy and (spin-orbit resolved) ab initio calculations of the ground and excited state potentials along the C-I stretch coordinate, RC-I. The textbook description of the near UV photochemistry of CH3I, i.e., σ*←n excitation to the 3Q0+ state, followed by direct dissociation (to yield spin-orbit excited iodine atom (I*) products) or by non-adiabatic coupling via a conical intersection (CI) with the 1Q1 potential (to yield ground state iodine (I) atoms) is shown to provide a good zero-order model for aryl iodide photochemistry also. However, the aryl halides also possess occupied π and low-lying π* orbitals, and have lower (C2v or Cs) symmetry than CH3I. Both of these factors introduce additional subtleties. For example, excitations to and predissociation of ππ* excited states provide additional routes to I products, most obviously at long UV wavelengths. nσ*/πσ* configuration mixing stabilizes the (analogue of the) 3Q0+ potential energy surface (PES), to an extent that scales with the degree of fluorination; the corresponding 4A1 PES in C6F5I is actually predicted to exhibit a minimum at extended RC-I. This has the effect of extending the long wavelength threshold for forming I* products. The lowered symmetry enables an additional (sloped) CI with the 5A2 (9A″ in 2,4-difluorobenzene) PES, which provides an extra non-adiabatic route to (fast) ground state I atoms when populating the 4A1 PES at shorter UV excitation wavelengths. [ABSTRACT FROM AUTHOR]

Following 157 nm photoexcitation of amorphous solid water and polycrystalline water ice, photodesorbed water molecules (H2O and D2O), in the ground vibrational state, have been observed using resonance-enhanced multiphoton ionization detection methods. Time-of-flight and rotationally resolved spectra of the photodesorbed water molecules were measured, and the kinetic and internal energy distributions were obtained. The measured energy distributions are in good accord with those predicted by classical molecular dynamics calculations for the kick-out mechanism of a water molecule from the ice surface by a hot hydrogen (deuterium) atom formed by photodissociation of a neighboring water molecule. Desorption of D2O following 193 nm photoirradiation of a D2O/H2S mixed ice was also investigated to provide further direct evidence for the operation of a kick-out mechanism. [ABSTRACT FROM AUTHOR]

The photophysics of gas phase pyrazole (C3N2H4) and 2H-1,2,3-triazole (C2N3H3) molecules following excitation at wavelengths in the range 230 nm≥λphot≥193.3 nm has been investigated using the experimental technique of H (Rydberg) atom photofragment translational spectroscopy. The findings are compared with previous studies of pyrrole (C4N1H5) and imidazole (C3N2H4), providing a guide to H atom loss dynamics in simple N-containing heterocycles. CASPT2 theoretical methods have been employed to validate these findings. Photoexcitation of pyrazole at the longest wavelengths studied is deduced to involve π*←π excitation, but photolysis at λphot≤214 nm is characterized by rapid N–H bond fission on a 1πσ* potential energy surface. The eventual pyrazolyl radical products are formed in a range of vibrational levels associated with both the ground (2A2) and first excited (2B1) electronic states as a result of nonadiabatic coupling at large N–H bond lengths. The excitation energy of the lowest 1πσ* state of pyrazole is found to be significantly higher in energy than that of pyrrole and imidazole. Similar studies of 2H-1,2,3-triazole reveal that the lowest 1πσ* state is yet higher in energy and not accessible following excitation at λphot≥193.3 nm. The N–H bond strength of pyrazole is determined as 37 680±40 cm-1, significantly greater than that of the N–H bonds in pyrrole and imidazole. The correlation between the photochemistry of azoles and the number and position of nitrogen atoms within the ring framework is discussed in terms of molecular symmetry and orbital electron density. A photodissociation channel yielding H atoms with low kinetic energies is also clearly evident in both pyrazole and 2H-1,2,3-triazole. Companion studies of pyrazole-d1 suggest that these slow H atoms arise primarily from the N–H site, following π*←π excitation, and subsequent internal conversion and/or unintended multiphoton absorption processes. [ABSTRACT FROM AUTHOR]

Methanol is typically observed within water-rich interstellar ices and is a source of interstellar organic species. Following the 157 nm photoexcitation of solid methanol at 90 K, desorbed CH3(v=0) and OH(v=0,1) radicals have been observed in situ, near the solid surface, using resonance-enhanced multiphoton ionization (REMPI) detection methods. Time-of-flight and rotationally resolved REMPI spectra of the desorbed species were measured, and the respective fragment internal energy and kinetic energy distributions were obtained. Photoproduction mechanisms for CH3 and OH radicals from solid methanol are discussed. The formation of O(1D and 3P) atoms and H2O was investigated, but the yield of these species was found to be negligible. CH3 products arising following the photoexcitation of water-methanol mixed ice showed similar kinetic and internal energy distributions to those from neat methanol ice. [ABSTRACT FROM AUTHOR]

The production of H2 in highly excited vibrational and rotational states (v=0–5, J=0–17) from the 157 nm photodissociation of amorphous solid water ice films at 100 K was observed directly using resonance-enhanced multiphoton ionization. Weaker signals from H2(v=2,3 and 4) were obtained from 157 nm photolysis of polycrystalline ice, but H2(v=0 and 1) populations in this case were below the detection limit. The H2 products show two distinct formation mechanisms. Endothermic abstraction of a hydrogen atom from H2O by a photolytically produced H atom yields vibrationally cold H2 products, whereas exothermic recombination of two H-atom photoproducts yields H2 molecules with a highly excited vibrational distribution and non-Boltzmann rotational population distributions as has been predicted previously by both quantum-mechanical and molecular dynamics calculations. [ABSTRACT FROM AUTHOR]