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C-I bond extension and fission following ultraviolet (UV, 262 nm) photoexcitation of 2- and 3-iodothiophene is studied using ultrafast time-resolved extreme ultraviolet (XUV) ionization in conjunction with velocity map ion imaging. The photoexcited molecules and eventual I atom products are probed by site-selective ionization at the I 4d edge using intense XUV pulses, which induce multiple charges initially localized to the iodine atom. At C-I separations below the critical distance for charge transfer (CT), charge can redistribute around the molecule leading to Coulomb explosion and charged fragments with high kinetic energy. At greater C-I separations, beyond the critical distance, CT is no longer possible and the measured kinetic energies of the charged iodine atoms report on the neutral dissociation process. The time and momentum resolved measurements allow determination of the timescales and the respective product momentum and kinetic energy distributions for both isomers, which are interpreted in terms of rival 'direct' and 'indirect' dissociation pathways. The measurements are compared with a classical over the barrier model, which reveals that the onset of the indirect dissociation process is delayed by ∼1 ps relative to the direct process. The kinetics of the two processes show no discernible difference between the two parent isomers, but the branching between the direct and indirect dissociation channels and the respective product momentum distributions show isomer dependencies. The greater relative yield of indirect dissociation products from 262 nm photolysis of 3-iodothiophene ( cf. 2-iodothiophene) is attributed to the different partial cross-sections for (ring-centred) π∗ ← π and (C-I bond localized) σ∗ ← (n/π) excitation in the respective parent isomers.

Coulomb explosion imaging (CEI) methods are finding ever-growing use as a means of exploring and distinguishing the static stereo-configurations of small quantum systems (molecules, clusters, etc ). CEI experiments initiated by ultrafast (femtosecond-duration) laser pulses also allow opportunities to track the time-evolution of molecular structures, and thereby advance understanding of molecular fragmentation processes. This Perspective illustrates two emerging families of dynamical studies. 'One-colour' studies (employing strong field ionisation driven by intense near infrared or single X-ray or extreme ultraviolet laser pulses) afford routes to preparing multiply charged molecular cations and exploring how their fragmentation progresses from valence-dominated to Coulomb-dominated dynamics with increasing charge and how this evolution varies with molecular size and composition. 'Two-colour' studies use one ultrashort laser pulse to create electronically excited neutral molecules (or monocations), whose structural evolution is then probed as a function of pump-probe delay using an ultrafast ionisation pulse along with time and position-sensitive detection methods. This latter type of experiment has the potential to return new insights into not just molecular fragmentation processes but also charge transfer processes between moieties separating with much better defined stereochemical control than in contemporary ion-atom and ion-molecule charge transfer studies.

We present results from an experimental ion imaging study into the fragmentation dynamics of 1-iodopropane and 2-iodopropane following interaction with extreme ultraviolet intense femtosecond laser pulses with a photon energy of 95 eV. Using covariance imaging analysis, a range of observed fragmentation pathways of the resulting polycations can be isolated and interrogated in detail at relatively high ion count rates (∼12 ions shot -1 ). By incorporating the recently developed native frames analysis approach into the three-dimensional covariance imaging procedure, contributions from three-body concerted and sequential fragmentation mechanisms can be isolated. The angular distribution of the fragment ions is much more complex than in previously reported studies for triatomic polycations, and differs substantially between the two isomeric species. With support of simple simulations of the dissociation channels of interest, detailed physical insights into the fragmentation dynamics are obtained, including how the initial dissociation step in a sequential mechanism influences rovibrational dynamics in the metastable intermediate ion and how signatures of this nuclear motion manifest in the measured signals.

Diketopyrrolopyrroles are a popular class of electron-withdrawing unit in optoelectronic materials. When combined with electron donating side-chain functional groups such as thiophenes, they form a very broad class of donor-acceptor molecules: thiophene-diketopyrrolopyrroles (TDPPs). Despite their widescale use in biosensors and photovoltaic materials, studies have yet to establish the important link between the electronic structure of the specific TDPP and the critical optical properties. To bridge this gap, ultrafast transient absorption with 22 fs time resolution has been used to explore the photophysics of three prototypical TDPP molecules: a monomer, dimer and polymer in solution. Interpretation of experimental data was assisted by a recent high-level theoretical study, and additional density functional theory calculations. These studies show that the photophysics of these molecular prototypes under visible photoexcitation are determined by just two excited electronic states, having very different electronic characters (one is optically bright, the other dark), their relative energetic ordering and the timescales for internal conversion from one to the other and/or to the ground state. The underlying difference in electronic structure alters the branching between these excited states and their associated dynamics. In turn, these factors dictate the fluorescence quantum yields, which are shown to vary by ∼1-2 orders of magnitude across the TDPP prototypes investigated here. The fast non-radiative transfer of molecules from the bright to dark states is mediated by conical intersections. Remarkably, wavepacket signals in the measured transient absorption data carry signatures of the nuclear motions that enable mixing of the electronic-nuclear wavefunction and facilitate non-adiabatic coupling between the bright and dark states.

Multi-mass velocity map imaging studies of charged fragments formed by near infrared strong field ionization together with covariance map image analysis offer a new window through which to explore the dissociation dynamics of several different highly charged parent cations, simultaneously - as demonstrated here for the case of CF 3 I Z + cations with charges Z ranging from 1 to at least 5. Previous reports that dissociative ionization of CF 3 I + cations yields CF 3 + , I + and CF 2 I + fragment ions are confirmed, and some of the CF 3 + fragments are deduced to undergo secondary loss of one or more neutral F atoms. Covariance map imaging confirms the dominance of CF 3 + + I + products in the photodissociation of CF 3 I 2+ cations and, again, that some of the primary CF 3 + photofragments can shed one or more F atoms. Rival charge symmetric dissociation pathways to CF 2 I + + F + and to IF + + CF 2 + products and charge asymmetric dissociations to CF 3 + I 2+ and CF 2 I 2+ + F products are all also identified. The findings for parent cations with Z ≥ 3 are wholly new. In all cases, the fragment recoil velocity distributions imply dissociation dynamics in which coulombic repulsive forces play a dominant role. The major photoproducts following dissociation of CF 3 I 3+ ions are CF 3 + and I 2+ , with lesser contributions from the rival CF 2 I 2+ + F + and CF 3 2+ + I + channels. The CF 3 2+ fragment ion images measured at higher incident intensities show a faster velocity sub-group consistent with their formation in tandem with I 2+ fragments, from photodissociation of CF 3 I 4+ parent ions. The measured velocity distributions of the I 3+ fragment ions contain features attributable to CF 3 I 5+ photodissociation to CF 3 2+ + I 3+ and the images of fragments with mass to charge ( m / z ) ratio ∼31 show formation of I 4+ products that must originate from parent ions with yet higher Z .

Recent experimental and computational advances have heralded huge progress in the range and the detail of the database pertaining to photoinduced C-H bond fission processes. This Perspective provides a snapshot of the current state of knowledge as determined via gas phase (i.e. isolated molecule) studies of the primary photochemistry of families of hydrocarbon molecules (alkynes, alkenes, alkanes, aromatics and selected heteroatom containing analogues) and the corresponding radicals (including saturated and unsaturated hydrocarbon radicals). Different families show different and, in many cases, understandable propensities for dissociating from an excited electronic state or following non-adiabatic coupling (i.e. internal conversion) to high vibrational levels of the ground electronic state. The Perspective seeks to emphasise the potentially vast range of behaviours (dissociation timescales, product energy disposals, etc.) that can be expected to accompany internal conversion, reflecting the extent to which the tuning coordinate (i.e. the nuclear motions that tune the energy separation between the excited and ground state) projects onto the dissociation coordinate of interest (i.e. the breaking of the C-H bond).

Norrish reactions are important photo-induced reactions in mainstream organic chemistry and are implicated in many industrially and biologically relevant processes and in the processing of carbonyl molecules in the atmosphere. The present study reports multi-reference electronic structure calculations designed to assess details of the potential energy profiles associated with the Norrish type-I and type-II reactions of a prototypical ketone 5-methyl-hexan-2-one. We show that the well-established 'triplet state mediated' reaction pathways following initial population of a singlet excited state can be complemented by (hitherto rarely recognized) 'singlet state only' Norrish type-I and type-II reaction mechanisms that involve no spin-forbidden transitions along the respective reaction paths, and suggest how the efficiencies of such reactions might be affected by strategic substitutions at selected sites within the parent ketone.

The availability of non-radiative decay mechanisms by which photoexcited molecules can revert to their ground electronic state, without experiencing potentially deleterious chemical transformation, is fundamental to molecular photostability. This Perspective Article combines results of new ab initio electronic structure calculations and prior experimental data in an effort to systematise trends in the non-radiative decay following UV excitation of selected families of heterocyclic molecules. We start with the prototypical uni- and bicyclic molecules phenol and indole, and explore the structural and photophysical consequences of incorporating progressively more nitrogen atoms within the respective ring structures en route to the DNA bases thymine, cytosine, adenine and guanine. For each of the latter, we identify low energy non-radiative decay pathways via conical intersections with the ground state potential energy surface accessed by out-of-plane ring deformations. This is followed by summary descriptions and illustrations of selected rival (electron driven H atom transfer) non-radiative excited state decay processes that demand consideration once the nucleobases are merely components in larger biomolecular systems like nucleosides, and both individual and stacked base-pairs.

H atom loss following near ultraviolet photoexcitation of gas phase 2-thiophenethiol molecules has been studied experimentally, by photofragment translational spectroscopy (PTS) methods, and computationally, by ab initio electronic structure calculations. The long wavelength (277.5 ≥ λ(phot) ≥ 240 nm) PTS data are consistent with S-H bond fission after population of the first (1)πσ* state. The partner thiophenethiyl (R) radicals are formed predominantly in their first excited Ã(2)A' state, but assignment of a weak signal attributable to H + R(X˜(2)A'') products allows determination of the S-H bond strength, D0 = 27,800 ± 100 cm(-1) and the Ã-X˜ state splitting in the thiophenethiyl radical (ΔE = 3580 ± 100 cm(-1)). The deduced population inversion between the à and X˜ states of the radical reflects the non-planar ground state geometry (wherein the S-H bond is directed near orthogonal to the ring plane) which, post-photoexcitation, is unable to planarise sufficiently prior to bond fission. This dictates that the dissociating molecules follow the adiabatic fragmentation pathway to electronically excited radical products. π* ← π absorption dominates at shorter excitation wavelengths. Coupling to the same (1)πσ* potential energy surface (PES) remains the dominant dissociation route, but a minor yield of H atoms attributable to a rival fragmentation pathway is identified. These products are deduced to arise via unimolecular decay following internal conversion to the ground (S0) state PES via a conical intersection accessed by intra-ring C-S bond extension. The measured translational energy disposal shows a more striking change once λ(phot) ≤ 220 nm. Once again, however, the dominant decay pathway is deduced to be S-H bond fission following coupling to the (1)πσ* PES but, in this case, many of the evolving molecules are deduced to have sufficiently near-planar geometries to allow passage through the conical intersection at extended S-H bond lengths and dissociation to ground (X˜) state radical products. The present data provide no definitive evidence that complete ring opening can compete with fast S-H bond fission following near UV photoexcitation of 2-thiophenethiol.

The photoisomerisation dynamics following excitation to the S1 electronic state of two structurally related heterocyclic molecules, α-pyrone and coumarin, in acetonitrile solution have been probed by time-resolved vibrational absorption spectroscopy. Following irradiation at 310 nm, α-pyrone relaxes rapidly from its initially excited state, with a quantum yield for parent molecule reformation of 68%. Probing the antisymmetric ketene stretch region between 2100 cm(-1) and 2150 cm(-1) confirms the presence of at least two isomeric ring-opened photoproducts, which are formed highly vibrationally excited and relax on a picosecond timescale. Following vibrational cooling, a secondary, thermally driven, isomerisation is observed with a 1.8(1) ns time constant. In contrast, coumarin reforms the parent molecule with essentially 100% efficiency following excitation at 330 nm. The conical intersections driving the non-radiative relaxation of α-pyrone have been investigated using an automated search algorithm. The two lowest energy conical intersections possess remarkably similar structures to the two energetically accessible conical intersections reported previously for coumarin, suggesting that the differing photochemistry is the result of dynamical effects occurring after passage through these intersections.

We report a combination of experimental (velocity map imaging measurements of the methyl (Me) radical products) and ab initio electronic structure studies that explore the influence of substituents (Y) on the dynamics of S-Me bond fission following excitation to the first excited S1 states of thioanisole and three 4-substituted thioanisoles (4-YPhSMe, with Y = H, Me, MeO and CN). In all bar the case that Y = CN, the resulting 4-YPhS products are found to be formed predominantly in their excited (Ã) electronic state. In all cases, the relative yield of X̃ state products increases upon tuning to shorter excitation wavelengths and, in the specific case of bare thioanisole (as found previously by Lim and Kim, Nat. Chem., 2010, 2, 627), jumps when exciting on the parent resonance assigned to the S1(v7a = 1) level. Two conical intersections (CIs) in the RS-Me stretch coordinate are crucial to rationalising all of the observed dynamics. The first, (CI-1, between the diabatic (1)ππ* and dissociative (1)πσ* potential energy surfaces (PESs) at RS-Me∼ 2 Å) lies above the S1(v = 0) level in energy, and the calculated minimum energy path through this barrier involves substantial deviations from planarity in all bar 4-CNPhSMe. Beyond this barrier, the potential is quite steeply repulsive, and Me + 4-YPhS(Ã) products are the inevitable products if the molecular framework is unable to re-planarise within the time it takes for the dissociating molecules to pass through the region of CI-2 (between the diabatic (1)πσ* and ground (S0) states) where the product electronic branching is determined. The gradual increase in the yield of 4-YPhS(X̃) radicals upon tuning to shorter photolysis wavelengths, the much increased branching into PhS(X̃) products when exciting the PhSMe (S1, v7a = 1) level and the dominance of 4-CNPhS(X̃) products in the specific case that Y = CN can all be understood in terms of a (relative) lowering of the effective barrier associated with CI-1, thereby allowing access to the dissociative region of the PES at closer-to-planar geometries.

We report data from a comprehensive investigation into the photodissociation dynamics of methyl iodide and ethyl iodide at several wavelengths in the range 236-266 nm, within their respective A-bands. The use of non-resonant single-photon ionization at 118.2 nm allows detection and velocity-map imaging of all fragments, regardless of their vibrotational or electronic state. The resulting photofragment kinetic energy and angular distributions and the quantum yields of ground-state and spin-orbit excited iodine fragments are in good agreement with previous studies employing state-selective detection via REMPI. The data are readily rationalised in terms of three competing dissociation mechanisms. The dominant excitation at all wavelengths studied is via a parallel transition to the (3)Q0 state, which either dissociates directly to give an alkyl radical partnered by spin-orbit excited iodine, or undergoes radiationless transfer to the (1)Q1 potential surface, where it dissociates to an alkyl radical partnered by iodine in its electronic ground state. Ground state iodine atoms can also be formed by direct dissociation from the (1)Q1 or (3)Q1 excited states following perpendicular excitation at the shorter and longer wavelength region, respectively, in the current range of interest. The extent of internal excitation of the alkyl fragment varies with dissociation mechanism, and is considerably higher for ethyl fragments from ethyl iodide photolysis than for methyl fragments from methyl iodide photolysis. We discuss the relative advantages and disadvantages of single-photon vacuum-ultraviolet ionization relative to the more widely used REMPI detection schemes, and conclude, in agreement with others, that single-photon ionization is a viable detection method for photofragment imaging studies, particularly when studying large molecules possessing multiple fragmentation channels.

The heterocyclic ring-opening dynamics of thiophenone and furanone dissolved in CH3CN have been probed by ultrafast transient infrared spectroscopy. Following irradiation at 267 nm (thiophenone) or 225 nm (furanone), prompt (τ < 1 ps) ring-opening is confirmed by the appearance of a characteristic antisymmetric ketene stretching feature around 2150 cm(-1). The ring-opened product molecules are formed highly vibrationally excited, and cool subsequently on a ∼6.7 ps timescale. By monitoring the recovery of the parent (S0) bleach, it is found that ∼60% of the initially photoexcited thiophenone molecules reform the parent molecule, in stark contrast with the case in furanone where there is less than 10% parent bleach recovery. Complementary ab initio calculations of potential energy cuts along the S-C([double bond, length as m-dash]O) and O-C([double bond, length as m-dash]O) ring-opening coordinate reveals insights into the reaction mechanism, and the important role played by dissociative (n/π)σ* states in the UV-induced photochemistry of such heterocyclic systems.

The photodissociation dynamics of ethyl bromide and ethyl iodide cations (C2H5Br(+) and C2H5I(+)) have been studied. Ethyl halide cations were formed through vacuum ultraviolet (VUV) photoionization of the respective neutral parent molecules at 118.2 nm, and were photolysed at a number of ultraviolet (UV) photolysis wavelengths, including 355 nm and wavelengths in the range from 236 to 266 nm. Time-of-flight mass spectra and velocity-map images have been acquired for all fragment ions and for ground (Br) and spin-orbit excited (Br*) bromine atom products, allowing multiple fragmentation pathways to be investigated. The experimental studies are complemented by spin-orbit resolved ab initio calculations of cuts through the potential energy surfaces (along the RC-Br/I stretch coordinate) for the ground and first few excited states of the respective cations. Analysis of the velocity-map images indicates that photoexcited C2H5Br(+) cations undergo prompt C-Br bond fission to form predominantly C2H5(+) + Br* products with a near-limiting 'parallel' recoil velocity distribution. The observed C2H3(+) + H2 + Br product channel is thought to arise via unimolecular decay of highly internally excited C2H5(+) products formed following radiationless transfer from the initial excited state populated by photon absorption. Broadly similar behaviour is observed in the case of C2H5I(+), along with an additional energetically accessible C-I bond fission channel to form C2H5 + I(+) products. HX (X = Br, I) elimination from the highly internally excited C2H5X(+) cation is deemed the most probable route to forming the C2H4(+) fragment ions observed from both cations. Finally, both ethyl halide cations also show evidence of a minor C-C bond fission process to form CH2X(+) + CH3 products.

We report a combined experimental (H (Rydberg) atom photofragment translational spectroscopy) and theoretical (ab initio electronic structure and vibronic coupling calculations) study of the effects of symmetry on the photodissociation dynamics of phenols. Ultraviolet photoexcitation to the bound S1((1)ππ*) state of many phenols leads to some O-H bond fission by tunneling through the barrier under the conical intersection (CI) between the S1 and dissociative S2((1)πσ*) potential energy surfaces in the R(O-H) stretch coordinate. Careful analysis of the total kinetic energy release spectra of the resulting products shows that the radicals formed following S1 ← S0 excitation of phenol and symmetrically substituted phenols like 4-fluorophenol all carry an odd number of quanta in vibrational mode ν(16a), whereas those deriving from asymmetrically substituted systems like 3-fluorophenol or 4-methoxyphenol do not. This contrasting behavior can be traced back to symmetry. Symmetrically substituted phenols exist in two equivalent rotamers, which interconvert by tunneling through the barrier to OH torsional motion. Their states are thus best considered in the non-rigid G4 molecular symmetry group, wherein radiationless transfer from the S1 to S2 state requires a coupling mode of a2 symmetry. Of the three a2 symmetry parent modes, the out-of-plane ring puckering mode ν(16a) shows much the largest interstate coupling constant in the vicinity of the S1/S2 CI. The nuclear motions associated with ν(16a) are orthogonal to the dissociation coordinate, and are thus retained in the radical products. Introducing asymmetry (even a non-linear substituent in the 4-position) lifts the degeneracy of the rotamers, and lowers the molecular symmetry to Cs. Many more parent motions satisfy the reduced (a'') symmetry requirement to enable S1/S2 coupling, the most effective of which is OH torsion. This motion 'disappears' on O-H bond fission; symmetry thus imposes no restriction to forming radical products with vibrational quantum number v = 0. The present work yields values for the O-H bond strengths in 3-FPhOH and 4-MeOPhOH, and recommends modest revisions to the previously reported O-H bond strengths in other asymmetrically substituted phenols like 3- and 2-methylphenol and 4-hydroxyindole.

This article explores the extent to which insights gleaned from detailed studies of molecular photodissociations in the gas phase (i.e. under isolated molecule conditions) can inform our understanding of the corresponding photofragmentation processes in solution. Systems selected for comparison include a thiophenol (p-methylthiophenol), a thioanisole (p-methylthioanisole) and phenol, in vacuum and in cyclohexane solution. UV excitation in the gas phase results in RX-Y (X = O, S; Y = H, CH3) bond fission in all cases, but over timescales that vary by ~4 orders of magnitude - all of which behaviours can be rationalised on the basis of the relevant bound and dissociative excited state potential energy surfaces (PESs) accessed by UV photoexcitation, and of the conical intersections that facilitate radiationless transfer between these PESs. Time-resolved UV pump-broadband UV/visible probe and/or UV pump-broadband IR probe studies of the corresponding systems in cyclohexane solution reveal additional processes that are unique to the condensed phase. Thus, for example, the data clearly reveal evidence of (i) vibrational relaxation of the photoexcited molecules prior to their dissociation and of the radical fragments formed upon X-Y bond fission, and (ii) geminate recombination of the RX and Y products (leading to reformation of the ground state parent and/or isomeric adducts). Nonetheless, the data also show that, in each case, the characteristics (and the timescale) of the initial bond fission process that occurs under isolated molecule conditions are barely changed by the presence of a weakly interacting solvent like cyclohexane. These condensed phase studies are then extended to an ether analogue of phenol (allyl phenyl ether), wherein UV photo-induced RO-allyl bond fission constitutes the first step of a photo-Claisen rearrangement.

Deactivation of excited electronic states through coupling to dissociative (1)πσ* states in heteroaromatic systems has received considerable attention in recent years, particularly as a mechanism that contributes to the ultraviolet (UV) photostability of numerous aromatic biomolecules and their chromophores. Recent studies have expanded upon this work to look at more complex species, which involves understanding competing dynamics on two different (1)πσ* potential energy surfaces (PESs) localized on different heteroatom hydride coordinates (O-H and N-H bonds) within the same molecule. In a similar spirit, the work presented here utilizes ultrafast time-resolved velocity map ion imaging to study competing dissociation pathways along (1)πσ* PESs in mequinol (p-methoxyphenol), localized at O-H and O-CH(3) bonds yielding H atoms or CH(3) radicals, respectively, over an excitation wavelength range of 298-238 nm and at 200 nm. H atom elimination is found to be operative via either tunneling under a conical intersection (CI) (298 ≥ λ ≥ 280 nm) or ultrafast internal conversion through appropriate CIs (λ ≤ 245 nm), both of which provide mechanisms for coupling onto the dissociative state associated with the O-H bond. In the intermediate wavelength range of 280 ≥ λ ≥ 245 nm, mediated H atom elimination is not observed. In contrast, we find that state driven CH(3) radical elimination is only observed in the excitation range 264 ≥ λ ≥ 238 nm. Interpretation of these experimental results is guided by: (i) high level complete active space with second order perturbation theory (CASPT2) calculations, which provide 1-D potential energy cuts of the ground and low lying singlet excited electronic states along the O-H and O-CH(3) bond coordinates; and (ii) calculated excitation energies using CASPT2 and the equation-of-motion coupled cluster with singles and doubles excitations (EOM-CCSD) formalism. From these comprehensive studies, we find that the dynamics along the O-H coordinate generally mimic H atom elimination previously observed in phenol, whereas O-CH(3) bond fission in mequinol appears to present notably different behavior to the CH(3) elimination dynamics previously observed in anisole (methoxybenzene).

This article reports the striking interplay between the molecular structure and the photodissociation dynamics of catechol (a key dihydroxybenzene), identified using a combination of electronic spectroscopy, hydrogen (Rydberg) atom photofragment translational spectroscopy, density functional theory and second order approximate coupled cluster methods. We describe how the non-planar (C(1) symmetry) ← planar (C(s) symmetry) geometry change during S(1) (1(1)ππ*) ←S(0) excitation in catechol, as well as the presence of internal hydrogen bonding, can perturb the photodissociation dynamics relative to that of phenol (a monohydroxybenzene), particularly with respect to O-H bond fission via the lowest dissociative (1)πσ* state. For λ(phot) > 270 nm, O-H bond fission (of the non hydrogen bonded hydroxyl moiety) is deduced to proceed via H atom tunnelling from the photo-prepared 1(1)ππ* state into the lowest (1)πσ* state of the molecule. The vibrational energy distribution in the resulting catechoxyl product changes notably as λ(phot) is tuned on resonance with either the v' = 0, m(2)' = 1(+) or m(2)' = 2(+) torsional levels of the photo-prepared 1(1)ππ* state: the product state distribution is highly sensitive to the degree of OH torsional excitation (m(2)) prepared during photo-excitation. It is deduced that such torsional excitation can be redistributed very efficiently into ring puckering (and likely also in-plane ring stretch) vibrations as the molecule tunnels to its repulsive 1(1)πσ* state and dissociates. These observations can be rationalised by consideration of the photo-prepared nuclear wavefunctions. Analysis of the product vibrational energy distribution also reveals that the O-H bond strength of the non hydrogen bonded O-H moiety in catechol, D(0)(H-catechoxyl) ≤ 27 480 ± 50 cm(-1), ∼2500 cm(-1) lower than that of the sole O-H bond in bare phenol. As a consequence, the vertical excitation energy of the 1(1)πσ* state in catechol is reduced relative to that in phenol, yielding a particularly broad distribution of product vibrations for λ(phot) < 270 nm. This study highlights the interplay between molecular geometry and redistribution of vibrational energy during ultraviolet photolysis of phenols.

The photodesorption of H(2)O in its vibrational ground state, and of OH radicals in their ground and first excited vibrational states, following 157 nm photoexcitation of amorphous solid water has been studied using molecular dynamics simulations and detected experimentally by resonance-enhanced multiphoton ionization techniques. There is good agreement between the simulated and measured energy distributions. In addition, signals of H(+) and OH(+) were detected in the experiments. These are inferred to originate from vibrationally excited H(2)O molecules that are ejected from the surface by two distinct mechanisms: a direct desorption mechanism and desorption induced by secondary recombination of photoproducts at the ice surface. This is the first reported experimental evidence of photodesorption of vibrationally excited H(2)O molecules from water ice.

H (Rydberg) atom photofragment translational spectroscopy (HRA-PTS) and complete active space with second order perturbation theory (CASPT2) methods have been used to explore the competing N-H and O-H bond dissociation pathways of 4- and 5-hydroxyindoles (HI) and methoxyindoles (MI). When 4-HI was excited to bound (1)L(b) levels, (λ(phot) ≤ 284.893 nm) O-H bond fission was demonstrated by assignment of the structure within the resulting total kinetic energy release (TKER) spectra. By analogy with phenol, dissociation was deduced to occur by H atom tunnelling under the barrier associated with the lower diabats of the (1)L(b)/(1)πσ*((OH)) conical intersection (CI). No evidence was found for a significant N-H bond dissociation yield at these or shorter excitation wavelengths (284.893 ≥ λ(phot) ≥ 193.3 nm). Companion studies of 4-MI revealed different reaction dynamics. In this case, N-H bond fission is deduced to occur at λ(phot) ≤ 271.104 nm, by direct excitation to the (1)πσ*((NH)) state. Analysis of the measured TKER spectra implies a mechanism wherein, as in pyrrole, the (1)πσ*((NH)) state gains oscillator strength by intensity borrowing from nearby bound states with higher oscillator strengths. HRA-PTS studies of 5-HI, in contrast, showed no evidence for O-H bond dissociation when excited on (1)L(b) levels. The present CASPT2 calculations assist in rationalizing this observation: the area underneath the (1)L(b)/(1)πσ* CI diabats in 5-HI is ~60% greater than the corresponding area in 4-HI and O-H bond dissociation by tunnelling is thus much less probable. Only by reducing the wavelength to ≤ 255 nm were signs of N-H and/or O-H bond dissociation identified. By comparison with companion 5-MI studies, we deduce little O-H bond fission in 5-HI at λ(phot) > 235 nm and that N-H bond fission is the dominant source of H atoms in the wavelength region 255 > λ(phot) > 235 nm. The very different dissociation dynamics of 4- and 5-HI are traced to the position of the -OH substituent, and its effect on the overall electronic structure.

With nanotextured surfaces and interfaces increasingly being encountered in technological and biomedical applications, there is a need for a better understanding of frictional properties involving such surfaces. Here we report friction measurements of several nanostructured surfaces using an Atomic Force Microscope (AFM). These nanostructured surfaces provide well defined model systems on which we have tested the applicability of Amontons' laws of friction. Our results show that Amontonian behaviour is observed with each of the surfaces studied. However, no correlation has been found between measured friction and various surface roughness parameters such as average surface roughness (R(a)) and root mean squared (rms) roughness. Instead, we propose that the friction coefficient may be decomposed into two contributions, i.e., μ = μ(0) + μ(g), with the intrinsic friction coefficient μ(0) accounting for the chemical nature of the surfaces and the geometric friction coefficient μ(g) for the presence of nanotextures. We have found a possible correlation between μ(g) and the average local slope of the surface nanotextures., (This journal is © the Owner Societies 2011)

A recent review (Ashfold et al., Phys. Chem. Chem. Phys., 2010, 12, 1218) highlighted the important role of dissociative excited states formed by electron promotion to σ* orbitals in establishing the photochemistry of many molecular hydrides. Here we extend such considerations to molecular halides, with a particular focus on iodobenzene. Two experimental techniques (velocity mapped ion imaging (VMI) and time resolved infrared (IR) diode laser absorption) and electronic structure calculations have been employed in a comprehensive study of the near ultraviolet (UV) photodissociation of gas phase iodobenzene molecules. The VMI studies yield the speeds and angular distributions of the I((2)P(3/2)) and I*((2)P(1/2)) photofragments formed by photolysis in the wavelength range 330 ≥λ≥ 206 nm. Four distinct dissociation channels are observed for the I((2)P(3/2)) atom products, and a further three channels for the I*((2)P(1/2)) fragments. The phenyl (Ph) radical partners formed via one particular I* product channel following excitation at wavelengths 305 ≥λ≥ 250 nm are distributed over a sufficiently select sub-set of vibrational (v) states that the images allow resolution of specific I* + Ph(v) channels, identification of the active product mode (ν(10), an in-plane ring breathing mode), and a refined determination of D(0)(Ph-I) = 23,390 ± 50 cm(-1). The time-resolved IR absorption studies allow determination of the spin-orbit branching ratio in the iodine atom products formed at λ = 248 nm (ϕ(I*) = [I*]/([I] + [I*]) = 0.28 ± 0.04) and at 266 nm (ϕ(I*) = 0.32 ± 0.05). The complementary high-level, spin-orbit resolved ab initio calculations of sections (along the C-I bond coordinate) through the ground and first 19 excited state potential energy surfaces (PESs) reveal numerous excited states in the energy range of current interest. Except at the very shortest wavelength, however, all of the observed I and I* products display limiting or near limiting parallel recoil anisotropy. This encourages discussion of the fragmentation dynamics in terms of excitation to states of A(1) total symmetry and dissociation on the 2A(1) and 4A(1) (σ* ← n/π) PESs to yield, respectively, I and I* products, or via non-adiabatic coupling to other σ* ← n/π PESs that correlate to these respective limits. Similarities (and differences) with the available UV photochemical data for the other aryl halides, and with the simpler (and more thoroughly studied) iodides HI and CH(3)I, are summarised.

The last few years have seen a surge in interest (both theoretical and experimental) in the photochemistry of heteroaromatic molecules (e.g. azoles, phenols), which has served to highlight the importance of dissociative excited states formed by electron promotion to sigma* molecular orbitals. Such excited states--which, for brevity, are termed pi sigma* states in this Perspective article--may be populated by direct photo-excitation (though the transition cross-sections are intrinsically small), or indirectly, by non-adiabatic coupling from an optically 'bright' excited state (e.g. an excited state resulting from pi* <--pi excitation). The analogous pi sigma* excited states in prototypical hydride molecules like H(2)O and NH(3) have long been recognised. They have served as test-beds for developing concepts like Rydbergisation, conical intersections (CIs) between potential energy surfaces, and for investigating the ways in which non-adiabatic couplings at such CIs influence the eventual photofragmentation dynamics. This Perspective article seeks to highlight the continuity of behaviour revealed by the earlier small molecule studies and by the more recent studies of heteroaromatic systems, and to illustrate the photochemical importance of pi sigma* excited states in many broad families of molecules. Furthermore, the dynamical influence of such excited states is not restricted to closed shell species; the Article concludes with a brief consideration of the consequences of populating sigma* orbitals in free radical species, in molecular cations, and in dissociative electron attachment processes.

The photodissociation of jet-cooled 4-, 3- and 2-methylphenol molecules has been investigated using the experimental techniques of resonance enhanced multiphoton ionisation and H (Rydberg) atom photofragment translational spectroscopy. O-H bond fission is found to occur, via a repulsive (1)pisigma state, in a manner analogous to that occurring in phenol and 4-fluorophenol. Excitation to the (1)pipi manifold results in H-atom loss either directly (via a (1)pipi/(1)pisigma conical intersection) or indirectly, following internal conversion to the ground state and subsequent coupling to the (1)pisigma state via a second conical intersection at extended O-H bond lengths. The resulting methylphenoxyl radicals are created with specific vibrational excitation, reflecting the nuclear distortions required to access the (1)pisigma potential energy surface and the geometry changes induced by subsequent H atom loss. The position of the methyl group on the benzene ring is observed to influence the product vibrational energy disposal-not least through its influence on the mode(s) that are activated as a result of coupling to the repulsive (1)pisigma state. O-H bond strengths are reported for 4-, 3- and 2-methylphenol. These are in good agreement with values derived from recent combustion calorimetry studies and serve to highlight the relative destabilisation of the radical caused by methyl substitution at the 3-position.

Ion imaging methods have enabled identification of three mechanisms by which (79)Br(+) and (35)Cl(+) fragment ions are formed following one-color multiphoton excitation of BrCl molecules in the wavelength range 324.6 > lambda > 311.7 nm. Two-photon excitation within this range populates selected vibrational levels (v'= 0-5) of the [X (2)Pi(1/2)]5ssigma Rydberg state. Absorption of a third photon results in branching between (i) photoionization (i.e. removal of the Rydberg electron-a traditional 2 + 1 REMPI process) and (ii)pi*<--pi excitation within the core, resulting in formation of one or more super-excited states with Omega= 1 and configuration [A (2)Pi(1/2)]5ssigma. The fate of the latter states involves a further branching. They can autoionize (yielding BrCl(+)(X (2)Pi) ions in a wider range of v(+) states than formed by direct 2 + 1 REMPI). Further, one-photon absorption by the parent ions resulting from direct ionization or autoionization leads to formation of Br(+) and (energy permitting) Cl(+) fragment ions. Alternatively, the super-excited molecules can fragment to neutral atoms, one of which is in a Rydberg state. Complementary ab initio calculations lead to the conclusion that the observed [Cl**[(3)P(J)]4s + Br/Br*] products result from direct dissociation of the photo-prepared super-excited states, whereas [Br**[(3)P(J)]5p + Cl/Cl*] product formation involves interaction between the [A (2)Pi(1/2)]5ssigma and [X (2)Pi(1/2)]5psigma Rydberg potentials at extended Br-Cl bond lengths. Absorption of one further photon by the resulting Br** and Cl** Rydberg atoms leads to their ionization, and thus their appearance in the Br(+) and Cl(+) fragment ion images.

The experimental techniques of H (Rydberg) atom photofragment translational spectroscopy and resonance-enhanced multiphoton ionisation time-of-flight spectroscopy have been used to investigate the dynamics of H atom loss processes from gas phase 4-fluorophenol (4-FPhOH), 4-chlorophenol (4-ClPhOH) and 4-bromophenol (4-BrPhOH) molecules, following excitation at many wavelengths, lambda(phot), in the range between their respective S(1)-S(0) origins (284.768 nm, 287.265 nm and 287.409 nm) and 216 nm. Many of the Total Kinetic Energy Release (TKER) spectra obtained from photolysis of 4-FPhOH show structure, the analysis of which reveals striking parallels with that reported previously for photolysis of bare phenol (M. G. D. Nix, A. L. Devine, B. Cronin, R. N. Dixon and M. N. R. Ashfold, J. Chem. Phys., 2006, 125, 133318). The data demonstrates the importance of O-H bond fission, and that the resulting 4-FPhO co-fragments are formed in a select fraction of their available vibrational state density. All spectra recorded at lambda(phot)> or = 238 nm show a feature centred at TKER approximately 5500 cm(-1). These H atom fragments show no recoil anisotropy, and are rationalised in terms of initial S(1)<-- S(0) (pi* <--pi) excitation and subsequent dissociation via two successive radiationless transitions: internal conversion to ground (S(0)) state levels carrying sufficient O-H stretch vibrational energy to allow efficient transfer to (and round) the Conical Intersection (CI) between the S(0) and S(2)((1)pi sigma*) Potential Energy Surfaces (PESs) at larger R(O-H), en route to H atoms and ground state 4-FPhO products. The vibrational energy disposal in the 4-FPhO products indicates that parent mode nu(16a) promotes non-adiabatic coupling at the S(0)/S(2) CI. Spectra recorded at lambda(phot)< or = 238 nm reveal a faster (but still isotropic) distribution of recoiling H atoms, centred at TKER approximately 12 000 cm(-1), attributable to H + 4-FPhO products formed when the optically excited (1)pi pi* molecules couple directly with the (1)pi sigma* PES. Parent mode nu(16b) is identified as the dominant coupling mode at the S(1)((1)pi pi*)/S(2)((1)pi sigma*) CI, and the resulting 4-FPhO radical co-fragments display progressions in nu(18b) (the C-O in-plane wagging mode) and nu(7a) (an in-plane ring breathing mode involving significant C-O stretching motion). Analysis of all structured TKER spectra yields a C-F bond dissociation energy: D(0)(H-OC(6)H(4)F) = 29 370 +/- 50 cm(-1). The photodissociation of 4-ClPhOH shows many similarities, though the 4-ClPhO products formed together with faster H atoms at shorter wavelengths (lambda(phot)< or = 238 nm, by coupling through the S(1)/S(2) CI) show activity in an alternative ring breathing mode (nu(19a) rather than nu(7a)). Spectral analysis yields D(0)(H-OC(6)H(4)Cl) = 29 520 +/- 50 cm(-1). H atom formation via O-H bond fission is (at best) a very minor channel in the photolysis of 4-BrPhOH at all wavelengths investigated. Time-dependent density functional theory calculations suggest that this low H atom yield is because of competition from the alternative C-Br bond fission channel, and that the analogous C-Cl bond fission may be responsible for the weakness of the one photon-induced H atom signals observed when photolysing 4-ClPhOH at longer wavelengths.

High resolution time-of-flight (TOF) measurements of the D atom fragments arising in the near ultraviolet (UV) photodissociation of deuterated pyrrole are reported. Structures evident in the measured TOF spectra are all interpretable in terms of N-D bond fission, and population of selected vibrational states of the pyrrolyl-d(4) co-fragment -- thereby clarifying previous uncertainties regarding the branching into different vibronic states of the pyrrolyl radical following UV excitation of pyrrole.

The photodissociation dynamics of 2,5-dimethylpyrrole (2,5-DMP) has been investigated following excitation at 193.3 nm and at many near ultraviolet (UV) wavelengths in the range 244 < lambda(phot) < 282 nm using H Rydberg atom photofragment translational spectroscopy (PTS). Complementary UV absorption and, at the longest excitation wavelengths, one photon resonant multiphoton ionisation spectra of 2,5-DMP are reported also; analysis of the latter highlights the role of methyl torsional motions in promoting the parent absorption. The deduced fragmentation dynamics show parallels with that reported recently (B. Cronin, M. G. D. Nix, R. H. Qadiri and M. N. R. Ashfold, Phys. Chem. Chem. Phys., 2004, 6, 5031) for the bare pyrrole molecule. Excitation at the longer wavelengths leads to (vibronically induced) population of the 1(1)A(2)(pisigma*) excited state of 2,5-DMP, but once lambda(phot) decreases to approximately 250 nm stronger, dipole allowed transitions start to become apparent in the parent absorption. All total kinetic energy release (TKER) spectra of the H + 2,5-dimethylpyrrolyl (2,5-DMPyl) fragments measured at lambda(phot)> or=244 nm show a structured fast component, many of which are dominated by a peak with TKER approximately 5100 cm(-1); analysis of this structure reveals lambda(phot) dependent population of selected vibrational levels of 2,5-DMPyl, and enables determination of the N-H bond strength in 2,5-DMP: D(0) = 30 530 +/- 100 cm(-1). Two classes of behaviour are proposed to account for details of the observed energy partitioning. Both assume that N-H bond fission involves passage over (or tunnelling through) a small exit channel barrier on the 1(1)A(2) potential energy surface, but differ according to the vibrational energy content of the photo-prepared molecules. Specific parent out-of-plane skeletal modes that promote the 1(1)A(2)-X(1)A(1) absorption appear to evolve adiabatically into the corresponding vibrations of the 2,5-DMPyl products. Methyl torsions can also promote the 1(1)A(2)<-- X(1)A(1) absorption in 2,5-DMP, and provide a means of populating a much higher density of excited vibrational levels than in pyrrole. Such excited levels are deduced to dissociate by redistributing the minimum amount of internal energy necessary to overcome the exit channel barrier in the N-H dissociation coordinate. Coupling with the ground state surface via a conical intersection at extended N-H bond lengths is proposed as a further mechanism for modest translational --> vibrational energy transfer within the separating products. The parent absorption cross-section increases considerably at wavelengths approximately 250 nm, and PTS spectra recorded at lambda(phot)< or = 254 nm display a second, unstructured, peak at lower TKER. As in pyrrole, this slower component is attributed to H atoms from the unimolecular decay of highly vibrationally excited ground state molecules formed via radiationless decay from photo-excited states lying above the 1(1)A(2) state.

Ion imaging methods are making ever greater impact on studies of gas phase molecular reaction dynamics. This article traces the evolution of the technique, highlights some of the more important breakthroughs with regards to improving image resolution and in image processing and analysis methods, and then proceeds to illustrate some of the many applications to which the technique is now being applied--most notably in studies of molecular photodissociation and of bimolecular reaction dynamics.

Density functional theory-based electronic structure computations on small models of the diamond {100} surface have enabled prediction of the energetics and activation parameters of a number of plausible mechanistic steps for boron incorporation into, and boron loss from, the growing diamond surface. Initial proving calculations for the carbon-only case show, as in previous work, that the rate-limiting step for diamond growth involves opening of a five-membered ring species, and subsequent closure to form six-membered rings as in bulk diamond. The five-membered ring intermediate arises following 2 x 1 reconstruction of the {100} surface, or at steps on the {111} surface. Diamond growth arises as a result of successful competition between the ring-opening step and a two-carbon loss step, both of which involve significant activation barriers. In the boron case, we find that BH(x) (x = 0-3) species can all bind to radical sites on the diamond {100} surface to form stable adducts. Interconversion between the surface bound BH, species is facile at the H and H2 number densities and temperatures typical for diamond CVD conditions. B incorporation can occur by a ring expansion mechanism, as in the all-carbon case, and by direct insertion of surface bound BH (and B) species into the C-C bond on the diamond {100} surface. BH(x) loss processes identified include release of surface bound BH3 and/or CH2BH species into the gas phase. Both B incorporation into, and B loss from, the diamond {100} surface are deduced to be significantly less energy demanding than the corresponding carbon addition and loss processes.

Photoelectron imaging and time of flight mass spectrometry are used to study the multiphoton ionization and dissociation of pyrrole and its cation following excitation at 243 nm and at 364.7 nm. Our results confirm the 8.2 eV ionization potential of pyrrole and the 9.2 eV ionization threshold for forming the 2B1 first excited state of the cation. Prompt photolysis of the N-H bond in neutral pyrrole following one-photon excitation to its 1 1A2 neutral excited state is inferred from analysis of the two-photon photoelectron spectrum recorded at 243 nm, confirming the findings of recent translational spectroscopy studies. Facile dissociation of the pyrrole cation is also observed following excitation at 243 nm; analysis of the fragment cations indicates the operation of a complex dissociation mechanism involving dual bond fission and possible migration of the H atom originally bonded to the nitrogen heteroatom.

Absorption cross sections have been measured for the 2(0)(2)4(0)(3) and 2(0)(3)4(0)(1) vibrational bands of the [formula: see text] electronic transition of formaldehyde in the wavelength range 313-320 nm. Accurate values are of considerable importance for atmospheric monitoring and to understand the photochemistry of this compound. The 0.10 cm(-1) FWHM wavenumber resolution of the experiments is determined by the bandwidth of the ultraviolet laser used, and is a factor of 10 or more higher than any previously reported data. The absorption cross section data are thus obtained at a spectrometer resolution close to the Doppler broadening limit at 294 K of 0.07 cm(-1) FWHM, for isolated rotational lines, but lifetime broadening effects contribute a further approximately 0.5 cm(-1) of width. Our spectral resolution is thus higher than required to resolve the sharpest spectral features and, as a consequence, the cross sections peak at greater values than previous studies of these structured rovibronic bands conducted at much lower spectrometer resolutions. Previous data can be quantitatively reproduced by convolution of the newly obtained spectra with lower-resolution instrument functions. Pressure broadening of regions of the spectra in the presence of up to 500 Torr of N2 is examined and the effects on peak absorption cross sections are very small. The influence of reduced temperature on the spectrum is also explored through experimental measurements and spectral simulations.