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Two dimensional electronic spectroscopy has proved to be a valuable experimental technique to reveal electronic excitation dynamics in photosynthetic pigment-protein complexes, nanoscale semiconductors, organic photovoltaic materials, and many other types of systems. It does not, however, provide direct information concerning the spatial structure and dynamics of excitons. 2D infrared spectroscopy has become a widely used tool for studying structural dynamics but is incapable of directly providing information concerning electronic excited states. 2D electronic-vibrational (2DEV) spectroscopy provides a link between these domains, directly connecting the electronic excitation with the vibrational structure of the system under study. In this work, we derive response functions for the 2DEV spectrum of a molecular dimer and propose a method by which 2DEV spectra could be used to directly measure the electronic site populations as a function of time following the initial electronic excitation. We present results from the response function simulations which show that our proposed approach is substantially valid. This method provides, to our knowledge, the first direct experimental method for measuring the electronic excited state dynamics in the spatial domain, on the molecular scale. [ABSTRACT FROM AUTHOR]

Changes in the electronic structure of pigments in protein environments and of polar molecules in solution inevitably induce a re-adaption of molecular nuclear structure. Both changes of electronic and vibrational energies can be probed with visible or infrared lasers, such as two-dimensional electronic spectroscopy or vibrational spectroscopy. The extent to which the two changes are correlated remains elusive. The recent demonstration of two-dimensional electronic-vibrational (2DEV) spectroscopy potentially enables a direct measurement of this correlation experimentally. However, it has hitherto been unclear how to characterize the correlation from the spectra. In this paper, we present a theoretical formalism to demonstrate the slope of the nodal line between the excited state absorption and ground state bleach peaks in the spectra as a characterization of the correlation between electronic and vibrational transition energies. We also show the dynamics of the nodal line slope is correlated to the vibrational spectral dynamics. Additionally, we demonstrate the fundamental 2DEV spectral line-shape of a monomer with newly developed response functions. [ABSTRACT FROM AUTHOR]

Two-dimensional electronic-vibrational (2DEV) spectroscopy is an experimental technique that shows great promise in its ability to provide detailed information concerning the interactions between the electronic and vibrational degrees of freedom in molecular systems. The physical quantities 2DEV is particularly suited for measuring have not yet been fully determined, nor how these effects manifest in the spectra. In this work, we investigate the use of the center line slope of a peak in a 2DEV spectrum as a measure of both the dynamic and static correlations between the electronic and vibrational states of a dye molecule in solution. We show how this center line slope is directly related to the solvation correlation function for the vibrational degrees of freedom. We also demonstrate how the strength with which the vibration on the electronic excited state couples to its bath can be extracted from a set of 2DEV spectra. These analytical techniques are then applied to experimental data from the laser dye 3,3'-diethylthiatricarbocyanine iodide in deuterated chloroform, where we determine the lifetime of the correlation between the electronic transition frequency and the transition frequency for the backbone C==C stretch mode to be ~1.7 ps. Furthermore, we find that on the electronic excited state, this mode couples to the bath ~1.5 times more strongly than on the electronic ground state [ABSTRACT FROM AUTHOR]

The photochemistry of the water molecule has revealed a wealth of quantum phenomena, which arise from the involvement of several coupled electronic states with very different potential energy surfaces. Most recently, dissociation from single rotational levels of its C1B1 state near 124 nm has been attributed to a vibronically coupled decay via the lower Ã-state surface, despite a large vertical energy gap of 2.8 eV. Similar conclusions have been reached for subsequent experimental data for D2O. The present paper presents further experimental data for HOD and for both the H+OD(X) and D+OH(X) products. Unlike the cases for H2O and D2O, the vibrational populations for hydroxyl products do not follow a smooth distribution with v(OH/OD). In particular, for OH there is a clear alternation in population for all the strong peaks, with odd v favoured over even v. These experimental data are analysed using new MRCI+Q calculations, which have been used to generate potential surfaces and associated non-adiabatic matrix elements for transition from the adiabatic C-state to lower unbound potential surfaces; and hence, to guide dynamical calculations using time-dependent wavepackets. It is concluded that although there is a minor contribution from the C→ à decay route, the major route follows C→ 1A2 → Ã. This is mediated through two regions of near degeneracy of the elusive 1A2 surface with C for short bonds ca. 0.8 Å; and between 1A2 and à with long bonds ≥2 Å, thereby bridging the vertical energy gap. The striking population alternation for the D+OH(X) products is attributed to dynamic symmetry breaking on the 1A2 surface. [ABSTRACT FROM AUTHOR]

The dynamics of reactions of CN radicals with cyclohexane, d12-cyclohexane, and tetramethylsilane have been studied in solutions of chloroform, dichloromethane, and the deuterated variants of these solvents using ultraviolet photolysis of ICN to initiate a reaction. The H(D)-atom abstraction reactions produce HCN (DCN) that is probed in absorption with sub-picosecond time resolution using ∼500 cm-1 bandwidth infrared (IR) pulses in the spectral regions corresponding to C-H (or C-D) and C≡N stretching mode fundamental and hot bands. Equivalent IR spectra were obtained for the reactions of CN radicals with the pure solvents. In all cases, the reaction products are formed at early times with a strong propensity for vibrational excitation of the C-H (or C-D) stretching (v3) and H-C-N (D-C-N) bending (v2) modes, and for DCN products there is also evidence of vibrational excitation of the v1 mode, which involves stretching of the C≡N bond. The vibrationally excited products relax to the ground vibrational level of HCN (DCN) with time constants of ∼130-270 ps (depending on molecule and solvent), and the majority of the HCN (DCN) in this ground level is formed by vibrational relaxation, instead of directly from the chemical reaction. The time-dependence of reactive production of HCN (DCN) and vibrational relaxation is analysed using a vibrationally quantum-state specific kinetic model. The experimental outcomes are indicative of dynamics of exothermic reactions over an energy surface with an early transition state. Although the presence of the chlorinated solvent may reduce the extent of vibrational excitation of the nascent products, the early-time chemical reaction dynamics in these liquid solvents are deduced to be very similar to those for isolated collisions in the gas phase. The transient IR spectra show additional spectroscopic absorption features centered at 2037 cm-1 and 2065 cm-1 (in CHCl3) that are assigned, respectively, to CN-solvent complexes and recombination of I atoms with CN radicals to form INC molecules. These products build up rapidly, with respective time constants of 8-26 and 11-22 ps. A further, slower rise in the INC absorption signal (with time constant >500 ps) is attributed to diffusive recombination after escape from the initial solvent cage and accounts for more than 2/3 of the observed INC. [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]

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]

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]

The potential energy surfaces (PESs) and S1-T1 spin-orbit coupling matrix element (SOCME) surfaces are investigated for the toluene–X weakly bound clusters (X=Ne, Ar, and Kr). Calculations of the vibrational wave functions using a one-dimensional stretch model are presented and used to determine vibrationally averaged values of the SOCMEs. Our ab initio theoretical results compare well with intersystem crossing rates derived from recent experimental fluorescence lifetime data [Doyle et al., J. Chem. Phys. 122, 194315 (2005)]. Vibrational averaging is shown to change the absolute magnitude of the calculated SOCMEs, but the ratio between them remains very similar to that of the single-point values calculated at the minima of the PESs. [ABSTRACT FROM AUTHOR]

The wavelength dependences of C-Y and O-H bond fission following ultraviolet photoexcitation of 4-halophenols (4-YPhOH) have been investigated using a combination of velocity map imaging, H Rydberg atom photofragment translational spectroscopy, and high level spin-orbit resolved electronic structure calculations, revealing a systematic evolution in fragmentation behaviour across the series Y = I, Br, Cl (and F). All undergo O-H bond fission following excitation at wavelengths λ <= 240 nm, on repulsive ((n/π)σ*) potential energy surfaces (PESs), yielding fast H atoms with mean kinetic energies ∼11 000 cm-1. For Y = I and Br, this process occurs in competition with prompt C-I and C-Br bond cleavage on another (n/π)σ* PES, but no Cl/Cl* products unambiguously attributable to one photon induced C-Cl bond fission are observed from 4-ClPhOH. Differences in fragmentation behaviour at longer excitation wavelengths are more marked. Prompt C-I bond fission is observed following excitation of 4-IPhOH at all λ ≤ 330 nm; the wavelength dependent trends in I/I* product branching ratio, kinetic energy release, and recoil anisotropy suggest that (with regard to C-I bond fission) 4-IPhOH behaves like a mildly perturbed iodobenzene. Br atoms are observed when exciting 4-BrPhOH at long wavelengths also, but their velocity distributions suggest that dissociation occurs after internal conversion to the ground state. O-H bond fission, by tunnelling (as in phenol), is observed only in the cases of 4-FPhOH and, more weakly, 4-ClPhOH. These observed differences in behaviour can be understood given due recognition of (i) the differences in the vertical excitation energies of the C-Y centred (n/π)σ* potentials across the series Y = I < Br < Cl and the concomitant reduction in C-Y bond strength, cf. that of the rival O-H bond, and (ii) the much increased spin-orbit coupling in, particularly, 4-IPhOH. The present results provide (another) reminder of the risks inherent in extrapolating photochemical behaviour measured for one molecule at one wavelength to other (related) molecules and to other excitation energies. [ABSTRACT FROM AUTHOR]

Changes in the electronic structure of pigments in protein environments and of polar molecules in solution inevitably induce a re-adaption of molecular nuclear structure. Both changes of electronic and vibrational energies can be probed with visible or infrared lasers, such as two-dimensional electronic spectroscopy or vibrational spectroscopy. The extent to which the two changes are correlated remains elusive. The recent demonstration of two-dimensional electronic-vibrational (2DEV) spectroscopy potentially enables a direct measurement of this correlation experimentally. However, it has hitherto been unclear how to characterize the correlation from the spectra. In this paper, we present a theoretical formalism to demonstrate the slope of the nodal line between the excited state absorption and ground state bleach peaks in the spectra as a characterization of the correlation between electronic and vibrational transition energies. We also show the dynamics of the nodal line slope is correlated to the vibrational spectral dynamics. Additionally, we demonstrate the fundamental 2DEV spectral line-shape of a monomer with newly developed response functions.

This article reports a comprehensive study of the mechanisms of H atom loss in aniline (C(6)H(5)NH(2)) following ultraviolet excitation, using H (Rydberg) atom photofragment translational spectroscopy. N-H bond fission via the low lying (1)pi sigma(*) electronic state of aniline is experimentally demonstrated. The (1)pi sigma(*) 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 (lambda(phot)) in the range 293.859 nm > or = lambda(phot) > or = 193.3 nm yields H atom loss via a range of mechanisms. With lambda(phot) resonant with the 1(1)pi pi(*) <-- S(0) origin (293.859 nm), H atom loss proceeds via, predominantly, multiphoton excitation processes, resonantly enhanced at the one photon energy by the first (1)pi pi(*) excited state (the 1(1)pi pi(*) state). Direct excitation to the first few quasibound vibrational levels of the (1)pi sigma(*) state (at wavelengths in the range 269.513 nm > or = lambda(phot) > or = 260 nm) induces N-H bond fission via H atom tunneling through an exit barrier into the repulsive region of the (1)pi sigma(*) PES, forming anilino (C(6)H(5)NH) radical products in their ground electronic state, and with very limited vibrational excitation; the photo-prepared vibrational mode in the (1)pi sigma(*) state generally evolves adiabatically into the corresponding mode of the anilino radical upon dissociation. However, as the excitation wavelength is reduced (lambda(phot) < 260 nm), N-H bond fission yields fragments with substantially greater vibrational excitation, rationalized in terms of direct excitation to 1(1)pi pi(*) levels, followed by coupling to the (1)pi sigma(*) PES via a 1(1)pi pi(*)/(1)pi sigma(*) conical intersection. Changes in product kinetic energy disposal once lambda(phot) approaches approximately 230 nm likely indicate that the photodissociation pathways of aniline proceed via direct excitation to the (higher) 2(1)pi pi(*) state. Analysis of the anilino fragment vibrational energy disposal-and thus the concomitant dynamics of (1)pi sigma(*) state mediated photodissociation-provides a particularly interesting study of competing sigma(*) <-- pi and pi(*) <-- pi 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, D(0)(H-anilino) = 31630+/-40 cm(-1).