In this paper we consider the inclusion of the spin-orbit interaction in surface hopping molecular dynamics simulations to take into account spin forbidden transitions. Two alternative approaches are examined. The spin-diabatic one makes use of eigenstates of the spin-free electronic Hamiltonian and of Ŝ(2) and is commonly applied when the spin-orbit coupling is weak. We point out some inconsistencies of this approach, especially important when more than two spin multiplets are coupled. The spin-adiabatic approach is based on the eigenstates of the total electronic Hamiltonian including the spin-orbit coupling. Advantages and drawbacks of both strategies are discussed and illustrated with the help of two model systems.

Benzophenone is a prototype molecule for photochemistry in the triplet state through its high triplet yield and reactivity. We have investigated its dynamics of triplet formation under the isolated gas phase conditions via femtosecond and nanosecond time resolved photoelectron spectroscopy. This represents the complete evolution from the excitation in S2 to the final decay of T1 to the ground state S0. We have found that the triplet formation can be described almost as a direct process in preparing T1, the lowest reacting triplet state, from the S1 state after S2 → S1 internal conversion. The molecule was also deposited by a pick-up technique on cold argon clusters providing a soft relaxation medium without evaporation of the molecule and the mechanism was identical. This cluster technique is a model for medium influenced electronic relaxation and provides a continuous transition from the isolated gas phase to the relaxation dynamics in solution.

The trans-azobenzene molecule is thought to prefer a planar C2h geometry, in gas phase as well as in solution, according to the most recent computational studies. As a consequence, the weak n-->pi* absorption band is forbidden by symmetry at the equilibrium geometry, and its intensity depends on the effect of the vibrational motions on the electronic structure. In this computational study, we determine the contribution of the vibrational modes to the oscillator strength, taking into account the anharmonicity, the thermal distributions, and the solvent effects. The good agreement of our results with the measured absorption spectrum confirms the C2h equilibrium structure of trans-azobenzene, with a relatively easy torsion of the phenyl groups around the N--C bonds. We also address the question of the polarization of this transition, which is a preliminary step to interpret the time-resolved fluorescence anisotropy measurements [C.-W. Chang et al., J. Am. Chem. Soc., 126, 10109 (2004)], a very sensitive probe of solvent effects on the excited state dynamics.

The S1( 1 *) state of the (dominant) syn-conformer of 2-chlorophenol (2-ClPhOH) in the gas phase has a sub-picosecond lifetime, whereas the corresponding S1 states of 3-and 4-ClPhOH have lifetimes that are, respectively, ~2 and ~3-orders of magnitude longer. A range of experimental techniques -electronic spectroscopy, ultrafast time-resolved photoion and photoelectron spectroscopies, H Rydberg atom photofragment translational spectroscopy, velocity map imaging and time-resolved Fourier transform infrared emission spectroscopyas well as electronic structure calculations (of key regions of the multidimensional ground (S0) state potential energy surface (PES) and selected cuts through the first few excited singlet PESs) have been used in the quest to explain these striking differences in excited state lifetime. The intramolecular OH ---Cl hydrogen bond specific to syn-2-ClPhOH is key. It encourages partial charge transfer and preferential stabilisation of the diabatic 1 * potential (relative to that of the 1 * state) upon stretching the C-Cl bond, with the result that initial C-Cl bond extension on the adiabatic S1 PES offers an essentially barrierless internal conversion pathway via regions of conical intersection with the S0 PES. Intramolecular hydrogen bonding is thus seen to facilitate the type of heterolytic dissociation more typically encountered in solution studies.3

We report on the single photoionization of jet-cooled benzophenone using a tunable source of VUV synchrotron radiation coupled with a photoion/photoelectron coincidence acquisition device. The assignment and the interpretation of the spectra are based on a characterization by ab initio and density functional theory calculations of the geometry and of the electronic states of the cation. The absence of structures in the slow photoelectron spectrum is explained by a congestion of the spectrum due to the dense vibrational progressions of the very low frequency torsional mode in the cation either in pure form or in combination bands. Also a high density of electronic states has been found in the cation. Presently, we estimate the experimental adiabatic and vertical ionization energy of benzophenone at 8.80 ± 0.01 and 8.878 ± 0.005 eV, respectively. The ionization energy as well as the energies of the excited states are compared to the calculated ones.