Ab initio nonadiabatic dynamics involving conical intersection combined with Wigner distribution approach to ultrafast spectroscopy illustrated on Na3F2 cluster

We present a theoretical study of a femtosecond photo isomerization process due to a nonadiabatic radiationless decay from the first excited state thr ough a conical intersection occurring in one of the nonstoichiometric halid e-deficient clusters with one excess electron (Na3F2). This is an extension of the adiabatic dynamics study presented in the accompanying paper [J. Ch em. Phys. 114, 2106 (2001)] for other members of the NanFn-1 series charact erized by a strong ionic bonding for which the "frozen ionic bonds" approxi mation has been justified, allowing consideration of the optical response o f the single excess electron in the effective field of the other electrons. In this contribution we outline the extension of the ab initio Wigner-dist ribution approach to nonadiabatic molecular dynamics which combines the Wig ner-Moyal representation of the vibronic density matrix with the ab initio multistate molecular dynamics in the ground- and excited electronic states including the nonadiabatic coupling computed "on the fly" in connection wit h the fewest-switches hopping algorithm. This scheme allows accounting for temperature-dependent initial conditions, for the propagation in the excite d state and in the ground state after the passage through the conical inter section, and for probing in the cationic ground state as well as for derivi ng analytic expressions for the pump-probe signals which utilize an ensembl e of classical trajectories obtained at low computational demand. Our appro ach permits investigation of the photo isomerization through the conical in tersection due to the long amplitude motion in the Na3F2 system in full com plexity, taking into account all degrees of freedom. After breaking of one metallic and of one ionic bond the conical intersection occurs at the linea r geometry and involves states of different symmetry which differ in the tr anslocation of the one excess electron or positive charge localized at the Na atom from one end to the other of the system and separates two isomers w ith C-s and C-2v structures. From the analysis of the nonadiabatic dynamics , the time scales for the metallic bond breaking of similar to 90 fs and fo r the ionic bond breaking of similar to 220 fs, for the passage through the conical intersection after similar to0.4 ps and for the internal vibration al energy redistribution (IVR) of more than 0.9 ps for the individual isome rs, have been determined. The simulated fs pump-probe signals confirm the a bove results and provide the information about the experimental conditions such as laser frequencies and pulse duration under which bond breaking of d ifferent type as well as the population of each of the two isomers after th e passage through the conical intersection can be identified. In this contr ibution we show that the mechanism of the photo isomerization at a conical intersection due to a long amplitude motion can occur in atomic clusters an d is not necessarily limited to organic photochemistry. (C) 2001 American I nstitute of Physics.