Ab initio adiabatic dynamics involving excited states combined with Wignerdistribution approach to ultrafast spectroscopy illustrated on alkali halide clusters

We investigate the ultrafast multistate nuclear dynamics involving adiabati c electronic excited states of nonstoichiometric halide deficient clusters (NanFn-1) characterized by strong ionic bonding and one excess electron, wh ich is localized either in the halide vacancy or on the alkali atom attache d to the ionic subunit depending on the cluster size. For this purpose we d eveloped an ab initio adiabatic nuclear dynamics approach in electronic exc ited and ground states "on the fly" at low computational demand by introduc ing the "frozen ionic bonds" approximation, which yields an accurate descri ption of excited states considering the excitation of the one excess electr on in the effective field of the other n-1 valence electrons involved in th e ionic bonding. We combined this multistate dynamics approach with the Wig ner-Moyal representation of the vibronic density matrix forming the ab init io Wigner distribution approach to adiabatic dynamics. This method allows t he simulation of femtosecond NeExPo-pump-probe and NeExNe-pump-dump signals based on an analytic formulation which utilizes temperature-dependent grou nd-state initial conditions (Ne), an ensemble of trajectories carried out o n the electronic excited state (Ex) for the investigation of the dynamics o f the system, and either the cationic (Po) or the ground state (Ne) for the probing step. The choice of the systems has been made in order to determin e the time scales of processes involving (i) metallic bond breaking such as during the dynamics in the first excited state of Na2F, and (ii) fast geom etric relaxation leaving the bonding frame intact as during the dynamics in the first excited state of Na4F3. The bond-breaking process via a conical intersection involving nonadiabatic dynamics will be presented in the accom panying paper [Hartmann , J. Chem. Phys. 114, 2123 (2001)]. The dynamics in the first excited state of Na2F from triangular-to linear-to triangular st ructure gives rise to fast geometric relaxation due to Na-Na bond breaking at the time scale of similar to 90 fs but no signature of internal vibratio nal energy redistribution (IVR) is present in NeExNe-pump-dump signals sinc e the broken metallic bond prevents the coupling between stretching and ben ding modes. Instead, anharmonicities of the bending periodic motion have be en identified. In contrast, in the case of Na4F3, which is the smallest fin ite system for a surface F-center prototype of bulk color centers, after th e geometric relaxation in the excited state of similar to 100 fs leading to the deformed cuboidal type of structure without breaking of bonds, differe nt types of IVR have been identified in NeExNe signals by tuning the dump l aser: one-mode selective energy leaving IVR, resonant, and restricted energ y arriving IVR corresponding to the selection of different parts of the pha se space. Dissipative IVR could not be identified in NeExNe signals of Na4F 3 at low initial temperature on the time scale up to 2 ps in spite of 15 de grees of freedom. Due to similar structural and electronic properties such as F centers in bulk, these findings can serve as guidance for establishing the time scales for geometric relaxation and IVR in excited states of larg er systems. (C) 2001 American Institute of Physics.