THEORETICAL EXPLORATION OF ULTRAFAST SPECTROSCOPY OF SMALL CLUSTERS

摘要

The central issue in femtosecond (fs) time resolved spectroscopy of clusters is the investigation of geometrical relaxation and internal vibrational redistribution (IVR) after optical excitation in a non-equilibrium configuration of nuclei by laser photoelectron excitation and corresponding time delayed probing by multiphoton-ionization. For this purpose we have developed a multi state ab initio molecular dynamics (involving ground as well as adiabatic and non adiabatic excited electronic states) on the time scale of nuclear motion, using the time evolution of a thermal ensemble in Wigner representation. The combination of ab initio quantum chemical methods used for the molecular dynamics "on the fly" and the Wigner distribution approach for the description of the motion of the nuclei allowed us the accurate determination of pump-probe and pump-dump signals also under temperature dependent initial conditions. The connections between simulated pump-probe signals and the underlying dynamics of nuclei as well as time scales for different processes has been explicitly established for the examples of the Ag_3~-/Ag_3/Ag_3~+ systems and compared with experimental NeNePo (negative ion - to neutral - to positive ion) signals. Our simulations reproduced the experimental NeNePo results and determined in addition to the time scales of geometrical relaxation the conditions under which the resonant or dissipative IVR as well as vibrational coherence can be found in the experimental pump-probe signals. This can be realized in the zero electron kinetic energy NeNePo-ZEKE experiments, which are in progress. The method has been recently extended to the analysis of the timescales as well as of the dynamics in excited electronic states of the non-stoichiometric Na_nF_(n-1)clusters with the single excess valence electron. The time scales of the structural relaxation in excited electronic states vs. intramolecular vibrational relaxation processes can be determined as illustrated on the example of Na_4F_3. This is the first study of the system with 16 degrees of freedom for which the dynamics in the excited states have been carried out without the precalculation of the energy surface. The investigated systems represent important test cases for providing the conceptual framework of ultrafast dynamics in finite systems.