Theoretical exploration of ultrafast spectroscopy of small clusters
Authors: Bonačić‐Koutecký V. , Hartmann Michael , Pittner Jiří , van Dam Huub 
Published: August 2002

The central issue in femtosecond (fs) time resolved spectroscopy of clusters is the investigation of geometric relaxation and internal vibrational redistribution (IVR) after optical excitation in a nonequilibrium configuration of nuclei by laser photoelectron excitation, and corresponding time delayed probing by multiphoton‐ionization. For this purpose, we have developed multistate ab initio molecular dynamics involving adiabatic ground and excited electronic states, as well as nonadiabatic coupling between them, using the time evolution of initial thermal ensemble in Wigner representation. The combination of ab initio quantum chemical methods, used for the adiabatic and nonadiabatic 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 connection between simulated pump‐probe signals and the underlying dynamics of nuclei involving adiabatic electronic ground states has been first established for the example of the Ag\documentclass{article}\pagestyle{empty}\begin{document}$_{3}^{-}$\end{document}/Ag3/Ag\documentclass{article}\pagestyle{empty}\begin{document}$_{3}^{+}$\end{document} systems, and compared with experimental negative‐to‐neutral‐to‐positive NeNePo pump‐probe signals. Our simulations reproduced the experimental NeNePo results and determined, in addition to the timescales of geometric relaxation, the conditions under which the resonant or dissipative IVR, as well as vibrational coherence, should 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 above combination of methods has been recently extended to the analysis of the timescales as well as of the dynamics in excited electronic states of the nonstoichiometric NanFn−1 (n=2–4) clusters with the single excess valence electron. Our approach allows the simulation of femtosecond NeExPo‐pump‐probe and NeExNe‐pump‐dump signals, based on an analytic formulation which utilizes temperature dependent ground state initial conditions of neutral system (Ne); an ensemble of trajectories carried out either on the adiabatic electronic excited state (Ex), or on both the excited and the ground states through nonadiabatic coupling in connection with the fewest switching hopping algorithm for the investigation of the dynamics of the system; and either the cationic (Po) or the neutral ground state (Ne) for the probing step. The choice of the systems has been made in order to determine the timescales of processes involving (1) fast geometric relaxation leaving the bonding frame intact versus IVR, as during the adiabatic dynamics in the first excited state of Na4F3, being the smallest prototype of F‐colored centers in the bulk; and (2) the photo‐isomerization process through the conical intersection during nonadiabatic dynamics due to the long amplitude motion, as in the Na3F2 cluster after breaking of one metallic and one ionic bond, representing the first example of a five atomic cluster in the gas phase exhibiting conical intersection between the ground and the first excited state. In both cases, full complexity of the problem has been considered taking into account all degrees of freedom. The investigated systems represent important test cases for providing the conceptual framework of ultrafast dynamics in finite systems. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001