Ultrafast Dynamics of Small Clusters on the Time Scale of Nuclear Motion

Abstract

We present a theoretical study of the multistate dynamics of the Ag3 -/Ag3/Ag3 + system on the time scale of nuclear motion, as explored by femtosecond pump−probe negative ion-to neutral-to positive ion (NENEPO) spectroscopy. The dynamics of the Ag3 cluster initiated from its linear transition state involves configurational relaxation, intracluster collision, and onset of IVR, resonant and dissipative IVR and vibrational equilibration, whose time scales were determined. Our analysis reveals that theory is essential for the elucidation of the rich dynamic information regarding geometrical change, completion of IVR, and vibrational coherence effects in the NENEPO femtosecond signals.

Femtosecond Time‐Resolved Geometry Relaxation and Ultrafast Intramolecular Energy Redistribution in Ag2Au

“…[7,8,[22][23][24][25] In the framework of NeNePo spectroscopy the following steps are needed: preparation of an initial ensemble in the anionic ground state of the cluster, one-photon detachment by the pump pulse, propagation of the system on the neutral ground state, and detection in the cationic state by a time-delayed ionizing probe pulse. For this purpose precalculated high-quality energy surfaces for small systems can be used if available, [7,8] but ab initio MD "on the fly" offers a more general alternative since it is applicable to large systems.…”

“…Activation enables the system to propagate toward the transition state on the reaction coordinate, as pointed out by Wigner and by Eyring. With the advent of femtosecond spectroscopy, real-time interrogation of the transition state of chemical reactions became amenable to experimental [3][4][5] and theoretical [6][7][8] study. The underlying idea involved the preparation of a transition state of a chemical reaction by a vertical optical excitation or ionization process, which resulted in a nonequilibrium nuclear configuration of a stable collision complex, and probing its temporal evolution by laser-induced fluorescence, photoelectron spectroscopy, or resonance-enhanced multiphoton ionization spectroscopy.…”

“…The underlying idea involved the preparation of a transition state of a chemical reaction by a vertical optical excitation or ionization process, which resulted in a nonequilibrium nuclear configuration of a stable collision complex, and probing its temporal evolution by laser-induced fluorescence, photoelectron spectroscopy, or resonance-enhanced multiphoton ionization spectroscopy. [3][4][5][6][7][8] In this respect, some of the present authors advanced a vertical one-photon photodetachment technique to prepare a transition state of a neutral mass-selected cluster and its subsequent interrogation by (one-photon or twophoton) ionization. [4,5] Negative ion-to-neutral-to-positive ion (NeNePo) pump-probe femtosecond spectroscopy was first applied to explore the nuclear dynamics of the transition state of the Ag 3 cluster in the nonequilibrium linear structure ( 2 S þ g state), and was later also extended to larger metal clusters [9] and molecules.…”

“…[10] In the case of Ag 3 , the relaxation dynamics include sequential processes of configurational relaxation to the triangular structure ( 2 B 2 state), intracluster collisions, and IVR. [5,7,8,[11][12][13] The process of IVR in an isolated molecule or cluster involves vibrational energy flow from an initially excited vibrational mode, or a wave packet of such vibrational modes, to other types of nuclear motion. [14] IVR is the manifestation of quantum coherence imposed by the preparation process on the initial state.…”

“…[17] As a three-atom molecule or cluster is the simplest system in which IVR can occur, particular attention has been devoted to these systems. [7,8,13] Herein, we report joint experimental and theoretical results on the bimetallic noble metal trimer Ag 2 Au, which allow determination of the influence of a heavy atom on the timescales of processes and of lowered symmetry on the nuclear dynamics. From the experimental side it will be shown that, by NeNePo spectroscopy under conditions close to zero electron kinetic energy (ZEKE), the timescales of different processes such as geometric relaxation versus IVR can be revealed.…”