Ultrafast dynamics in atomic clusters: Analysis and control

Abstract

We present a study of dynamics and ultrafast observables in the frame of pump-probe negative-to-neutral-to-positive ion (NeNePo) spectroscopy illustrated by the examples of bimetallic trimers Ag2Au ؊ ͞Ag2Au͞Ag2Au ؉ and silver oxides Ag3O 2 ؊ ͞Ag3O2͞Ag3O 2 ؉ in the context of cluster reactivity. First principle multistate adiabatic dynamics allows us to determine time scales of different ultrafast processes and conditions under which these processes can be experimentally observed. Furthermore, we present a strategy for optimal pump-dump control in complex systems based on the ab initio Wigner distribution approach and apply it to tailor laser fields for selective control of the isomerization process in Na3F2. The shapes of pulses can be assigned to underlying processes, and therefore control can be used as a tool for analysis.ultrafast ab initio dynamics ͉ NeNePo spectroscopy ͉ optimal control I nvestigation of femtosecond dynamical processes in elemental clusters and their control by tailored laser fields are of fundamental importance for learning how the interplay of size, structures, and laser fields can be used to manipulate optical properties and reactivity of these species (1). This research area involving combination of laser-selective femtochemistry (2-5) with the functionality of nanostructures opens new perspectives for basic research and numerous technical applications. In particular, exploration of clusters in the size regime in which each atom counts is attractive, because in this regime structures and the numbers of atoms directly determine size-selective properties (6-12). Another important aspect is that the study of ultrafast dynamics in clusters with finite densities of states allows for separation of time scales of nuclear motion (1). Therefore, the identification of different ultrafast processes such as geometric relaxation, internal vibrational relaxation (IVR), different photoionization pathways, fragmentation, etc. becomes attainable (13-19). Moreover, optimization of the laser fields permits one to manipulate these processes by favoring or suppressing some of the chosen channels. In both contexts, the role of theory is essential from conceptual as well as from predictive point of view.Theory not only determines time scales of different processes and predicts ultrafast observables, but also finds conditions under which they can be experimentally realized (13). Moreover, the analysis of shaped laser pulses and the comparison with experimentally optimized laser fields allows us to identify the underlying processes and therefore to use optimal control (20-23) as the tool for analysis (1,23).In this contribution we address both aspects by showing what we can learn (i) from the pump-probe signals and (ii) from the optimized pulse shapes. We also wish to show that for accurate simulations of pump-probe signals and for developing new control strategies for complex systems, the semiclassical limit of the Liouville formulation of quantum mechanics offers a suitable theoretical approach (1,14,15,23)....

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