Ultrafast dynamics in isolated molecules and molecular clusters

Abstract

During the past decade the understanding of photo-induced ultrafast dynamics in molecular systems has improved at an unforeseen speed and a wealth of detailed insight into the fundamental processes has been obtained. This review summarizes our present knowledge on ultrafast dynamics in isolated molecules and molecular clusters evolving after excitation with femtosecond pulses as studied by pump-probe analysis in real time. Experimental tools and methods as well as theoretical models are described which have been developed to glean information on primary, ultrafast processes in photophysics, photochemistry and photobiology. The relevant processes are explained by way of example-from wave packet dynamics in systems with a few atoms all the way to internal conversion via conical intersections in bio-chromophores. A systematic overview on characteristic systems follows, starting with diatomic and including larger organic molecules as well as various types of molecular clusters, such as micro-solvated chromophore molecules. For conciseness the focus is on molecular systems which remain unperturbed by the laser pulses-apart from the excitation and detection processes as such. Thus, only some aspects of controlling and manipulating molecular reactions by shaped and/or very intense laser pulses are discussed briefly for particularly instructive examples, illustrating the perspectives of this prospering field. The material presented in this review comprises some prototypical examples from earlier pioneering work but emphasizes studies from recent years and covers the most important and latest developments until January 2006.

Monitoring the effect of a control pulse on a conical intersection by time-resolved photoelectron spectroscopy

“…Energy-resolved photoelectron angular distributions [eqn (17)] were computed for probing of the wavepackets in Fig. 4: (a) without the control pulse, P ðaÞ k j ðỹ k Þ, and (b) with the control pulse, P ðbÞ k j ðỹ k Þ.…”

“…The delay time between the pump and probe increases from left to right. For each frame, the polarization of the pump and probe lies along the vertical axis and the distributions are cylindrically symmetric around this axis because of the averaging of eqn (17). The distributions are also symmetric with respect to the horizontal axis because the probe pulse does not differentiate between molecules with r 1 4 r 2 and r 1 o r 2 .…”

“…Femtosecond time-resolved photoelectron spectroscopy is a versatile probe of ultrafast dynamics in molecules [17][18][19][20] and is particularly well suited for studies of wavepacket dynamics in nonadiabatic systems where nuclear and electronic modes are coupled. In fact, internal conversion in polyatomic molecules was among the earliest suggested application of this technique, 21 which was subsequently realized experimentally.…”

Electronic structure in real time: mapping valence electron rearrangements during chemical reactions

“…Since these original studies, the field of femtochemistry has rapidly evolved and it is not the aim of this perspective to review this development. Rather, the reader is referred to a number of excellent articles, reviews and books that have appeared over the years (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15).…”

“…This corresponds to preparing a nuclear wavepacket (magenta) by a coherent superposition of states on the dissociative potential energy curve (17). The evolution of the nuclear wavepacket can be tracked by probing at different nuclear distances or times Δt [pump-probe delay, see Figure 1 (a)] after excitation (1)(2)(3)(4)(5). In a simplified way, the molecular orbital (MO) energy level diagram often used to introduce molecular orbitals in text books in Figure 1 Note that any probe that is sensitive to the wavepacket evolution on the dissociative potential energy curve [ Figure 1 (b)] is suited to give a complete picture of the dissociation process.…”