Wavepacket pump-probe spectroscopy

The double proton transfer of [2,2 -Bipyridyl]-3,3 -diol is investigated by UV-visible pump-probe spectroscopy with 30 fs time resolution. We find characteristic wavepacket motions for both the concerted double proton transfer and the sequential proton transfer that occur in parallel. The coherent excitation of an optically inactive, antisymmetric bending vibration is observed demonstrating that the reactive process itself and not only the optical excitation drives the vibrational motions. We show by the absence of a deuterium isotope effect that the ESIPT dynamics is entirely determined by the skeletal modes and that it should not be described by tunneling of the proton. 

Unlike the case of simple diatomic molecules, the reaction coordinate in polyatomic molecules does not simply correspond to the change of a particular chemical bond. Therefore, it is not yet clear for polyatomic molecules how the observed wavepacket motion is related to the reaction coordinate. Study of such a coherent vibration in ultrafast reacting system is expected to give us a clue to reveal its significance in chemical reactions. In this study, we employed two-color pump-probe spectroscopy with ultrashort pulses in the 10-fs regime, and investigated the coherent nuclear motion of solution-phase molecules that undergo photodissociation and intramolecular proton transfer in the excited state. 

It is very likely that the metal-insulator transition, the unusual catalytic properties, the unusual degree of chemical reactivity, and perhaps even some of the ultramagnetic properties of metal clusters are all linked intimately with the dynamic, vibronic processes inherent in these systems. Consequently, the combination of pump-probe spectroscopy on the femtosecond time scale with theoretical calculations of wavepacket propagation on just this scale offers a tantalizing way to address this class of problems [5]. Here we describe the application of these methods to several kinds of metal clusters with applications to some specific, typical systems first, to the simplest examples of unperturbed dimers then, to trimers, in which internal vibrational redistribution (IVR) starts to play a central role and finally, to larger clusters, where dissociative processes become dominant. 

Since the development of ultrashort lasers, nudear wavepacket dynamics of various matters have attracted continuing attention [1,2]. The research targets extend from gas phase molecules [3, 4] to molecules in solution [5, 6], and solids [7]. In general, an excitation of matter by an ultrashort pulse with sufficient bandwidth leads to the creation of coherence between vibrational (or vibronic) eigenstates [1]. The induced nuclear wavepacket then starts to evolve on a certain potential energy surface and the dynamics is probed by a suitable pump-probe spectroscopy. The direct time-domain observation of the nudear motion provides us with valuable information on photochemical reaction dynamics, vibrational excitation/relaxation mechanisms, electron-vibration (phonon) coupling, and so on. 

Pump-Probe Spectroscopy, Photon Echoes and Vibrational Wavepackets ... 

Takeuchi T, Tahara T (2005) Coherent nuclear wavepacket motions in ultrafast excited-state intramolecular proton transfer sub-30-fs resolved pump-probe absorption spectroscopy of 10-hydroxybenzo[h]quinoline in solution. J Phys Chem A 109 10199-10207... 

H. Rabitz The information in the recurrence time alone is minimal. However, the temporal structure of the recurrence signal contains detailed information on the surface explored by the wandering scout wavepacket during its excursion. Further experiments may be necessary to follow (i.e., track) the wavepacket through its excursion over the potential surface. Such pump-probe experiments go beyond conventional spectroscopy. 

The earliest pulsed laser quantum beat experiments were performed with nanosecond pulses (Haroche, et al., 1973 Wallenstein, et al., 1974 see review by Hack and Huber, 1991). Since the coherence width of a temporally smooth Gaussian 5 ns pulse is only 0.003 cm-1, (121/s <-> 121 cm"1 for a Gaussian pulse) nanosecond quantum beat experiments could only be used to measure very small level splittings [e.g. Stark (Vaccaro, et al., 1989) and Zeeman effects (Dupre, et al., 1991), hyperfine, and extremely weak perturbations between accidentally near degenerate levels (Abramson, et al., 1982 Wallenstein, et al., 1974)]. The advent of sub-picosecond lasers has expanded profoundly the scope of quantum beat spectroscopy. In fact, most pump/probe wavepacket dynamics experiments are actually quantum beat experiments cloaked in a different, more pictorial, interpretive framework,... 

Chapter 3 treats nuclear motions on the adiabatic potential energy surfaces (PES). One of the most powerful and simplest means to study chemical dynamics is the so-called ab initio molecular dynamics (or the first principle dynamics), in which nuclear motion is described in terms of the Newtonian d3mamics on an ab initio PES. Next, we review some of the representative time-dependent quantum theory for nuclear wavepackets such as the multiconfigurational time-dependent Hartree approach. Then, we show how such nuclear wavepacket d3mamics of femtosecond time scale can be directly observed with pump>-probe photoelectron spectroscopy. 

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