Pump-probe spectroscopy dynamics

Schwartz, B. J. and Rossky, P. J. Pump-probe spectroscopy of the hydrated electron a quantum molecular dynamics simulation, J. Chem.Phys., 101 (1994), 6917-6926... 

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. 

An additional piece of information can be obtained by studying a synthetic compound derived from the GFP chromophore (1-28) fluorescing at room temperature. In Fig. 3a we show the chemical structure of the compound that we studied in dioxan solution by pump-probe spectroscopy. If we look at the differential transmission spectra displayed in Fig. 3b, we observed two important features a stimulated emission centered at 508 nm and a huge and broad induced absorption band (580-700 nm). Both contributions appear within our temporal resolution and display a linear behavior as a function of the pump intensity in the low fluences limit (<1 mJ/cm2). We note that the stimulated emission red shifts with two characteristic time-scales (500 fs and 10 ps) as expected in the case of solvation dynamics. We conclude that in the absence of ESPT this chromophore has the same qualitative dynamical behavior that we attribute to the relaxed anionic form. 

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. 

Y. Tanimura and S. Mukamel. Multistate quantum Fokker-Planck approach to nonadiabatic wave packet dynamics in pump-probe spectroscopy. J. Chem. Phys., 101 3049, 1994. 

The vibrational spectra of strongly associated liquids display broad bands that are difficult to analyze. The structural and dynamical information contained in these spectral features provided a field of speculation over the last decades. Obviously additional experimental evidence is urgently needed for a reliable interpretation. In this chapter, nonlinear pump-probe spectroscopy with intense ultrashort tunable pulses is considered... 

Abstract A challenging task in surface science is to unravel the dynamics of molecules on surfaces associated with, for example, surface molecular motion and (bimolecular) reactions. As these processes typically take place on femtosecond time scales, ultrafast lasers must be used in these studies. We demonstrate two complementary approaches to study these ultrafast molecular dynamics at metal surfaces. In the first, the molecules are studied after desorbing from the surface initiated by a laser pulse using the so called time-of-flight technique. In the second approach, molecules are studied in real time during their diffusion over the surface by using surface-specific pump-probe spectroscopy. 

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. 

Figure 4. Scheme of multistate fs dynamics for NeNePo pump-probe spectroscopy of Agj/ Ag4/Ag4 with structures and energy intervals for the pump and probe steps (A). Simulated NeNePo-ZEKE signals for the 50 K initial condition ensemble (B) at the probe energy of 6.41 eV and a pulse duration of 50 fs (C). Normal modes responsible for relaxation leading to oscillatory behavior of the signal are also shown [49]. 

Figure 8. Absorption spectra for two isomers I and II of Na3p2 obtained from one electron frozen ionic bonds approximation [46] (upper part). Scheme of the multistate fs dynamics for NeExPo pump-probe spectroscopy of NasF2 including conical intersection with structures and energy intervals for the pump and probe steps [46]. See color insert.

Theoretical papers on effects directly observable in the very short time regime are notable in this years collection. The theory of femtosecond pump-probe spectroscopy of ultrafast Internal conversion processes in polyatomic molecules has been developed using the behaviour of the excited pyrazine molecule as an example . The solvation dynamics for an ion pair in a polar solvent can now be examined by the time dependence of fluorescence and by direct observation of photoinduced charge... 

All these studies with femtosecond pulses on the primary photochemical processes of rhodopsin were done by means of transient absorption (pump probe) spectroscopy [10]. However, absorption spectroscopy may not be the best way to probe the excited-state dynamics of rhodopsin, because other spectral features, such as ground-state depletion and product absorption, are possibly superimposed on the excited-state spectral features (absorption and stimulated emission) in the obtained data. Each spectral feature may even vary in the femtosecond time domain, which provides further difficulty in analyzing the data. In contrast, fluorescence spectroscopy focuses only on the excited-state processes, so that the excited-state dynamics can be observed more directly. 

Abstract. We present a novel instrument combining femtosecond pump-probe spectroscopy with broadband detection and confocal microscopy. The system has 200-fs temporal resolution and 300-nm spatial resolution. We apply the instrument to map excited state dynamics in thin films of polyfluorene-polymethylethacrylate blends. 

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,... 

---