Dynamics from Pump-Probe Spectroscopy

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. 

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 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.

Figure 10.3 Result of time-resolved pump-probe spectroscopy in combination with TOF-MS. The graphs show femtosecond ionization signals for four DNA and RNA bases, namely cytosine, guanine, thymine, and uracil. The transient of dimethylether (DME) was displayed along with the transient of guanine for comparison [8]. Reprinted with permission from Canuel, C., Mans, M., Piuzzi, F, Tardivel, B., Dimicoli, ., Elhanine, M. (2005) Excited States Dynamics of DNA and RNA Bases Characterization of a Stepwise Deactivation Pathway in the Cas Phase. y. Chem. Phys. 122 074316. Copyright (2005) AlP Publishing EEC...

Two examples which emerged from a fruitful cooperation of theory and experiment are discussed in this book. First, once again the K2 molecule excited to its A state acts as a model system (Sect. 3.1.5). Its special spectroscopic properties combined with the dynamics induced by femtosecond state preparation facilitate the transition from pump probe to control spectroscopy. The intensity of the pump pulse serves as the control parameter, allowing the forcing of the molecule to perform either the A state vibrations or its ground state dynamics. 

Time-resolved spectroscopy has become an important field from x-rays to the far-IR. Both IR and Raman spectroscopies have been adapted to time-resolved studies. There have been a large number of studies using time-resolved Raman [39], time-resolved resonance Raman [7] and higher order two-dimensional Raman spectroscopy (which can provide coupling infonuation analogous to two-dimensional NMR studies) [40]. Time-resolved IR has probed neutrals and ions in solution [41, 42], gas phase kmetics [42] and vibrational dynamics of molecules chemisorbed and physisorbed to surfaces [44]- Since vibrational frequencies are very sensitive to the chemical enviromnent, pump-probe studies with IR probe pulses allow stmctiiral changes to... 

Ethylene glycol is a very viscous liquid and the molecule presents two close OH groups. It has to be noticed that, among all the different solvents studied by pulse radiolysis, the transition energy of the solvated electron absorption band is maximum in liquid ethylene glycol. For these reasons, the electron in EG seems to have a special behaviour and it is of great interest to study the dynamics of the formation of equilibrated solvated electron. Within this context, the present communication deals with the dynamics of solvation in EG of electrons produced by photoionisation of the solvent at 263 nm. The formation of solvated electrons is followed by pump-probe transient absorption spectroscopy in the visible spectral range from 425 to 725 nm and also in near IR. For the first time, the absorption spectrum of the precursor of the equilibrated electron is observed in EG. Our results are shortly compared by those obtained in water and methanol. 

---