Pump-probe studies

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... 

III. PUMP-PROBE STUDIES OF PROTEIN-LIGAND BINDING... 

During the past few years, our group has performed some experiments that enable to propose new interpretations of previous pump-probe studies on clusters. We want to present here a short summary of these interpretations to enlighten two main ideas ... 

A. E. Kondo, W. J. Meath, S. H. Nilar, and A. J. Thakkar, Chem. Phys., 186, 375 (1994). Pump-Probe Studies of the Effects of Permanent Dipoles in One- and Two-Colour Molecular Excitations. 

Baumert, T., Engel, V., Rottgermann, C., Strunz, W.T., and Gerber, G. (1992). Femtosecond pump-probe study of the spreading and recurrence of a vibrational wave packet in Na2, Chem. Phys. Lett. 191, 639-644. 

Woutersen S, Emmerichs U, Bakker HJ. A femtosecond midinfrared pump-probe study of hydrogen bonding in ethanol. J Chem Phys 1997 107 1483-1490. 

It should be stressed that miiltiphoton pump-probe studies are frequently carried out at high excitation density this may result in a bulk thermal spike that considerably changes both the electron thermahzation and geminate recombination dynamicsIt seems likely that irreproducible reports of imusual spectral features and exotic short-lived intermediates are traced to the effects of such thermal spikes. 

Temperature-independent anharmonic decay rates of high-frequency modes of myoglobin have been observed in time-resolved spectroscopic studies. Pump-probe vibrational spectra of the amide I band, between 1600 and 1700 cm 1, measured at temperatures from 6 K to 310 K reveal decay rates ranging only from 0.5 to 1 ps-1 [25], similar to the values we calculate. Similarly, pump-probe studies on myoglobin-CO reveal that the decay of the CO stretch, about 1950 cm is also essentially independent of temperature over the same temperature range [28]. 

Thus, the first trials provided rather confusing results on the primary processes of rhodopsin photoisomerization. The first group then reported several femtosecond pump probe studies, one after another, which involved the measurement of bovine rhodopsin with a wide spectral window [47] and the measurements of 9-cis rhodopsin [48] and 13-demethyl rhodopsin [49]. In addition, they observed oscillatory features with a period of 550 fs (60 cm-1) on the kinetics at probe wavelengths within the photoproduct absorption band of rhodopsin, whose phase and amplitude demonstrate that they are the result of nonstationary vibrational motion in... 

Such features of a beatnote demonstrate the presence of more than one oscillation frequency. In Fig. 15.6 (a), the Fourier transforms of the oscillatory signals are plotted for the 3 spectral positions. There are 3 prominent frequency components, a strong doublet with maxima at 145 and 170 cm i and a much weaker component around 50 cm h Comparative pump-probe studies of OH/OH dimers reveal a similar doublet at 145 and 170 cm i with slightly changed relative intensities of the two components. The 50 cm i component is practically absent in the OH/OH... 

In these two examples of phenol and naphthol-ammonia clusters, we have evidenced the role played by evaporation phenomena that are expected to be present in a lot of clusters pump-probe studies and ean not only hide but replace the reactive processes. Since absorption... 

Equations (20) and (21) predict that the anisotropy should initially decay rapidly from r(0) S 0.4 with lifetime 2F, then decay more slowly from r = 0.4 with dynamics that resemble Forster kinetics in the limit of weak coupling. No anisotropies r(0) > 0.4 have been reported in numerous pump-probe studies of energy transport in bacterial and green plant antennae, conducted at time resolutions varying from around 10 psec to about 300 fsec, For these systems, (2F) therefore appears to be bounded from above by (conservatively) approximately 1 psec. Anisotropies r(0) 0.7 have been observed in magnesium tetraphenylporphyrin, a system to which the homodimer model appears to be directly applicable. 

We continue in the next two sections with two separate applications of the basic mechanisms discussed here (i) A thorough interpretation of a recent pump-probe study of the dissociative ionization of H2 and (ii) A study of control scenarios for molecular cooling and vibrational population transfer using ZWRs and the related concept of EPs. 

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