Relaxation processes, picosecond laser

An intense femtosecond laser spectroscopy-based research focusing on the fast relaxation processes of excited electrons in nanoparticles has started in the past decade. The electron dynamics and non-linear optical properties of nanoparticles in colloidal solutions [1], thin films [2] and glasses [3] have been studied in the femto- and picosecond time scales. Most work has been done with noble metal nanoparticles Au, Ag and Cu, providing information about the electron-electron and electron-phonon coupling [4] or coherent phenomenon [5], A large surface-to-volume ratio of the particle gives a possibility to investigate the surface/interface processes. 

With the development of shorter and shorter laser pulses, the inter- and intramolecular relaxation processes in the nano- and picosecond windows have been progressively resolved in real time down to the vibrational and reactive dynamics in the femtosecond window [1]. These observations have... 

With the advent of picosecond and subsequently femosecond laser techniques, it became possible to study increasingly fast chemical reactions, as well as related rapid solvent relaxation processes. In 1940, the famous Dutch physicist, Kramers [40], published an article on frictional effects on chemical reaction rates. Although the article was occasionally cited in chemical kinetic texts, it was largely ignored by chemists until about 1980. This neglect was perhaps due mostly to the absence or sparsity of experimental data to test the theory. Even computer simulation experiments for testing the theory were absent for most of the intervening period. 

In this chapter results of the picosecond laser photolysis and transient spectral studies on the photoinduced electron transfer between tryptophan or tyrosine and flavins and the relaxation of the produced ion pair state in some flavoproteins are discussed. Moreover, the dynamics of quenching of tryptophan fluorescence in proteins is discussed on the basis of the equations derived by the present authors talcing into account the internal rotation of excited tryptophan which is undergoing the charge transfer interaction with a nearby quencher or energy transfer to an acceptor in proteins. The results of such studies could also help to understand primary processes of the biological photosynthetic reactions and photoreceptors, since both the photoinduced electron transfer and energy transfer phenomena between chromophores of proteins play essential roles in these systems. 

Picosecond laser spectroscopy offers direct access to molecular vibrational (T,) and phase (Tj) relaxation as well as orientational dynamics of molecules, t,< . In contrast to experiments on vibrational relaxation in liquids, all those on the reorientational process have been confined to large polyatomic molecules, particularly dye molecules in probe solvents for pragmatic reasons. Since the slip boundary conditions are also a sensitive function of the shape of the molecules and solute-solvent interactions, there is some uncertainty in deciding whether or not it is the local interaction in terms of the solvation volume, or the boundary condition, or both, that varies for a given molecule in a range of liquids such as the... 

We can indeed claim that this is an example of photoselective laser chemistry. The competition between relaxation and reaction of photoex-cited electrons in clusters represented in Fig. 14(b) is reminiscent of the competition in many laser-induced chemical processes, stimulated by the selective absorption of one or more photons, such as photodissociation, photoionization, isomerization, and so forth in polyatomic molecules, where the coupling of many vibrational modes provides energy randomization and relaxation on picosecond time scales. 

Other work using picosecond laser spectroscopy has shown that these reactions proceed via a solvent intermediate, M(CO)5(solvent), which forms in a few picoseconds after the laser pulse and then decays to products. Lee and Harris have observed formation of the solvated species Cr(CO)5(C5H,2) with t = 17 ps and the decay of the vibrationally excited Cr(CO)j with t 21 ps (apparently at ambient temperature). These observations are at variance with those of Spears and co-woikers, who claim that the bare Cr(CO)j persists on the 100-ps time scale at 22°C. Hopkins and co-workers have used resonance Raman detection to show that the 100-ps process is due to thermal relaxation of the excited vibrational state, probably of Cr(CO)5(CgH,2). 

Besides excitation and probing with infrared laser pulses the CARS technique (Sect.8.4) is a promising technique to study these relaxation processes. An example is the measurement of the dephasing process of the OD stretching vibration in heavy water D2O by CARS [11.111]. The pump at w = Wl is provided by an amplified 80-fs dye-laser pulse form a CPM ring dye laser. The Stokes pulse at Wg is generated by a synchronized tunable picosecond dye laser. The CARS signal at = 2wL-Wg is detected as a function of the time delay between the pump and probe pulses. 

To estimate the flow relaxation time scale, we simply set Fstress =0 in the preceding equation. This is valid because the picosecond laser pulses, as well as the time scales associated with all the physical processes such as thermal expansion and density change that initiate the flow process, are all much shorter than the flow response time. 

Eck V./Josef Holzwarth, A. Genz Iodine Laser Temperature J.F. Holzwarth, V. Eck, A. Genz, Iodine Laser Temperature Jump from Picoseconds to Seconds Relaxation Processes of Phospholipid Bilayers. In Bayley, P.M./R. Dale (Eds.), Spectroscopy and Dynamics of Biological Systems, London 1984, p. 351-377. 

Because the high quantum yield originates from the high-rate isomerization, which competes with other relaxation processes in the excited state of rhodopsin, ultrafast laser spectroscopies were applied to investigate the isomerization process of the retinal chromophore. Picosecond time-resolved spectroscopy was appHed to the photochemistry of rhodopsin, and the formation of the primary intermediates was reported, such as photorhodopsin and bathorhodopsin at room temperature. - - However, the time resolution needed to be improved in order to detect the cis-tram isomerization process in the excited state of rhodopsin. The direct observation of the rhodopsin excited state was reported in 1991, in which the primary intermediate photorhodopsin formed from the excited state of rhodopsin within 200 fs. Later, the effects of oscillatory features with a period of 550 fs (60 cm ) on the formation kinetics of photorhodopsin, were observed, suggesting that the primary step in vision is a vibrationally coherent process. 

This type of laser produces output pulses which are typically between 1 and 10 ns duration and are well suited to provide initial excitation in the study of the decay of excited states and other transient effects in small molecules. Many fundamental processes, however, occur on a time scale much shorter than the 1—10 ns resolution available with dye lasers of the type discussed above. These processes, such as the relaxation of large biological molecules and dyes in solution, exciton decay and migration in solids, charge-transfer and other non-radiative transfer processes between molecules, and many more, take place on a picosecond time scale. 

A simplified view of the early processes in electron solvation is given in Figure 7. Initially, electron pulse radiolysis was the main tool for the experimental study of the formation and dynamics of electrons in liquids (Chapter 2), first in the nanosecond time range in viscous alcohols [23], later in the picosecond time range [24,25]. Subsequently, laser techniques have achieved better time resolution than pulse radiolysis and femtosecond pump-probe laser experiments have led to observations of the electron solvation on the sub-picosecond to picosecond time scales. The pioneering studies of Migus et al. [26] in water showed that the solvation process is complete in a few hundreds of femtoseconds and hinted at the existence of short-lived precursors of the solvated electron, absorbing in the infrared spectral domain (Fig. 8). The electron solvation process could thus be depicted by sequential stepwise relaxation cascades, each of the successive considered species or... 

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