Relaxation processes, ultrafast

Rini M, Dreyer J, Nibbering ETJ et al (2003) Ultrafast vibrational relaxation processes induced by intramolecular excited state hydrogen transfer. Chem Phys Lett 374 13-19... 

Ultrafast charge separation and radiationless relaxation processes from S2 excited electronic states of directly linked Zinc-porphyrin-acceptor dyads... 

For Ag, the decay time values were found similar to those reported in ref. [1, 2] providing information about the electron-phonon scattering. For Fe203, several other phenomena could cause the OD changes at the ultrafast time scale. The sub-picosecond and picosecond decay times allow to take into account hot electron thermalization [4] and subsequent fast relaxation processes such as exciton formation or surface traps filling [6]. 

Time resolved coherent anti-Stokes Raman spectroscopy of condensed matter has been recently extended to the femtosecond domain allowing direct and detailed studies of the fast relaxation processes of molecular vibrations in liquids. The vibrational phase relaxation (dephasing) is a fundamental physical process of molecular dynamics and has attracted considerable attention. Both experimental and theoretical studies have been performed to understand microscopic processes of vibrational dephasing. Developments in ultrafast coherent spectroscopy enables one now to obtain direct time-domain information on molecular vibrational dynamics. Femtosecond time-resolved coherent anti-Stokes Raman scattering measuring systems have been constructed (see Sec. 3.6.2.2.3) with an overall time resolution of less than 100 fs (10 s). Pioneering work has been per-... 

Cyclobutanones in acetic acid undergo a regiospecific photoconversion into 2-acetoxy-5-alkoxytetrahydrofurans with retention of configuration at the migrating a-position, and an investigation of ultrafast relaxation processes in A, A -dimethylaminobenzylidene indan-l,3-dione as a molecular film has shown that formation and vibronic relaxation of the exciton states occurs in less than 100 fs. Equilibration of the two trapped exciton states seems to occur within 20 ps. 

Following photo excitation a solution sample returns to thermal equilibrium by a variety of photochemical and photophysical processes. The faster processes, e.g. vibrational relaxation and solvent relaxation, have only recently begun to be studied by direct kinetic methods (1-5). Picosecond emission spectroscopy has been especially useful in elucidating these ultrafast processes (1,/3, 5). As electronically excited molecules relax, their fluorescence spectrum shows time dependence that reflects the relaxation processes. 

Ultrafast charge separation and radiationless relaxation processes from S2 excited electronic states of directly linked Zinc-porphyrin-acceptor dyads S. Taniguchi, N. Malaga, H. Chosrowjan, N. Yoshida and A. Osuka... 

Abstract The density matrix method is a powerful theoretical technique to describe the ultrafast processes and to analyze the femtosecond time-resolved spectra in the pump-probe experiment. The dynamics of population and coherence of the system can be described by the evolution of density matrix elements. In this chapter, the applications of density matrix method on internal conversion and vibrational relaxation processes will be presented. As an example, the ultfafast internal conversion process of Jt jt nn transition of pyrazine will be presented,... 

Fig. 6.97 Pump-and-probe technique for the measurements of ultrafast relaxation processes...

Figure 22.29 schematically shows the model originally proposed [182] for the ultrafast energy relaxation processes in PDPA films. It contains two relaxation channels [183,184] ionic, by IS and covalent, by 2Ag, which is populated following an ultrafast phonon-assisted internal conversion from the photogenerated excitons. PAg at short time is thus due to transitions from 2Ag (dark) excitons. As in long-chain polyenes [185] and f-(CH)jj [186] these excitons are subject to ultrafast recombination dynamics and this explains the ultrafast decay dynamics seen in Figure 22.28a. In degenerate ground state polymers 2Ag is unstable with respect to the formation of soliton excitations and therefore undergoes fission into two neutral SS pairs, 2Ag => 2(S° -I- S°) [18,184], followed by further separation...

In an alternative approach, exact (numerical) time-dependent quantum wave-packet methods have been employed since the early eighties of the last century to explore the d3mamics of ob-initio-haseA models of conical intersections, see Refs. 6-8 for reviews. It has been found by these calculations that the fundamental dissipative processes of population and phase relaxation at femtosecond time scales are clearly expressed already in fewmode systems, if a directly accessible conical intersection of the PE surfaces is involved. The results strongly support the idea that conical intersections provide the microscopic mechanism for ultrafast relaxation processes in polyatomic molecules. " More recently, these calculations have been extended to describe photodissociation and photoisomerization processes associated with conical intersections. The latter are particularly relevant for our understanding of the elementary mechanisms of photochemistry. 

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