Vibronic relaxation

Other early work, which continues to this day, involved vibronic relaxation [6] of large colored molecules such as chrysene [19], pyrene [20] and perylene [21], due to the relative ease of using visible or near-UV light to pump and probe these systems (see example C3.5.6.5 below). 

S j where denotes an electronic singlet state) of a coloured molecule [6]. Vibronic relaxation may be... 

In rare gas crystals [77] and liquids [78], diatomic molecule vibrational and vibronic relaxation have been studied. In crystals, VER occurs by multiphonon emission. Everything else held constant, the VER rate should decrease exponentially with the number of emitted phonons (exponential gap law) [79, 80] The number of emitted phonons scales as, and should be close to, the ratio O/mQ, where is the Debye frequency. A possible complication is the perturbation of the local phonon density of states by the diatomic molecule guest [77]. 

Vibronic relaxation of XeF in solid Ar at 25 K was studied by pumping vibronic transitions with a subpicosecond UV pulse, and detecting frequency-resolved emission with a fast optical gate [25]. XeF has two sites in Ar, one... 

Kr. In the B-emitting states, a slower stepwise relaxation was observed. Figure C3.5.5 shows the possible modes of relaxation for B-emitting XeF and some experimentally detennined time constants. Although a diatomic in an atomic lattice seems to be a simple system, these vibronic relaxation experiments are rather complicated to interiDret, because of multiple electronic states which are involved due to energy transfer between B and C sites. 

Figure C3.5.5. Vibronic relaxation time constants for B- and C-state emitting sites of XeF in solid Ar for different vibrational quantum numbers v, from [25]. Vibronic energy relaxation is complicated by electronic crossings caused by energy transfer between sites.

Figure C3.5.10. Frequency-dependent vibronic relaxation data for pentacene (PTC) in naphthalene (N) crystals at 1.5 K. (a) Vibrational echoes are used to measure VER lifetimes (from [99]). The lifetimes are shorter in regime I, longer in regime II, and become shorter again in regime III. (b) Two-colour pump-probe experiments are used to measure vibrational cooling (return to the ground state) from [1021.

The first type of interaction, associated with the overlap of wavefunctions localized at different centers in the initial and final states, determines the electron-transfer rate constant. The other two are crucial for vibronic relaxation of excited electronic states. The rate constant in the first order of the perturbation theory in the unaccounted interaction is described by the statistically averaged Fermi golden-rule formula... 

The reactions of electron transfer and vibronic relaxation are ubiquitous in chemistry and many review papers have dealt with them in detail (see, e.g., Ovchinnikov and Ovchinnikova [1982], Ulstrup [1979]), so we discuss them to the extent that the nuclear tunneling is involved. 

Sarkar, N., Takeuchi, S., and Tahara, T. 1999. Vibronic relaxation of polyatomic molecule in nonpolar solvent. J. Phys. Chem. A 103 4808. 

Processes associated with electron transfer and/or vibronic relaxation are ubiquitous in chemistry, and many review papers have discussed them... 

The effects of the vibronic coupling competing with vibrational and/or vibronic relaxation on the femtosecond pump-probe stimulated emission spectra of molecules in condensed phases have been investigated. Taking into account vibronic and vibrational relaxation and vibronic coupling in molecular terms, the coupled master equations have been briefly derived for... 

To first order, we consider the molecular structure of the surface layers to be identical to that of the bulk layers. Consequently, all the characteristics corresponding to short-range intralayer interactions (e.g. Davydov splitting, vibrational frequencies, excitonic band structure, vibronic relaxations are similar for bulk and surface layers). In fact, we shall see that even slight changes may be detected. They will be analyzed in Section III.C, devoted to surface reconstruction. Therefore, our crystal model consists of (a,b) monolayers translated in energy relative to the bulk excitation by 206, 10, and 2cm-1 for the first three layers, as indicated in Fig. 3.5. No other changes are considered in this first-order crystal model. 

The high-energy excitation, hw, > h(o2 + hS20, is also due to the vibronic components at 390 and 1400 cm- . If we privilege the vibronic relaxation by fission and the creation of one vibration hQ0, then the relaxation of the incident photon h(ot leads to a state about 100 cm-1 above the observed emission. The excitation spectrum due to the vibronic component at 1400 cm-1 (see Fig. 3.18) shows that the relaxation by creation of other vibrations contributes also, since no threshold structure is observed around the value hco2 + 1400 cm-1. This conclusion is also consistent with the vibronic analysis of the bulk (Section II.B.3). 

---