Multiphonon vibrational relaxation results

This relative ease of electronic relaxation as compared with multiphonon vibrational relaxation was qualitatively interpreted using phonon Franck-Condon arguments. Change in the vibrational state of the guest molecule requires in general very little change in the equilibrium positions of the lattice atoms, and this results in poor Franck-Condon factors for multiphonon vibrational relaxation. Electronic transitions, on the other hand, are often accompanied by considerable changes in electron density distri-... 

Picosecond laser pulses in the UV range do not result in better ablation behavior than nanosecond laser pulses. This is different for doped polymers. Experiments with doped PMMA (an IR-absorber, i.e., IR-165 for ablation with near-IR laser and diazomeldrum s acid (DMA) for ablation with UV lasers) with nanosecond and picosecond laser irradiation in the UV (266 nm) and near-IR (1064 nm) range have shown that, in the IR, neat features could be produced with picosecond laser irradiation, while nanosecond irradiation only results in rough surface features [105]. This corresponds well with the different behavior of the two absorbers. With IR-165 the polymer is matrix is heated by a fast vibrational relaxation and multiphonon up-pumping [106]. This leads to a higher temperature jump for the picosecond irradiation, which causes ablation, while for nanosecond pulses only lower temperatures are reached. 

The strong dependence of x upon the host lattice and the strong isotopic effect, which are contrary to the predictions of the so-called energy-gap law for multiphonon relaxation processes, indicate a relaxation mechanism with transfer of the vibrational energy into rotational energy of the impurity molecule rather than transfer into phonons of the host lattice. Theoretical models for vibrational relaxation of diatomic molecules in solids, which describe the experimental results for NH and ND, have been presented [4 to 9]. 

This results in the well-known exponential energy gap law that is common to several different types of multiphonon radiationless and vibrational relaxation processes. 

In this paper, the multiphonon relaxation of a local vibrational mode and the non-radiative electronic transitions in molecular systems and in solids are considered using this non-perturbative theory. Results of model calculations are presented. According to the obtained results, the rate of these processes exhibits a critical behavior it sharply increases near specific (critical) value(s) of the interaction. Also the usual increase of the non-radiative transition rate with temperature is reversed at certain value of the non-diagonal interaction and temperature. For a weak interaction, the results coincide with those of the perturbation theory. 

Fig. 13.2 The relaxation of different vibrational levels of the ground electronic state of 2 in a sohd Ar matrix. Analysis of these results indicates that the relaxation of the v < 9 levels is dominated by radiative decay and possible transfer to impurities. The relaxation ofthe upper levels probably takes place by the multiphonon mechanism discussed here. (From A. Salloum and H. Dubust, Chem. Phys. 189, 179 (1994).)...

Most of the earlier theoretical studies dealt with the simplest relaxation mechanism where the internal vibrational energy of the guest is dissipated directly into the delocalized and harmonic lattice phonons. The common results of these works " were, as we mentioned above, predictions of a strong temperature dependence for the relaxation and an exponential decrease in the rates with the size of the vibrational frequency. The former result has its origin in stimulated phonon emission the conversion of vibrational energy into lattice phonons is greatly facilitated if some excited phonon states are thermally populated. The energy-gap law is due to the fact that the order of the multiphonon relaxation increases with the size of... 

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