Excitation transfer collisions Vibrational-Rotational

Unlike the case of collision-induced vibrational energy transfer, collision induced rotational energy transfer seems to be free of strong restrictions on the changes in the rotational quantum numbers. When account is taken of the spectral widths of the excitation sources used, the nature of the rotation-vibration structure in the fluorescence and absorption spectra, and the possibility of resonant ener f transfer in the collision, it is concluded that the studies of Bj aniline are the weakest, those of B2 benzene better, and those of glyoxal the best available. With this hierarchy of quality of information kept in mind, the following weaker conclusions can also be obtained from the studies cited. 

A-B relative or external motion undergo free-free transitions (E., E. + dE.) (Ej Ej+ dE within the translational continuum, while the structured particles undergo bound-bound (excitation, de-excitation, excitation transfer) or bound-free (ionization, dissociation) transitions = (a, 3) ->/= (a, (3 ) in their internal electronic, vibrational or rotational structure. The transition frequency (s ) for this collision is... 

Since each sp)ecies at high temperatures has an mtemal energy component (vibration, rotation, electronic excitation) energy is transfere in the course of an inelastic collision when the particle becomes unexcited. This effect is taken into account by the internal thermal conductivity which is determined from the theory of... 

For the spectroscopy of vibrational-rotational transitions in molecules the laser-excited fluorescence is generally not the most sensitive tool, as was discussed at the end of Sect. 1.3.1. Optoacoustic spectroscopy, on the other hand, is based on colli-sional energy transfer and is therefore not applicable to molecular beams, where collisions are rare or even completely absent. For the infrared spectroscopy of molecules in a molecular beam therefore a new detection technique has been developed, which relies on the collision-free conditions in a beam and on the long radiative lifetimes of vibrational-rotational levels in the electronic ground state [84-86]. 

Both linear and nonlinear Raman spectroscopy can be combined with time-resolved detection techniques when pumping with short laser pulses [8.781. Since Raman spectroscopy allows the determination of molecular parameters from measurements of frequencies and populations of vibrational and rotational energy levels, time-resolved techniques give information on energy transfer between vibrational levels or on structural changes of short-lived intermediate species in chemical reactions. One example is the vibrational excitation of molecules in liquids and the collisional energy transfer from the excited vibrational modes into other levels or into translational energy of the collision partners. These processes proceed on picosecond to femtosecond time scales [8.77,8.79]. 

For hyperthermal collisions for which the kinetic energy is in the range of a few eV, most of the transferred energy goes into electronic and vibrational excitation and only a small fraction goes into rotational excitation. Hence the collisions are rotationally quasielastic so that F(0) and k are only very weakly dependent upon the rotational energy. Letting... 

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