Excitation transfer collisions Vibrational-Translational

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... 

Comparison of the collision numbers given above for vibration-vibration transfer with those for vibration-translation, given in Section 4, shows that in many cases vibration-vibration transfer between two resonant or near-resonant modes is much more efficient than vibration-translation transfer from either. This applies equally to homomolecular and heteromolecular collisions, and carries the interesting consequence that the quickest route for vibrational excitation of upper levels from the ground level by homomolecular collisions is an initial vibration-translation excitation to the v = 1 level, followed by successive vibration-vibration transfers to higher levels. Because of the selection rule, Av = 1,... 

For calculating the nonequilibrium flow of polyatomic gases, the DSMC method is currently the most widely used technique. In the DSMC approach, one usually employs some empirical models [4], which very much resemble a -BGK-type relaxation model with a constant relaxation time. There are only a few attempts due to Koura [5] and Erofeev [6], for example, which employ the so-called trajectory calculations, where a realistic modeling of inelastic collisions with translational-rotational (T-R) transfer of energy is made. These computations require enormous CPU time. For vibrational excitations, no trajectory calculations with the DSMC method have been attempted to date. The physical reality is such that the vibrational term becomes important at much higher temperature than the rotational term. For the N2 molecule the quanta of vibrational temperature is 3,340 K and the quanta of rotational temperature is 2.89 K the corresponding quanta for the O2 molecule are 2,230 and 2.1 K, respectively. 

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]. 

Quantitative analyses of the energetics of the neutral reaction products were not performed. However, our ion retarding potential analyses indicate some energy transfer from translational to internal modes, mainly vibrational excitation. At collision energies >4.5 eV, these reactions could also produce neutral atomic products. 

Michaels C A, Lin Z, Mullin A S, Tapalian H C and Flynn G W 1997 Translational and rotational excitation of the C02(00°0) vibrationless state in the collisional quenching of highly vibrationally excited perfluorobenzene evidence for impulsive collisions accompanied by large energy transfers J. Chem. Phys. 106 7055-71... 

A molecule vibrationally excited by absorption of a laser photon can convert its excitation energy into translational (F - T transfer), rotational (F-> i ), vibrational (V- F) or even electronic energy (F- ) of the collision partners. 

In addition to the processes just discussed that yield vibrationally and rotationally excited diatomic ions in the ground electronic state, vibrational and rotational excitations also accompany direct electronic excitation (see Section II.B.2.a) of diatomic ions as well as charge-transfer excitation of these species (see Section IV.A.l). Furthermore, direct vibrational excitation of ions and molecules can take place via charge transfer in symmetric ion molecule collisions, as the translational-to-internal-energy conversion is a sensitive function of energy defects and vibrational overlaps of the individual reactant systems.312-314... 

The photofragmentation that occurs as a consequence of absorption of a photon is frequently viewed as a "half-collision" process (16)- The photon absorption prepares the molecule in assorted rovibrational states of an excited electronic pes and is followed by the half-collision event in which translational, vibrational, and rotational energy transfer may occur. It is the prediction of the corresponding product energy distributions and their correlation to features of the excited pes that is a major goal of theoretical efforts. In this section we summarize some of the quantum dynamical approaches that have been developed for polyatomic photodissociation. For ease of presentation we limit consideration to triatomic molecules and, further, follow in part the presentation of Heather and Light (17). 

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