Collision cross section induced transitions

In the electronic ground states of molecules collision-induced transitions represent, for most experimental situations, the dominant mechanism for the redistribution of energy, because the radiative processes are generally too slow. In cases where a nonequilibrium distribution has been produced (for example, by chemical reactions or by optical pumping), these collisions try to restore thermal equilibrium. The relaxation time of the system is determined by the absolute values of collision cross sections. 

The level Ei of the molecule A can be depopulated not only by spontaneous emission but also by collision-induced radiationless transitions (Fig. 2.23). The probability dT f /dt of such a transition depends on the density of the collision partner B, on the mean relative velocity v between A and B, and on the collision cross section for an inelastic collision that induces the transition Ei Ek in the molecule A... 

The Intensity ratio between these satellite lines" and the main fluorescence line yields the absolute cross sections for the different collision-induced rotational transitions, provided the He pressure and the lifetime of the excited Na state are known. Both parameters have been measured, giving cross sections between = 1.5 A for A/ = 1 transitions and 6 = 0.3 A for A/ = 5... 

A S state of BaO by seven visible Ar laser lines, Sakurai etal. studied rotational and vibrational transitions induced by collisions with He, Ar and Nj. From measurements of the pressure-dependent lifetime (r = o = 12 3 /tsec), the authors found for the electronic quenching cross section of the A 2 state by He the value 6 = 0.25. ... 

The self-quenching rate constant of 21 x 10 cm /molecule s that was measured in these studies is unusually large (69). Quenching by various gases and collision induced rotational transitions have also been measured (68,69). The quenching cross sections by He and Ar in a supersonic jet increase at very low temperature (70), indicating the formation of collision complexes. ... 

Potential energy surfaces of weakly bound dimers and trimers are the key quantities needed to compute transition frequencies in the high resolution spectra, (differential and integral) scattering cross sections or rate coefficients describing collisional processes between the molecules, or some thermodynamic properties needed to derive equations of state for condensed phases. However, some other quantities governed by weak intermolecular forces are needed to describe intensities in the spectra or, more generally, infrared and Raman spectra of unbound (collisional complexes) of two molecules, and dielectric and refractive properties of condensed phases. These are the interaction-induced (or collision-induced) dipole moments and polarizabilities. 

Perturbations affect the rate of absorption and emission of radiation in a fully understood and exactly calculable manner. They also affect the rates of chemical and collisional population/depopulation processes, but in a less easily estimated way. Perturbation effects on steady-state populations can be very large and level-specific. Although collision-induced transitions and chemical reactions are not governed by rigorous selection rules as are electric dipole transitions and perturbation interactions, some useful propensity rules have been suggested theoretically and confirmed experimentally. Gelbart and Freed (1973) suggested that the cross sections for collision-induced transitions between two different electronic states, E and E, are... 

Fast electrons with kinetic energy T > 100 e.v. excite and ionize molecules predominantly in optical collisions inducing the same type of transitions of valence electrons as does the absorption of photons. This abundant type of primary activations shows, therefore, a very close connection with the optical spectra of molecules. Thus, foundations are laid for the use of the so-called optical approximation in radiation chemistry, as suggested by Platzman (13). More explicitly, the theory of cross sections for optical collisions leads in first approximation to the following formula for the yield of a particular electronic transition n ... 

Is it possible to correlate the cross-sections for collision-induced s-l transitions with the intramolecular s-l coupling strength Do the strongly mixed states play a specific role of gates between s and / manifolds ... 

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