Fast Non-Adiabatic Mechanisms of VT Relaxation

Vibrational relaxation is slow in adiabatic collisions when there is no chemical interaction between colliding partners. For example, the probability of deactivation of a vibrationally excited N2 molecule in collision with another N2 molecule can be as low as 10 at room temperature. The vibrational relaxation process can be much faster in non-adiabatic collisions, when the colliding partners interact chemically. 

VT relaxation in molecular collisions with atoms and radicals can be illustrated by relaxation of vibrationally excited N2 on atomic oxygen (Andreev Nikitin, 1976 see Fig. 2-35). The interval between degenerated electronic terms grows when an atom approaches the molecule. When the energy interval becomes equal to avibrational quantum, non-adiabatic relaxation (the so-called vibronic transition, Fig. 2-35) can take place. The temperature dependence of this relaxation is not significant, and typical rate coefficients are high lO -lO cm /s. Sometimes, as in the case of relaxation on alkaline atoms, the non-adiabatic VT-relaxation rate coefficients reach those of gas-kinetic collisions, that is, about 10 ° cm /s. 

Specific fast mechanisms of VT relaxation take place in collisions of polyatomic molecules and are discussed by Keimedy Fridman (2004), and in more detail by Rusanov, Fridman, and Sholin (1986). 

The kinetics of plasma-chemical reactions of vibrationally excited molecnles is determined not only by their concentration but mostly by the fraction of highly excited molecnles able to dissociate or participate in endothermic chemical reactions. The formation of highly vibrationally excited molecules at elevated pressures is due not to direct electron impact but to collisional energy exchange called W relaxation. Most conventional resonant W processes usually imply vibrational energy exchange between molecules of the same kind, for example, N2(w = 1) + N2(w = 0) N2(w = 0) - - = 1), and are char- 

The probability also depends on matrix elements of transitions m n and s 1  

---