Relaxation of Electronically Excited Atoms and Molecules

Relaxation of electronically excited atoms and molecules is due to different mechanisms. Superelastic collisions (energy transfer back to plasma electrons) and radiation are essential mostly in thermal plasma. Relaxation in collision with other heavy particles dominates in non-thermal discharges. Relaxation of electronic excitation into translational degrees 

Electronically excited atoms and molecules transfer energy not only into translational, but also into vibrational and rotational degrees of freedom, which is less adiabatic and faster. Fast relaxation takes place by formation of intermediate ionic complexes. For example, electronic energy transfer from excited metal atoms Me to vibrational excitation of nitrogen takes place for almost every collision  

Initial and final Me-N2 energy terms cross an ionic term (Fig. 2-36), which leads to a fast non-adiabatic relaxation transition (Bjerre Nikitirr, 1967). The maximum cross sections of such relaxation processes are presented in Table 2-25. 

Electronic excitation energy transfer processes can be effective only very close to a resonance (0.1 eV or less), which limits them to some specific collision partners. The He-Ne gas laser provides examples  

The interaction radius for colUsiorts of electronically excited atoms and molectrles is usually high, up to 1 nm. Cross sectiorrs, therefore, can obvionsly be very high if processes are close to resonance. For example, electronic excitation transfer from Hg atoms to sodium has a cross section reaching 10 cm.  

---