VIBRATIONAL-RELAXATION OF CO+(A2PI-I), CS(A1PI), AND C2(A3PI-G) IN HELIUM

Nonequilibrium vibrational distributions of CO+(A2Π,υ′=0-6), CS(A1Π,υ′=0-5), and C2(A3Π,υ′=0-6) were prepared by collisional processes in a 300-K helium flowing-afterglow apparatus. The vibrational band intensities of the electronic emission systems were used to obtain the steady-state vibrational distributions from 0.8 to 15 torr. Extensive vibrational relaxation by collisions with He was observed for CO+(A) and CS(A), but not for C2(A), over this pressure range. Electronic quenching of CS(A) probably is competitive with vibrational relaxation, even in helium. The data were fitted to relaxation models based upon Δυ = 1 collisional transitions by using the steady-state master equation formulation. The Δυ = 1 relaxation cross sections for CO+(A) and CS(A) with He are in the range of 0.01 of the gas kinetic values. The upper limit to the Δυ = -1 relaxation cross section for C2(A) is 5 × 10-3 of the gas kinetic value. Studies of the relaxation of CS(A1Π) in Ar were attempted, but electronic quenching appeared to dominate over vibrational relaxation. These results are compared to vibrational-translational relaxation of other electronically excited states. © 1979 American Chemical Society.