Relaxation within and from the (31/214151) and (3141/214251) Fermi dyads in acetylene: Vibrational energy transfer in collisions with C2H2, N2 and H2

Abstract

Infrared–ultraviolet double resonance (IRUVDR) experiments have been performed on samples of pure C₂H₂ and on C₂H₂ diluted in N₂ and H₂. Pulses of tunable IR radiation from an optical parametric oscillator (OPO) excited molecules of C₂H₂ to one component state of one of two Fermi dyads in the ¹Σ_g⁺ electronic ground state, i.e. (3₁/2₁4₁5₁)_II or (3₁4₁/2₁4₂5₁)_II, and tunable UV laser radiation was used to observe the evolution of population either in that state or in the other component state of the same Fermi dyad. In this paper,‡ rate coefficients are reported for two kinds of processes: (a) vibration-to-vibration (V–V) transfer between the two component states of the same Fermi dyad induced by collisions with C₂H₂, N₂ and H₂, and (b) vibrational relaxation from the coupled pair of Fermi dyad states in collisions with the same gases. In addition, populations have been observed in the (4₂5₁), (4₁5₁) and (4₂) states of C₂H₂ during relaxation from the (3₁4₁/2₁4₂5₁) dyad and rate coefficients for self-relaxation from the first two of these states have been derived. The results for both V–V intradyad transfer and for vibrational relaxation are discussed in terms of the vibrational matrix elements for such collision-induced transitions and differences in the mixing of the zero order states in the Fermi dyads.