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 1Σg+ 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.