Rotational energy transfer and rotationally specific vibration–vibration intradyad transfer in collisions of C2H2 X̃1Σg+(31/214151, J=10) with C2H2, Ar, He and H2

Abstract

Infrared–ultraviolet double resonance (IRUVDR) experiments have been performed on samples of pure C2H2 and on C2H2 diluted in Ar, He and H2. Pulses of tunable IR radiation from an optical parametric oscillator (OPO) excited molecules of C2H2 to the J = 10 rotational level of the lower component state (II) of the (31/214151)II Fermi dyad in the 1Σg+ electronic ground state of C2H2 and tunable UV radiation was used to record laser-induced spectra at short delays. In this way, state-to-state rate coefficients have been determined for two kinds of processes:§ (a) rotational energy transfer (RET) induced by collisions with C2H2, Ar, He and H2 from the initial level Ji = 10 to other levels (Jf = 2–8, 12–20) within the same component (II) of the (31/214151) Fermi dyad, and (b) intradyad transfer in C2H2–C2H2 collisions to specific levels (Jf = 2–14, 18) in the other component (I) of this Fermi dyad. Transfer from II to I is found to account for ca. 16% of the total relaxation from (II, Ji = 10). The distribution of state-to-state rate coefficients for RET becomes broader as the mass of the collision partner increases, in accord with the predictions of a simple classical model. Absolute values of the state-to-state rate coefficients are determined by scaling the results to the previously determined rate coefficients for rotational relaxation by the same collision partner. It is suggested that intradyad transfer is relatively facile because of the difference in the two diagonal terms in the vibrational matrix element for the transition, with the 〈214151∣V∣214151〉 component being larger than the 〈31∣V∣31〉 component.