Rotational energy transfer and rotationally specific vibration-vibration intradyad transfer in collisions of C₂H₂ X̃ ¹Σ_g⁺(3₁/2₁4 ₁5₁, J = 10) with C₂H₂, Ar, He and H₂

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 X̃ ¹Σ_g⁺ 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 J_i = 10 to other levels (J_f = 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 (J_f = 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, J_i = 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|2₁4₁5 ₁〉 component being larger than the 〈3₁|V|3₁〉 component.