Rotational state-to-state rate constants and pressure broadening coefficients for He-C2H2 collisions:: Theory and experiment

Converged close-coupling and coupled-states calculations were used to obtain state-to-state rate constants and pressure broadening coefficients for the collisional rotational (de-)excitation of C2H2 by He. The ab initio potential used in these calculations was previously computed by symmetry-adapted perturbation theory. The computed pressure broadening coefficients and total rate constants agree well with the available experimental data. In the experimental part of the paper stimulated Raman-pumping has been used to prepare acetylene in selected rotational states (j(i)=2 to 18 and j(i)=1 to 19 of the C equivalent to C stretching mode). The population decay in the prepared state and the transfer to other rotational states was monitored by laser induced fluorescence. The experimental data can be described by an infinite-order-sudden power law (IOS-P) or directly compared with the ab initio derived rate constants. The influence of multiple collisions possible at the relatively large pressure-delay-products employed has been taken into account by simulating the rotational energy transfer with a master equation. Experimentally we obtain a total rate constant for depopulation of k(tot)=10.89 +/- 0.07 mu s(-1) Torr(-1) and IOS-P fitting parameters of A=5.58 and gamma=0.96 in very good agreement with the ab initio calculated values. There we obtain k(tot)=10.69 +/- 0.09 mu s(-1) Torr(-1) and IOS-P fitting parameters of A=6.18 and gamma=0.96. Also the experimental state-to-state rotational energy transfer constants, which vary from 5.83 to 2.32 mu s(-1) Torr(-1) for endothermic Delta j=2 collisions, agree very well with the ab initio data. (C) 1999 American Institute of Physics. [S0021-9606(99)01430-0].