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

Abstract

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 (ji=2 to 18 and ji=1 to 19 of the C≡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 ktot=10.89±0.07μs−1 Torr−1 and IOS-P fitting parameters of A=5.58 and γ=0.96 in very good agreement with the ab initio calculated values. There we obtain ktot=10.69±0.09μs−1 Torr−1 and IOS-P fitting parameters of A=6.18 and γ=0.96. Also the experimental state-to-state rotational energy transfer constants, which vary from 5.83 to 2.32 μs−1 Torr−1 for endothermic Δj=2 collisions, agree very well with the ab initio data.

Experimental 2CH excitation in acetylene-containing van der Waals complexes

“…[78][79][80], but no resolved infrared or microwave spectrum has yet been reported, except very recently for C 2 D 2 -He [81]. Our attempts to observe the related 2CH spectrum using FANTASIOþ have all failed, so far.…”

BASECOL2012: A collisional database repository and web service within the Virtual Atomic and Molecular Data Centre (VAMDC)

“…For some other systems, such Ne-CO (Moszynski et al 1997) or Ne-C 2 H 2 (Bemish et al 1998), small scalings were necessary to quantitatively reproduce the experimental data. At the same time, it was shown that the ab initio potentials reproduce the (scarce) data for the state-to-state cross sections and rate constants; see, for instance, Heijmen et al (1997a) and Heijmen et al (1999a), respectively. Over the past decade, the complexity of the systems that can be treated by ab initio methods has A50, page 4 of 14 increased dramatically; see for instance, Bukowski et al (2007) and Jankowski et al (2012), who show the accuracy that can be reached for systems like the water dimer or CO-H 2 .…”

Ab initio potential energy surface and pressure broadening coefficients for the He–CH3F complex

“…We observed a series of rotational resonances at low scattering energies, below 20 cm À1 with a sharp one observed at energy lower than 2 cm À1 . Such resonances are known to result from Feshbach resonances when the total energy becomes equal to the energy of a quasi-bound level of the complex [4]. From the sharpness of the peak, one can conclude that the rotationally predissociative state is quite long lived.…”

“…The exchange-deformation energy was computed directly from the supermolecule Hartree-Fock interaction energy. The computational scheme for the pair interactions was the same as in our previous works (see, for instance, [4]). The SAPT interaction energies were always computed with the dimer basis set.…”

“…Not only is helium the simplest possible closed-shell monomer, it is also the second most abundant species (after molecular hydrogen) in the atmospheres of outer planets and in the interstellar gas. While accurate potentials of He with a linear molecule gave rise to a large number of spectroscopic, collisional, and thermodynamic studies [1][2][3][4], interactions of He with a polyatomic molecule are more scarce due to their increasing complexity [5,6], though they present a significant experimental and theoretical interest. For instance, recent experiments of van der Waals complexes of methyl fluoride with He open up the door of line broadening studies on a three-dimensional system.…”