Rotational excitation of HC_3N by H2 and He at low temperatures

Aims:Rates for rotational excitation of HC_3N by collisions with He atoms and H2 molecules are computed for kinetic temperatures in the range 5-20 K and 5-100 K, respectively.
Methods: These rates are obtained from extensive quantum and quasi-classical calculations using new accurate potential energy surfaces (PES). The HC_3N-He PES is in excellent agreement with the recent literature. The HC_3N-H2 angular dependence is approximated using 5 independent H2 orientations. An accurate angular expansion of both PES suitable for low energy scattering is achieved despite the severe steric hindrance effects by the HC_3N rod.
Results: The rod-like symmetry of the PES strongly favours even Δ J transfers and efficiently drives large Δ J transfers. Despite the large dipole moment of HC_3N, rates involving ortho-H2 are very similar to those involving para-H2, because of the predominance of the geometry effects. Except for the even Δ J propensity rule, quasi classical calculations are in excellent agreement with close coupling quantum calculations. As a first application, we present a simple steady-state population model that shows population inversions for the lowest HC3N levels at H2 densities in the range 10^4-10⁶ cm^-3.
Conclusions: .The HC_3N molecule is large enough to present an original collisional behaviour where steric hindrance effects hide the details of the interaction. This finding, combined with the fair accuracy of quasi classical rate calculations, is promising in view of collisional studies of larger molecules.

Table A.1 is only available in electronic form at http://www.aanda.org