Rotational excitation of HC

Aims.Rates for rotational excitation of ${\rm HC_3N}$ 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 ${\rm HC_3N}$–He PES is in excellent agreement with the recent literature. The ${\rm HC_3N}$–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 ${\rm HC_3N}$ rod. Results.The rod-like symmetry of the PES strongly favours even $\Delta J$ transfers and efficiently drives large $\Delta J$ transfers. Despite the large dipole moment of ${\rm HC_3N}$, rates involving ortho-H2 are very similar to those involving para-H2, because of the predominance of the geometry effects. Except for the even $\Delta J$ 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 104-106 cm-3. Conclusions.The ${\rm HC_3N}$ 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