Rotational excitation of HC3N by H2 and He at low temperatures

Aims. Rates for rotational excitation of HC3N by collisions with He atoms and H-2 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 HC3N - He PES is in excellent agreement with the recent literature. The HC3N - H-2 angular dependence is approximated using 5 independent H-2 orientations. An accurate angular expansion of both PES suitable for low energy scattering is achieved despite the severe steric hindrance effects by the HC3N 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 HC3N, 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 H-2 densities in the range 104- 106 cm(-3). Conclusions. The HC3N 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.