Quantum scattering of HC5N and <i>para</i> -H2 on a new potential energy surface

In the interstellar medium (ISM), non-local thermodynamic equilibrium situations are common due to low density, and one needs to consider the effect of molecular collisions in order to interpret the observations. Among the species detected in the ISM, cyanopolyynes, with the general molecular formula HC2n+1N (n = 1, 2, …), are characterized by large dipole moments and small rotational constants and constitute an indispensable class of candidates for the sensitive tracers of local density and temperature. We present a study of the collisional (de-) excitation of HC5N by para-H2 (p-H2) in its ground rotational state, namely HC5N (j1) + H2 (j2 = 0) → HC5N (j1') + H2 (j2' = 0), where j1 (or j1') and j2 (or j2') denote the initial (or final) rotational quantum numbers of HC5N and H2, respectively. We performed the quantum scattering calculations at low collision energy using a new four-dimensional ab initio potential energy surface. In the regime where p-H2 remains in its rotational ground state, converged cross sections did not require including excited rotational states of p-H2 in the rotational basis. State-to-state cross sections were computed by means of the quantum-mechanical close-coupling (CC) method and the coupled states (CS) approximation, and rate coefficients for the first 61 levels of HC5N were computed for the first time up to 20 K with the CC approach and up to 50 K with the CS method. CC and CS results were found to agree well at temperatures up to 20 K. These data should allow a more accurate derivation of the HC5N abundance in molecular clouds.