Rotational quenching of organic molecules by molecular hydrogen*

In order to characterize the chemical composition, isotopic enrichments and possibly, history of the interstellar medium, it is necessary to get an accurate estimate of the molecular densities of the polar molecules observed. Since the intensity of the spectral lines depends on both photonic and molecular collisions, knowing the quenching/excitation rates is an essential ingredient, as soon as the spontaneous photon emission rates becomes comparable to the collisional quenching rate. Many complex molecules are observed, and especially so, in pre-stellar and proto-stellar environments.

We shall present methodology and quenching rates for representaive very common molecules in the intersellar matter: carbenes C₃H₂, symmetric molecule CH₃CN, and heteroatomic molecule HNCO. We computed a precise ab initio Potential Energy Surface, by means of an ab initio CCSD(T)-F12a formalism. We conducted quantum dynamical scattering in order to get precise cross sections using a coupled-channel and coupled-states approach for solving the nuclear motion.
Rotationally inelastic cross sections as well as quenching rates are compared with optical quenching rates and relevance of computations are discussed.