Revisiting the rovibrational (de-)excitation of molecular hydrogen by helium

Abstract

The collisional (de-)excitation of H$_ $ by He plays an important role in the thermal balance and chemistry of various astrophysical environments, making accurate rate coefficients essential for interpreting observations of the interstellar medium. Our goal is to utilize a state-of-the-art potential energy surface (PES) to provide comprehensive state-to-state rate coefficients for He-induced transitions among rovibrational levels of H$_2$. We performed quantum scattering calculations for the H$_ $-He system. Thus, we were able to provide state-to-state rate coefficients for transitions between rovibrational levels of H$_ $, with internal energies up to simeq 15 000 cm$^ for temperatures ranging from 20 to 8 000 K. Our results demonstrate a good agreement with previous calculations for pure rotational transitions between low-lying rotational levels. However, we do find significant discrepancies for rovibrational processes involving highly-excited rotational and vibrational states. We attribute these differences to two key factors: 1) the broader range of intramolecular distances covered by ab initio points and 2) the superior accuracy of the PES, resulting from the utilization of the state-of-the-art quantum chemistry methods, compared to previous lower-level calculations. Radiative transfer calculations performed with the new collisional data indicate that the population of rotational levels in excited vibrational states experiences significant modifications, highlighting the critical need for this updated dataset in models of high-temperature astrophysical environments.