Rotational excitation of HOCO+ by helium at low temperature

Abstract

Context. It has been shown that the HOCO+ ion is present in interstellar space. As a large number of HOCO+ lines can be observed in the millimeter and submillimeter wavelengths, this molecule is a useful tracer for both the temperature and the density structure of the clouds. Modeling of the spectra will require accurate radiative and collisional rates of species of astrophysical interest. Aims. The paper focuses on the calculation of rotational excitation rate coefficients of HOCO+ by He, useful for studies of low-temperature environments. Methods. Cross sections are calculated using the quantum Coupled States approach for a total energy range 0-200 cm-1. These calculations are based on a new ab initio CEPA (coupled electron pair approach) potential energy surface. It was assumed that the HOCO+ ion was fixed at its theoretical equilibrium geometry. Results. Thermally averaged rate coefficients were calculated from the cross sections, at kinetic temperatures up to 30 K. © ESO 2007.