Collisional excitation of hyperfine levels of OH by hydrogen atoms

Abstract

ABSTRACT Observations of transitions between the hyperfine levels of the hydroxyl radical (OH) can provide crucial information on the physical conditions in interstellar clouds. Accurate modelling of the spectra requires calculated rate coefficients for the excitation of OH by H atoms, which is often present in molecular clouds in addition to the dominant H2 molecule. In this work, rate coefficients for the (de-)excitation of hyperfine levels of OH through collisions with hydrogen atoms are presented. In previous work, nuclear-spin-free scattering calculations were carried out; these took account of the fact that four electronic states (1A′, 1A″, 3A′, and 3A″) arise from the interaction of OH(X2Π) with H(2S). Because of the deep H2O($\tilde{X}^1A^{\prime }$) well, inelastic transitions can occur through direct collisions or by formation and decay of a collision complex. The rates of collision-induced hyperfine transitions were computed by the recoupling method and the MJ randomization approximations, respectively. These data will be useful in astrophysical models of OH excitation.