Near-infrared spectra of HCl highly diluted in liquid Ar show intense absorption in the P-R interbranch region, so-called Q-branch absorption. In spite of its relevance for the shape of the bands, its physical origin has been elusive to date. We employ molecular dynamics simulations to study the influence of some physical effects that could contribute to Q-branch absorption. We check that multipole-induced dipole induction mechanisms are not quantitatively relevant in this spectral region. We show that the particular characteristics of accurate HCl-Ar anisotropic potentials and the peculiar hindered rotational motion they provoke on the diatomic probe are essential to understand Q-branch absorption.
Vibration-rotation spectra of hydrogen halides in rare-gas liquids: Q-branch absorption