Evaluating Infrared Absorption Parameters for Low-Temperature Ices Using Reflection–Absorption Infrared Spectroscopy

We present infrared absorption coefficients, cross sections, and band strengths calculated from reflection-absorption infrared spectroscopy (RAIRS) data of molecular ices deposited on a highly oriented pyrolytic graphite (HOPG) surface at 28 K. We also describe an estimated conversion factor (encompassing considerations from the metal-surface selection rule and metal-surface enhancement factor for graphite) to explain how these parameters compare to those calculated using transmission infrared methods. Further, we discuss the limitations of this method and the assumptions which must be considered when evaluating absorption parameters from RAIRS data. The two species studied here (methyl acetate and methyl propanoate) were chosen to test these methods, as they are both astrochemically relevant and have previously been reported in the literature, providing a reliable benchmark.