Infrared spectroscopy of astrophysical ice analogs at oblique angles

Abstract

In astrochemical exploration, infrared (IR) spectroscopy is vital for understanding the composition and structure of ice in various space environments. This article explores the impact of incident angles on IR spectroscopy, focusing on molecular components present in interstellar and circumstellar ice mantles such as CO, CO$_2$, H$_2$O, CH$_3$OH, NH$_3$, CH$_4$, H$_2$S. The experiment involves changing the angle at which the infrared beam hits the surface used for ice deposition. It is important to measure the density of the ice layer accurately, especially for experiments that involve using different angles in infrared spectroscopy. Furthermore, the experimental methodology allowed us to derive the {\it effective} refraction index values in the infrared range for each ice component. Existing corrections typically consider geometric configurations but overlook the refractive index of the ice ($n$), a factor dependent on ice composition. The study reveals that the incident angle and the refractive index, determine the pathlength of the IR beam across the ice sample. This insight challenges conventional corrections, impacting the integrated absorption values of the IR bands and column densities. In addition, for certain ice components, variations in the incidence angle affect the longitudinal (LO) and transverse (TO) optical modes of the ice, leading to observable changes in the IR band profiles that provide information on the amorphous or crystalline structure of the ice. The practical implications of this work apply to experimental setups where normal IR measurements are unfeasible. Researchers using, for example, the standard 45$^{\circ}$ angle for IR spectroscopy, will benefit from a more accurate estimation of ice column density.