Influences on Infrared Spectra of Benzene Ices for Titan, Comets, and Beyond: Annealings, Artifacts, and Isosbestic Points

Abstract

Infrared (IR) spectra are needed for both laboratory and observational studies of extraterrestrial ices, both within the solar system, such as for trans-Neptunian objects, comets, and Titan, and in the interstellar medium. Here we present new transmission-IR measurements on solid benzene (C6H6), the simplest common aromatic compound. Spectra are shown with a greater combination of higher resolutions and temperatures than found in the literature. The influences of both warmings and annealings on amorphous benzene are described, and comparisons are made of the IR spectra of warmed and cooled crystalline benzene. Reversible variations with temperature are noted for several IR bands of crystalline benzene, with a suggestion for their use as a possible thermometer for remote sensing of extraterrestrial environments. Comparisons are made to both recent and older literature results, and an artifact produced by high vapor-phase condensation rates is identified. New density measurements are reported for crystalline benzene and used, along with IR-forbidden transitions, to identify amorphous and crystalline ices. It is suggested that differences in spectra of crystalline benzene ices at the same temperature do not necessarily imply partial crystallization.