Propanal, an interstellar aldehyde-first infrared band strengths and other properties of the amorphous and crystalline forms

Abstract

Chemical evolution in molecular clouds in the interstellar medium is well established, with the identification of over 200 molecules and molecular ions. Among the classes of interstellar organic compounds found are the aldehydes. However, laboratory work on the aldehydes has scarcely kept pace with astronomical discoveries as little quantitative solid-phase infrared (IR) data have been published on any of the aldehydes, and the same is true for important properties such as density, refractive indices, and vapour pressures. In this paper, we examine the IR spectra of solid propanal (HC(O)CH2CH3, propionaldehyde), along with several physical properties, for both the amorphous and crystalline forms of the compound. The quantitative measurements we report, such as IR intensities and optical constants, will be useful in laboratory investigations of the formation and evolution of propanal-containing ices, will serve as benchmark data for theoretical investigations, and will inform observational studies.