Densities, infrared band strengths, and optical constants of solid methanol

¹ Centro de Tecnologías Físicas, Universitat Politècnica de València, Plaza Ferrándiz-Carbonell, 03801 Alcoy, Spain
² Instituto de Estructura de la Materia, IEM-CSIC, Serrano 123, 28006 Madrid, Spain
e-mail: belen.mate@csic.es

Received: 21 May 2018
Accepted: 29 June 2018

Abstract

Contact. The increasing capabilities of space missions like the James Webb Space Telescope or ground-based observatories like the European Extremely Large Telescope demand high quality laboratory data of species in astrophysical conditions for the interpretation of their findings.

Aims. We provide new physical and spectroscopic data of solid methanol that will help to identify this species in astronomical environments.

Methods. Ices were grown by vapour deposition in high vacuum chambers. Densities were measured via a cryogenic quartz crystal microbalance and laser interferometry. Absorbance infrared spectra of methanol ices of different thickness were recorded to obtain optical constants using an iterative minimization procedure. Infrared band strengths were determined from infrared spectra and ice densities.

Results. Solid methanol densities measured at eight temperatures vary between 0.64 g cm⁻³ at 20 K and 0.84 g cm⁻³ at 130 K. The visible refractive index at 633 nm grows from 1.26 to 1.35 in that temperature range. New infrared optical constants and band strengths are given from 650 to 5000 cm⁻¹ (15.4–2.0 μm) at the same eight temperatures. The study was made on ices directly grown at the indicated temperatures, and amorphous and crystalline phases have been recognized. Our optical constants differ from those previously reported in the literature for an ice grown at 10 K and subsequently warmed. The disagreement is due to different ice morphologies. The new infrared band strengths agree with previous literature data when the correct densities are considered.