Infrared spectra, optical constants and temperature dependences of amorphous and crystalline benzene ices relevant to Titan

TL;DR

This study provides detailed laboratory measurements of benzene ice's spectral and optical properties across a range of temperatures relevant to Titan's atmosphere, aiding the interpretation of observational data.

Contribution

It offers the first comprehensive analysis of benzene ice's spectral characteristics and phase transitions at Titan-relevant temperatures, including refractive indices from 15 K to 130 K.

Findings

Benzene ice transitions from amorphous to crystalline between 120 K and 130 K.

Refractive indices of benzene ice were measured across 15 K to 130 K.

Spectral data now available for better Titan atmospheric modeling.

Abstract

Benzene ice contributes to an emission feature detected by the Cassini Composite InfraRed Spectrometer (CIRS) near 682 cm^{-1} in Titan's late southern fall polar stratosphere. It is as well one of the dominant components of the CIRS-observed High Altitude South Polar (HASP) ice cloud observed in Titan's mid stratosphere during late southern fall. Titan's stratosphere exhibits significant seasonal changes with temperatures that spatially vary with seasons. A quantitative analysis of the chemical composition of infrared emission spectra of Titan's stratospheric ice clouds relies on consistent and detailed laboratory transmittance spectra obtained at numerous temperatures. However, there is a substantial lack of experimental data on the spectroscopic and optical properties of benzene ice and its temperature dependence, especially at Titan-relevant stratospheric conditions. We have therefore analyzed in laboratory the spectral characteristics and evolution of benzene ice's vibrational modes at deposition temperatures ranging from 15 K to 130 K, from the far- to mid-IR spectral region (50 - 8000 cm^{-1}). We have determined the amorphous to crystalline phase transition of benzene ice and identified that a complete crystallization is achieved for deposition temperatures between 120 K and 130 K. We have also measured the real and imaginary parts of the ice complex refractive index of benzene ice from 15 K to 130 K. Our experimental results significantly extend the current state of knowledge on the deposition temperature dependence of benzene ice over a broad infrared spectral range, and provide useful new data for the analysis and interpretation of Titan-observed spectra.