High-temperature infrared spectroscopy of selected aromatics, PAHs, and olefins in the long-wave infrared range

Abstract

Extending the literature of high-temperature cross-sections is essential for the progress of high-temperature reaction-kinetics investigations and the advancement of exoplanet and distant star sensing. This study aims to contribute to the sparse literature of high-temperature cross-section data in the long-wave infrared range (LWIR) by presenting new absorption cross-section measurements of benzene, toluene, naphthalene, phenanthrene, propylene and 1,3-butadiene over 500 – 4000 cm^-1, at temperatures up to 1000 K and at atmospheric pressure.

A high-temperature optical cell which enables access to the LWIR “fingerprint” region was utilized and, for high-boiling-point species, a thermogravimetric analyzer (TGA) was used to prepare gas-phase mixtures with accurately quantified mole fractions. The presented novel experimental approach is of value as it may be extended to an array of other molecules and conditions in future studies.

The recorded vibrational bands in the LWIR range displayed spectral broadening and shifted towards longer wavelengths with increasing temperature, which are anticipated behaviors. In contrast to what is commonly assumed, integrated band intensities were found to mostly increase as a function of temperature over the covered temperature ranges. The experimental setup and methods were validated by comparing a subset of the measurements to data reported in the literature. The uncertainty in the reported results is estimated at ±5.2%.