# Experimental Approach to the Study of Anharmonicity in the Infrared Spectrum of Pyrene from 14 to 723 K

## TL;DR

This study experimentally investigates how anharmonicity affects the infrared spectrum of pyrene across a wide temperature range, providing empirical data and anharmonic factors relevant for astrophysical models.

## Contribution

It presents the first comprehensive experimental analysis of anharmonic effects on pyrene's IR spectrum from 14 to 723 K, including phase transition impacts.

## Key findings

- Identified phase transition effects on spectral features.
- Derived empirical anharmonic factors consistent with gas-phase data.
- Provided recommended anharmonic parameters for astrophysical modeling.

## Abstract

Quantifying the effect of anharmonicity on the infrared spectrum of large molecules such as polycyclic aromatic hydrocarbons (PAHs) at high temperature is the focus of a number of theoretical and experimental studies, many of them motivated by astrophysical applications. We recorded the infrared spectrum of pyrene C16H10 microcrystals embedded in a KBr pellet over a wide range of temperature (14 -723 K) and studied the evolution of band positions, widths and integrated intensities with temperature. We identified jumps for some of the spectral characteristics of some bands in the [423- 473]K range. These were attributed to a change of phase from crystal to molten in condensed pyrene, which appears to affect more strongly bands involving large CH motions. Empirical anharmonic factors that describe the evolution of band positions and widths with temperature were retrieved from both phases over an unprecedented temperature range. The derived values were found to be consistent with available gas-phase data. We provide recommended values for anharmonic factors and conclude about the interest of the methodology to provide data of interest for comparison with theoretical models and as inputs of models that simulate the infrared emission of astro-PAHs.