  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.
