Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous species in astrophysical environments such as star forming regions, galaxies and planetary nebulae in which they emit the Aromatic Infrared Bands (AIBs). Emission of the AIBs is triggered by the absorption of an UV photon via an electronic transition and a sequence of radiationless transitions converting most of the absorbed energy to a vibrational excitation in the electronic ground state. The hot molecules then relax by emitting IR photons, the resulting spectrum being dominated by a large number of hot bands, all slightly shifted with respect to the corresponding 1-0 fundamentals due to anharmonicity. The resulting bands are very broad and their interpretation complex.
We performed a benchmark study of the infrared (IR) spectroscopy of hot pyrene (C16H10) from a combined experimental and theoretical approach. Infrared spectra of pyrene microcrystals embedded in solid KBr pellet were recorded in a large temperature range (from 14 to 723 K) and the evolution of the band positions, widths and integrated band intensities with temperature were monitored (1). This large temperature window covers the crystalline and molten phases of pyrene and enables us to get insights into the effect of intermolecular forces on the IR spectra. Empirical anharmonicity factors were derived by fitting the experimental band positions with temperature.
We found that only bands involving large amplitude motions of peripheral H nuclei are significantly affected by phase change. In all other cases, the derived anharmonicity factors are consistent with available gas-phase data (2) and can be used in models describing the IR spectra of astronomical PAHs (3).
In parallel, we computed the anharmonic IR spectra of hot pyrene using both an ab-initio code that describes the connection between states with explicit consideration of the resonances (the AnharmoniCaOs code(4)) and a classical molecular dynamics (MD) approach based on the Density Functional based tight binding (DFTB) method (5) and available in the deMonNano package (6). We showed that both approaches are complementary (7). The first one gives a detailed picture of all sorts of couplings but is limited in the temperature one can reach (typically 600 K) due to its high computational cost. The MD approach provides the global shape of the finite temperature IR spectra but loses the detailed spectroscopic information such as resonances, hot bands or mode identification. It can however be run to much higher temperatures (1600K in our study), which is of relevance for astrophysical models.
In this talk I will present the methodologies and summarize the results that have been obtained so far. Some perspectives will be given.
 S. Chakraborty, G. Mulas, K. Demyk and C. Joblin J. Phys. Chem. A 2019 (under revision).
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 S. Chakraborty, G. Mulas, M. Rapacioli, C. Falvo and C. Joblin J. Chem. Phys 2019 (under preparation).