Anharmonic interstellar PAH molecules

Abstract

Polycyclic aromatic hydrocarbon (PAH) molecules are responsible for a family of features, the aromatic infrared bands, which are observed in many astronomical environments. The photophysics and photochemistry of PAHs in space is strongly influenced by anharmonicity, which has seldom been considered in astrophysical modeling because of the lack of available data or because too computationally demanding. Recent developments of second-order vibrational perturbation theory applied to density functional theory now allow the computation of the anharmonic quartic force field anharmonic spectra of medium-large molecules like PAHs in a time-efficient way. It is now possible to have a better grasp of the effects of anharmonicity on the vibrational spectrum of PAHs also thanks to the comparison with gas-phase, high-resolution experimental data. In this perspective, the quality of these anharmonic spectra for PAH molecules is reviewed and discussed in view of a robust astrochemical model for PAH molecules.anharmonic, density functional theory, polycyclic aromatic hydrocarbons | I N T R O D U C T I O NPolycyclic aromatic hydrocarbon (PAH) are a class of molecules which is very common on the Earth, being the byproduct of combustion. It is now known that PAHs are widespread in the entire Universe. They are accepted almost unequivocally as the carriers of a family of bands, the aromatic infrared bands (AIBs), detected in emission in the spectrum of astronomical objects ranging from dying stars to entire galaxies. [1] In space, PAH molecules absorb ultraviolet or visible photons, then undergo fast internal conversion by which the absorbed energy is transferred to the vibrational degrees of freedom. The energy is then released through a radiative cascade of the vibrational transitions down to the ground state, producing the AIBs. [2] As the average time for IR photon emission is much longer than the time for internal vibrational redistribution, the population of the vibrational states is in internal equilibrium, leading to emission between excited vibrational states rather than only fundamentals. Thus, the appearance of the AIBs spectrum (i.e., band position, band width, and rovibrational envelope) is strongly influenced by the anharmonicity of the potential energy surface of the molecules. The profile and general appearance of the AIB spectrum has been probed by multiple astronomical observations. TheAIBs not only show variation between different astronomical objects [3] but also spatially across a given single astronomical object. [4] This has led astronomers to hypothesize that these spectral variations are linked to the variation of local physical conditions which, in turn, affects the PAH population. The spectroscopy of PAH molecules is difficult to study experimentally in condition relevant to space due to their low vapor pressure; computational chemistry thus has been the major source of vibrational spectra. Databases exist containing the harmonic vibrational spectra of many different PAHs, up to molecules containi...

Laboratory Gas-phase Infrared Spectra of Two Astronomically Relevant PAH Cations: Diindenoperylene, and Dicoronylene,

“…As stated earlier, the IRMPD (absorption) process results in highly excited species and will therefore induce an additional (red) shift. The size of this shift depends on the level of anharmonicity and the temperature of the involved modes (Chen et al 2016;Candian & Mackie 2017). With this qualifying remark in mind, the comparison of our experimental results with the theoretical predictions is very encouraging with deviations of the order of a few percent.…”

Excitonic and charge transfer interactions in tetracene stacked and T-shaped dimers

“…Several potential advantages, such as low synthesis cost, have stimulated the search for organic optoelectronic materials. 1 Among these materials, a particularly important class is that of the polycyclic aromatic hydrocarbons (PAHs) due to their potential application as organic semiconductors 2 and their relevance for spintronics, 3 astrochemistry, 4 and nonlinear optics. 5 Tetracene and pentacene, in particular, are the focus of a renewed interest due to the singlet fission phenomenon 6 and the promise of increased efficiency in optoelectronic devices, with several reported theoretical [7][8][9][10] and experimental 11,12 studies focused on monomers and dimers of these molecules and their derivatives.…”

Computational challenges in Astrochemistry

“…The second step is understanding how these molecular species are formed and how they evolve chemically toward more complex species. In this scenario, molecular spectroscopy and computational chemistry play a crucial role . On one side, astronomical observations of spectroscopic signatures provide the unequivocal proof of the presence of chemical species; on the other side, quantum chemistry allows shedding light on their formation/transformation mechanisms, which are nowadays still object of debate and often not experimentally investigatable.…”