In silico spectroscopic studies of PAHs of astrochemical interest

Responsable du stage : Patrick Cassam-Chenaı̈ (
Équipe Interfaces des Mathématiques et Systèmes Complexes, Laboratoire J. A. Dieudonné, Faculté des Sciences, UCA-CNRS, Parc Valrose, 06108 Nice cedex 2.

Abstract: Polycyclic Aromatic Hydrocarbons (PAHs) are a family of organic compounds composed of aromatic rings of carbon atoms, whose peripheral valencies are saturated by hydrogen atoms. PAHs (or closely related species) are thought to be the carriers of the so-called Aromatic Infrared Bands (AIBs) at ∼3.3, 6.7, 7.7, 8.6, 11.3 µm. These features are among the strongest emission features observed in the interstellar medium in the infrared (IR), and
present some interesting variability from one observed source to another. Therefore, the IR spectroscopy of PAHs is of paramount importance in astrophysics and astrochemistry: A better understanding of it, is essential to infer from IR observations, informations about PAHs composition and physical parameters of their environment [1].

In our laboratory, we have developed a computer code, CONVIV, which solves the Schrödinger equation for molecular vibrational degrees of freedom with state-of-the-art accuracy [2]. It can be used to perform calculations of the infrared spectrum of molecules of arbitrary sizes. The internship will consist in using the CONVIV code to compute and interpret theoretically, the IR spectra of selected members of the PAH family.

A particular attention will be devoted to CH-stretching fundamental frequencies and their overtones which will be studied for small PAHs such as naphtalene, and pyrene. Concretely, the calculation will consists in, first, contracting together the CH-stretching vibrational modes, as well as possibly, other resonant modes, and then, in taking into account the remaining degrees of freedom as an effective field of second order [3]. The calculations will be confronted to available experimental data and astrophysical observations, in particular those from the James Webb Space Telescope (JWST) and from MATISSE already operating at the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory (ESO). Our aim is to gain a fine understanding of anharmonic effects in PAHs and to extrapolate the results to larger
PAHs, in order to extract as much information as possible from astrophysical spectra.

Prérequis: Basis of quantum mechanics and spectroscopy.

[1] G. Mulas, C. Falvo, P. Cassam-Chenaı̈, C. Joblin, J. Chem. Phys. 149, 144102 (2018).
[2] P. Cassam-Chenaı̈, J. Liévin, Journal of Computational Chemistry 27, 627-640 (2006).
[3] P. Cassam-Chenaı̈, G. Rousseau, A. Ilmane, Y. Bouret, M. Rey, J. Chem. Phys. 143,
034107 (2015).

Date de fin de publication : 28/02/2023