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

Laser spectroscopy of biomolecular systems : structure, interactions and selectivity

Internship for 1st and 2nd year Master student.
01 69 15 79 54
ISMO – Université Paris Saclay

Biological systems are gouverned by non covalent molecular interactions (H bonding, Van der Waals bonding) that can be probed by gas phase laser spectroscopy techniques experiments, in combination of theoretical molecular modeling methods at the quantum and classical levels.
Characterizing the interactions between peptides and carbohydrates is of prime importance in biology, as sugars play a major role in molecular recognition processes and peptides are models for sugar receptor proteins (lectins).
The goal of the internship will be to combine experiments and theory to evidence the structures of simple molecular systems forming a sugar-peptide complex.
The intern will be introduced to several experimental techniques used in many fields of experimental physical chemistry : lasers and optics, IR and UV spectroscopy, laser desorption of biomolecules, supersonic expansions to cool down the molecules and stabilize the complexes, mass spectrometry detection, vacuum techniques. In parallel, the simulation of the spectra to interpret the data and their interpretation may also be part of the project.

A new laser spectroscopy setup for exploring the mid infrared spectra of biomolecular systems.

tel. 0169157954
ISMO – Université Paris Saclay

Internship for 1st or 2nd year Master.
Vibrational spectroscopy is a tool of choice for probing the non-covalent molecular interactions that are at the center of interactions between biological molecules. In our laboratory, we have developed an expertise in the infrared (IR) spectroscopy of the elongation modes of the O-H, NH and O-D modes, in the gas phase. These modes, which absorb in the region of 2.5 to 4 um, are very important since they are at the center of these non-covalent interactions. For this purpose we have developed an IR-UV double resonance spectroscopy device in the near IR region. The proposed internship work consists in setting up a new double resonance laser spectroscopy setup to extend the accessible spectral range and obtain more information on large molecular structures. By a nonlinear optics approach based on frequency difference we will be able to observe the IR spectra over a range from 2.5 to 15 um. Another very promising approach will be based on Stimulated Raman double resonance spectroscopy – UV will give us access to even lower frequency modes. The new setup will be applied to observe the first mid-IR spectra of sugar based complex biomolecular assemblies and large hydrated clusters.