Spectroscopy of complex and out-of-equilibrium molecular systems

IR absorption spectrum of Naphtalene - Direct IR absorption spectrum of Naphtalene at 25K (Pirali et al. PCCP 15 10141 (2013)) with Planetary Nebula Kohoutek 4-55 recorded by Hubble Space Telescope. Credits: NASA/ JPL.

Infrared spectroscopy of semi-rigid molecular edifices allows to determine their equilibrium structure and to understand their vibrational and rotational dynamics.

Low temperatures (5 – 30 K) achieved in supersonic flows are critical to form and stabilize complexes and clusters held by weak van der Waals forces or hydrogen bonds. These temperatures are also useful to depopulate excited rovibrational states, thus simplifying the structure of the infrared spectrum that would be otherwise be very difficult to exploit at room temperature.

Rennes’ spectroscopic facilities, which operate in the near and mid infrared, were employed to investigate Argon atom solvation effects of small water clusters, and acquire the first direct absorption signature of the benzene dimer.

The investigation of very low frequency vibrational modes is performed with the help of the Jet-AILES instrument installed on the SOLEIL synchrotron and developed by a consortium of laboratories. Our consortium has obtained the rotation-vibration spectrum at 25 K of the CH out-of-plane bending mode of naphthalene (C10H8, two aromatics rings). We have also provided the evidence of the planar structure of the HF trimer through the spectrum recorded at 30 K of two of its intermolecular modes in the far infrared.

In parallel, we explore out of equilibrium phenomena induced by very high cooling rates achieved in adiabatic expansions. For instance we have demonstrated by FTIR spectroscopy the unexpected absence of nuclear spin relaxation of water in a supersonic expansion (collaboration with LPMAA). We also investigate with near-IR CRDS the strong decoupling between rotational and vibrational degrees of freedom characterized by hypersonic free jets. We have thus generated an Argon/CO flow for which rotation and vibration are characterized by temperatures of 7 and 2000 K, respectively. This strong imbalance is exploited for the simplified analysis of the rotational structure of hot bands involving highly excited vibrational states of methane.

Astrophysical applications

The astrophysical applications are plentiful. They include the formation and characterization of hydrocarbons for the dense atmosphere of Titan. The far infrared opens a way towards the identification in astrophysical environments of specific Polycyclic Aromatic Hydrocarbons (PAH). Out-of-equilibrium spectroscopy of species such as methane is of interest for hot atmospheres of exoplanets. The formation of carbon particles in hypersonic expansion of acetylene, beforehand partially dissociated by pyrolysis, aims to understand grain formation in circumstellar envelopes of late-type red giants


Hypersonic expansion - Hypersonic expansion of a gas initially heated at 2000 K, visualized by Pitot probe. The species seeded in the carrier gas are probed by FTIR or ultrasensitive CRDS.

Selected publications

Molecular complexes

  • The cyclic ground state structure of the HF trimer revealed by far infrared jet-cooled Fourier transform spectroscopy, P. Asselin;2 P. Soulard; B. Madebene; M. Goubet; T. R. Huet; R. Georges; O. Pirali; P. Roy, Physical Chemistry Chemical Physics, 2014, 16 (10), pp. 4797-4806, DOI
  • Direct absorption spectroscopy of benzene dimer. Chandrasekaran V, Biennier L, Arunan E, Talbi D, Georges R, J. Phys. Chem. A 115 (41) 11263-11268 (2011)
  • The (CH2)2O-H2O hydrogen bonded dimer. Ab initio calculations and FTIR spectroscopy from neon matrix, and a new supersonic jet experiment coupled to the infrared AILES beamline of synchrotron SOLEIL. Cirtog M, Asselin P, Soulard P, Tremblay B, Madebène B, Alikhani ME, Georges R, Moudens A, Goubet M, Huet TR, Pirali O, Roy P, J. Phys. Chem. A 115, 2523-2532 (2011)
  • Direct absorption spectroscopy of water clusters formed in a continuous slit nozzle expansion. Moudens A, Georges R, Goubet M, Makarewicz J, Lokshtanov SE, Vigasin AA, J. Chem. Phys. 131, 204312 (2009)

Stable molecules

  • High-resolution spectroscopy and analysis of the ν2 + ν3 combination band of SF6 in a supersonic jet expansion, V. Boudon; P. Asselin; P. Soulard; M. Goubet; T. R. Huet; R. Georges; O. Pirali; P. Roy, Molecular Physics, 2013, 111 (14-15), pp. 2154-2162, DOI
  • The far infrared spectrum of naphthalene characterized by high resolution synchrotron FTIR spectroscopy and anharmonic DFT calculations. Pirali O, Goubet M, Huet T R, Georges R, Soulard P, Asselin P, Courbe J, Roy P, Vervloet M, Phys. Chem. Chem. Phys. 15, 10141-10150 (2013), DOI (2013)
  • High-resolution spectroscopy and analysis of the ν3/2ν4 dyad of CF4. Boudon V, Mitchell J, Domanskaya A, Maul C, Georges R, Benidar A, Harter WG, Molecular Physics 109 (17-18), 2273-2290 (2011)