Large open-shell molecule of astrophysical interest

L. Coudert, O. Chitarra, B. Gans, M.-A. Martin-Drumel, O. Pirali (ISMO, UPSud)

This project aims at building high-resolution spectroscopic data bases for open-shell molecules of astrophysical significance. Spectroscopic results are still missing for such molecules because they are experimentally and theoretically more difficult to deal with than closed-shell molecules. These data bases should allow us to interpret radio astrophysical spectra and in some cases to carry out their first detection in the interstellar media. The present project includes experimental developments and theoretical research. One of the target molecules is the non-rigid open-shell radical CH2OH, which was recently detected in the lab.Results: Our sub-millimeter-wave spectrometer has been upgraded and the spectra of several new molecules could be observed for the first time including ethynethiol (HCCSH) which is a metastable isomer of thioketene.With the help of Jean-Christophe Loison (ISM, Bordeaux), we built a cell to produce the CH2OH and CH3O radicals by H atom abstraction, using methanol and atomic fluorine as precursors. This cell, shown on the left panel of the figure below, is 150 cm long and atomic fluorine, produced by a 2450 MHz microwave discharge, is injected through several holes so as to optimize radical production. The experimental signal of CH2OH was optimized varying discharge and gas flux conditions. The sub-millimeter-wave spectrum recorded near 386610 MHz is shown on the right panel of the figure below. The best signal is obtained with two injectors as the hyperfine structure of the = 8.5←7.5 transition can then clearly be seen.

A major result has since been obtained with this new experimental setup as more than two hundred new transitions of CH2OH could be recorded and assigned in terms of rotational-spin quantum numbers.Theoretical results concerning the potential energy surface of CH2OH were also obtained. It displays 8 saddle points including four equivalent C1 symmetry minima, two local maxima, and two C2v absolute maxima. The 2 lowest lying vibrational states display tunneling splittings of 0.035 à 44.809cm-1. These results are being used to develop a theoretical model allowing us to account for the spin-rotation coupling, the large amplitude motion effects and the hyperfine coupling.

Chitarra, Martin-Drumel, Gans, Loison, Spezzano, Lattanzi, Müller, and Olivier Pirali, Enabling CH2OH interstellar detection: Re-investigation of the rotation-tunneling spectrum of the radical, submitted to Astronomy & Astrophysics (2020)

Results achieved in the framework of the project  MACO-GT funded by topic emergence and carried out by  Laurent Coudert (ISMO)