Astronomical CH3+ rovibrational assignments. A combined theoretical and experimental study validating observational findings in the d203-506 UV-irradiated protoplanetary disk
Martin-Drumel, Marie-Aline; Van De Putte, Dries; Habart, Emilie; Peeters, Els; Sidhu, Ameek; Chown, Ryan; Alarcón, Felipe; Canin, Amélie; Schroetter, Ilane; Trahin, Boris; Dartois, Emmanuel; Rouillé, Gaël; Berné, Olivier; Goicoechea, Javier R.; Brünken, Sandra; Schlemmer, Stephan; Changala, P. Bryan; Chen, Ning L.; Le, Hai L.; Gans, Bérenger; Steenbakkers, Kim; Salomon, Thomas; Bonah, Luis; Jacovella, Ugo; Boyé-Péronne, Séverine; Alcaraz, Christian; Asvany, Oskar; Thorwirth, Sven
United States, France, Netherlands, Germany, Spain, Canada
Abstract
Context. The methyl cation (CH3+) has recently been discovered in the interstellar medium through the detection of 7 μm (1400 cm−1) features toward the d203-506 protoplanetary disk by the JWST. Line-by-line spectroscopic assignments of these features, however, were unsuccessful due to complex intramolecular perturbations preventing a determination of the excitation and abundance of the species in that source.
Aims: Comprehensive rovibrational assignments guided by theoretical and experimental laboratory techniques provide insight into the excitation mechanisms and chemistry of CH3+ in d203-506.
Methods: The rovibrational structure of CH3+ was studied theoretically by a combination of coupled-cluster electronic structure theory and (quasi-)variational nuclear motion calculations. Two experimental techniques were used to confirm the rovibrational structure of CH3+:(1) infrared leak-out spectroscopy of the methyl cation, and (2) rotationally resolved photoelectron spectroscopy of the methyl radical (CH3). In (1), CH3+ ions, produced by the electron impact dissociative ionization of methane, were injected into a 22-pole ion trap where they were probed by the pulses of infrared radiation from the FELIX free electron laser. In (2), neutral CH3, produced by CH3NO2 pyrolysis in a molecular beam, was probed by pulsed-field ionization zero-kinetic-energy photoelectron spectroscopy.
Results: The quantum chemical calculations performed in this study have enabled a comprehensive spectroscopic assignment of the v2+ and v4+ bands of CH3+ detected by the JWST. The resulting spectroscopic constants and derived Einstein A coefficients fully reproduce both the infrared and photoelectron spectra and permit the rotational temperature of CH3+ (T = 660 ± 80 K) in d203-506 to be derived. A beam-averaged column density of CH3+ in this protoplanetary disk is also estimated.