Astronomical CH3+ rovibrational assignments: A combined theoretical and experimental study validating observational findings in the d203-506 UV-irradiated protoplanetary disk

Context:The methyl cation (CH + 3) 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 CH + 3 in d203-506.Methods:The rovibrational structure of CH + 3 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 CH + 3 :1. Infrared leak-out spectroscopy of the methyl cation: CH + 3 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.2. Rotationally resolved photoelectron spectroscopy of the methyl radical (CH 3): Neutral CH 3 , produced by CH 3 NO 2 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 ν + 2 and ν + 4 bands of CH + 3 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 CH + 3 (T = 660 ± 80 K) in d203-506 to be derived. A beam-averaged column density of CH + 3 in this protoplanetary disk is also estimated.