High-resolution laboratory study of the ν 2 / ν 4 bending motion of CH 3 + and assignment of its astronomically observed lines
Year: 2026
Type: article
Abstract: Context . The methyl cation (CH 3 + ) was discovered through the detection of a 7 μm (1400 cm −1 ) band observed in the d203-506 protoplanetary disk in the Orion nebula using the MIRI-MRS spectrometer on board the JWST. This feature has now been reported in numerous astronomical sources. The initial detection was based on simulations of its coinciding and interacting bending modes ( ν 2 / ν 4 ) using coarse laboratory and theoretical data, and combined to astronomical data with an absolute wavelength calibration of only several cm −1 . A first spectroscopic assignment of the lines was enabled by later quantum chemical calculations. However, a full spectroscopic assignment of the astronomically observed features in the 7 μm range is awaiting high-resolution laboratory measurements and a more robust wavelength calibration of the MIRI-MRS data. Aims . We present a laboratory high-resolution study of the ν 2 / ν 4 bending vibrations of CH 3 + in order to provide accurate and calibrated data for the spectroscopic analysis of JWST detections of this species. Methods . The rovibrational structure of the ν 2 / ν 4 bending vibrations of CH 3 + was studied in a cryogenic 22-pole ion trap apparatus at 10 K. For the detection of the vibrational excitation, the efficient leak-out spectroscopy method was used, in combination with a highresolution quantum cascade laser operating at 7 μm. This was combined with MIRI-MRS data of d203-506 reduced with improved wavelength calibration. Results . We were able to measure 49 rovibrational lines of the ν 2 / ν 4 band up to J ″ = 6 between 1330 and 1440 cm −1 (6.9–7.5 μm) in high resolution. The measurement accuracy and precision of about 0.001 and 0.0007 cm −1 , respectively, allow spectroscopic constants and line predictions to be derived with high confidence, and thus permit accurate (Doppler velocity) diagnostics of CH 3 + lines with JWST.
Source: Astronomy and Astrophysics
Institutions: University of Cologne, Centre National de la Recherche Scientifique, Université Toulouse III - Paul Sabatier, Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, National Institute of Standards and Technology, Joint Institute for Laboratory Astrophysics, University of Colorado Boulder, Université Paris-Saclay, Institut des Sciences Moléculaires d'Orsay, Goethe University Frankfurt
Language: English
Subfield: Spectroscopy
Field: Chemistry
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,865
OpenAlex ID: W7166093293