High-resolution laboratory study of the ν₂/ν₄ bending motion of CH3+ and assignment of its astronomically observed lines
Abstract
Context: The methyl cation (CH3+) 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 (ν₂/ν₄) 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 ν₂/ν₄ bending vibrations of CH3+ in order to provide accurate and calibrated data for the spectroscopic analysis of JWST detections of this species. Methods: The rovibrational structure of the ν₂/ν₄ bending vibrations of CH3+ 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 ν₂/ν₄ 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 CH3+ lines with JWST.
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BibTeX
@article{salomon2026highresolution,
title = {{High-resolution laboratory study of the ν₂/ν₄ bending motion of CH$_3^+$ and assignment of its astronomically observed lines}},
author = {Thomas Salomon and Ilane Schroetter and Olivier Berné and P. Bryan Changala and Marie-Aline Martin-Drumel and Yoko Okada and Markus Röllig and Stephan Schlemmer and Oskar Asvany},
journal = {Astronomy \& Astrophysics - A\&A},
year = {2026},
month = sep,
doi = {10.1051/0004-6361/202660397},
url = {https://hal.science/hal-05746653v1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - High-resolution laboratory study of the ν₂/ν₄ bending motion of CH_3^+ and assignment of its astronomically observed lines
AU - Thomas Salomon
AU - Ilane Schroetter
AU - Olivier Berné
AU - P. Bryan Changala
AU - Marie-Aline Martin-Drumel
AU - Yoko Okada
AU - Markus Röllig
AU - Stephan Schlemmer
AU - Oskar Asvany
PY - 2026
DA - 2026/09/11
JO - Astronomy & Astrophysics - A&A
DO - 10.1051/0004-6361/202660397
UR - https://hal.science/hal-05746653v1
AB - 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 (ν₂/ν₄) 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 ν₂/ν₄ 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 ν₂/ν₄ 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 ν₂/ν₄ 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.
SN - 0004-6361
SN - 1432-0746
ER -