Infrared predissociation spectroscopy of protonated methyl cyanide, CH₃CNH⁺

Highlights

• Cryogenic action spectroscopy is applied to protonated methyl cyanide, CH₃CNH⁺, a potential interstellar molecule.

• Broad-range vibrational gas-phase spectra are recorded with FELIX.

• The extracted vibrational band positions allow to benchmark quantum-chemical calculations.

• Discussion on the influence of rare-gas tagging on the vibrational band positions.

• Prediction of rotational spectroscopic constants for low-lying vibrational states.

Abstract

The gas phase vibrational spectrum of CH₃CNH⁺ is investigated using a messenger infrared predissociation (IRPD) action spectroscopic method. Vibrational bands were recorded in the 300-1700 cm⁻¹ and 2000-3300 cm⁻¹ regions making use of the widely tunable free electron laser for infrared experiments, FELIX, coupled to a cryogenic ion trap instrument. Band assignments were aided by high-level quantum-chemical calculations, which showed excellent agreement with the experimental data. Effects of the neon atom used as messenger in the IRPD method are investigated in detail. The data presented here will support astronomical searches for the CH₃CNH⁺ ion in space, and provide a basis for high-resolution ro-vibrational and pure rotational studies in vibrationally excited states.