Gas-Phase Infrared Action Spectroscopy of CH2Cl+ and CH3ClH+: Likely Protagonists in Chlorine Astrochemistry

Abstract

Two fundamental halocarbon ions, CH₂Cl⁺ and CH₃ClH⁺, were studied in the gas phase using the FELion 22-pole ion trap apparatus and the Free Electron Laser for Infrared eXperiments (FELIX) at Radboud University, Nijmegen (the Netherlands). The vibrational bands of a total of four isotopologs, CH₂^35,37Cl⁺ and CH₃^35,37ClH⁺, were observed in selected wavenumber regions between 500 and 2900 cm^-1 and then spectroscopically assigned based on the results of anharmonic force field calculations performed at the CCSD(T) level of theory. As the infrared photodissociation spectroscopy scheme employed probes singly Ne-tagged weakly bound complexes, complementary quantum-chemical calculations of selected species were also performed. The impact of tagging on the vibrational spectra of CH₂Cl⁺ and CH₃ClH⁺ is found to be virtually negligible for most bands; for CH₃ClH⁺-Ne, the observations suggest a proton-bound structural arrangement. The experimental band positions as well as the best estimate rotational molecular parameters given in this work provide a solid basis for future spectroscopic studies at high spectral resolutions.

Keywords: action spectroscopy; astrochemistry; chlorine chemistry; ion trap; vibrational spectroscopy.