Near-infrared spectroscopy and anharmonic theory of the H 2 O + Ar 1,2 cation complexes.

Weakly bound complexes of the water radical cation with argon (H2 O+ Arn , n = 1,2) were generated by an electrical discharge/supersonic expansion and probed with mid- and near-infrared photodissociation spectroscopy in the 2050-4550 and 4850-7350 cm-1 regions. To elucidate these spectra, these complexes were studied computationally at the CCSD(T) level including anharmonicity with the VPT2 method. The comparison between experiment and predicted spectra demonstrates that the VPT2 method is adequate to capture most of the vibrational band positions and their intensities. In addition to the fundamentals, overtones of the symmetric and the asymmetric water stretches and their combination were detected. Additional bands were assigned to combinations of the overtone of the bound O-H stretch with multiple excitation levels of the intermolecular Ar-H stretch. H2 O+ Ar2 could not be dissociated in the higher frequency region (4850-7350 cm-1 ).