Structure and Vibrational Spectra of ArH⁺ (n = 2-3)

The structure and vibrational spectra of protonated Ar clusters ArH⁺ (n = 2-3) are studied using potential energy surfaces at the CCSD(T)/aug-cc-pVTZ level and basis set. Ar binding energies, as well as position isomerism in Ar₃H⁺, were investigated. In our previous work, the spectra of Ar₂H⁺ reveal a strong progression of combination bands, which involves the asymmetric Ar-H⁺ stretch with multiple quanta of the symmetric Ar-H⁺ stretch. In this work, insights on the origin of such progression were examined using an adiabatic model. In addition, contributions from mechanical and electrical anharmonicity on the progressions' intensities were also examined. Comparison of the calculated spectrum for the bare and Ar-tagged ions reveals that the reduction of the symmetry group, from D_∞h to either C_∞v or C_2v, results in a richer vibrational structure in the 500-1700 cm^-1 region. When compared with previously reported action spectra (D. C. McDonald III, D. T. Mauney, D. Leicht, J. H. Marks, J. A. Tan, J.-L. Kuo, and M. A. Duncan, J. Chem. Phys., 2016, 145, 231,101), it appears that the position isomers, because of the binding of the weakly bound Ar messenger, are needed to account for the additional bands in the infrared photodissociation spectrum for Ar₃H⁺. These findings demonstrate the active role of the messenger atom in relaxing some of the selection rules for the bare ion's vibrational transitions - resulting in an augmentation of the bands in the action spectrum.