Infrared photodissociation spectroscopy and anharmonic vibrational study of the HO4+ molecular ion.

Molecular cations of HO4+ and DO4+ are produced in a supersonic expansion. They are mass-selected, and infrared photodissociation spectra of these species are measured with the aid of argon-tagging. Although previous theoretical studies have modeled these systems as proton-bound dimers of molecular oxygen, infrared spectra have free OH stretching bands, suggesting other isomeric structures. As a consequence, we undertook extensive computational studies. Our conformer search used a composite method based on an economical combination of single- and multi-reference theories. Several conformers were located on the quintet, triplet, and singlet surfaces, spanning in energy of only a few thousand wavenumbers. Most of the singlet and triplet conformers have pronounced multiconfigurational character. Previously unidentified covalent-like structures (H—O—O—O—O) on the singlet and triplet surfaces likely represent the global minima. In our experiments, HO4+ is formed in a relatively hot environment, and similar exp...