High-resolution diode laser study of H2–H2O van der Waals complexes: H2O as proton acceptor and the role of large amplitude motion

Abstract

Three high-resolution rovibrational bands of ortho H2 with both para and ortho H2O are observed in a slit supersonic expansion, based on direct absorption of a tunable diode laser in the ν2 bend region of H2O near 1600 cm−1. Complexes containing para H2O are responsible for a Σ←Σ type band associated with intramolecular bending excitation of H2O, while complexes containing ortho H2O exhibit two bands associated with (i) the intramolecular HOH bend (Π←Π) and (ii) an inter+intramolecular combination band (Σ←Π) corresponding to simultaneous HOH bend plus internal rotor excitation. From high-resolution line broadening studies, each upper state has a different vibrational predissociation lifetime; for bend excited para H2O complexes it is 5.1(14) nsec, while for the bend excited state and bend+internal rotor combination state of ortho H2O, it is 2.53(14) and 1.85(33) nsec, respectively. Analysis of the spectra supplemented by 2D quantum calculations indicate large amplitude, slightly hindered internal rotation of the H2O subunit in the complex. Nevertheless, the internal rotor splittings yield potential parameters that suggest ortho H2–H2O is best described with the H2 predominantly pointing towards the O atom in a H2O proton acceptor geometry.