Overtone vibrational spectroscopy in H _2-H _2O complexes: A combined high level theoretical ab initio, dynamical and experimental study

摘要

First results are reported on overtone (v _(OH) 2 ← 0) spectroscopy of weakly bound H _2-H _2O complexes in a slit supersonic jet, based on a novel combination of (i) vibrationally mediated predissociation of H _2-H _2O, followed by (ii) UV photodissociation of the resulting H _2O, and (iii) UV laser induced fluorescence on the nascent OH radical. In addition, intermolecular dynamical calculations are performed in full 5D on the recent ab initio intermolecular potential of Valiron J. Chem. Phys. 129, 134306 (2008)10.1063/1.2988314 in order to further elucidate the identity of the infrared transitions detected. Excellent agreement is achieved between experimental and theoretical spectral predictions for the most strongly bound van der Waals complex consisting of ortho (I 1) H _2 and ortho (I 1) H _2O (oH _2-oH _2O). Specifically, two distinct bands are seen in the oH _2-oH _2O spectrum, corresponding to internal rotor states in the upper vibrational manifold of Σ and Π rotational character. However, none of the three other possible nuclear spin modifications (pH _2-oH _2O, pH _2-pH _2O, or oH _2-pH _2O) are observed above current signal to noise level, which for the pH _2 complexes is argued to arise from displacement by oH _2 in the expansion mixture to preferentially form the more strongly bound species. Direct measurement of oH _2-oH _2O vibrational predissociation in the time domain reveals lifetimes of 15(2) ns and 5(2) ns for the Σ and Π states, respectively. Theoretical calculations permit the results to be interpreted in terms of near resonant energy levels and intermolecular alignment of the H _2 and H _2O wavefunctions, providing insight into predissociation dynamical pathways from these metastable levels.