Overtone vibrational spectroscopy in H-2-H2O complexes: A combined high level theoretical ab initio, dynamical and experimental study
Résumé
First results are reported on overtone (v(OH) = 2 <- 0) spectroscopy of weakly bound H-2-H2O complexes in a slit supersonic jet, based on a novel combination of (i) vibrationally mediated predissociation of H-2-H2O, followed by (ii) UV photodissociation of the resulting H2O, 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 et al. [J. Chem. Phys. 129, 134306 (2008)] 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) H2O (oH(2)-oH(2)O). Specifically, two distinct bands are seen in the oH(2)-oH(2)O spectrum, corresponding to internal rotor states in the upper vibrational manifold of Sigma and Pi rotational character. However, none of the three other possible nuclear spin modifications (pH(2)-oH(2)O, pH(2)-pH(2)O, or oH(2)-pH(2)O) 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(2)O vibrational predissociation in the time domain reveals lifetimes of 15(2) ns and <5(2) ns for the Sigma and Pi 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 H2O wavefunctions, providing insight into predissociation dynamical pathways from these metastable levels.