Nuclear and electron dynamics in the photodissociation of water

Abstract

The photodissociation of water in its first absorption band is studied by photolyzing H₂O at 157 nm with an excimer laser. This dissociation proceeds directly to produce the electronic ground states of H and OH. Both nascent internal state distributions and alignment of the product OH (²Π) are probed by laser induced fluorescence. This is done with both warm (300 K) and cold (∼10 K) water. About 88% of the excess energy is translation, 10% vibration, about 2% rotation. The first three vibrational levels 0, 1, 2 have population ratios 1:1:0.15, respectively. The rotational distributions depend strongly upon the H₂O temperature and are very different for the upper and lower energy components of the Λ doublets, which are measured via Q and P, R lines, respectively. For Q lines, the distributions can be described by rotational temperatures which are 930 K for warm and 475 K for cold water, a surprising difference. For P,R lines strong deviations from Boltzmann behavior are found for cold H₂O. The spin distribution is almost statistical. A strong J dependent Λ‐doublet population inversion is found from cold H₂O, but there is no inversion from warm H₂O. The inversion provides a possible pump mechanism for the astronomical OH maser and is simply explained by approximate symmetry conservation. The orientation of the unpaired pπ lobe in OH in the upper Λ‐doublet state is measured to be perpendicular to the OH rotation plane. The J dependence of the inversion is explained by Λ‐doublet mixing in OH and quantitatively described in terms of the singly occupied pπ‐lobe in the excited water and the orientation of the corresponding singly occupied pπ‐lobe in OH. The alignment of OH is measured by polarizing both lasers. The large polarization effects are strongly dependent upon J and also upon the temperature of H₂O. It is shown that the dependence is related both to Λ‐doublet mixing and hyperfine structure of OH. For the cold H₂O the data indicate, despite the strong J dependence of both polarization and Λ‐doublet inversion, a completely planar dissociation process. It is shown that due to Λ‐doublet mixing the transition moment of Π molecules has a J dependent angle relative to the OH rotation plane which approaches the high J limit at the same rate that the molecule shifts from Hund’s case (a) to case (b). The model for the J dependence of the Λ‐doublet population and the polarization is important for chemical reactions, surface scattering and other processes where Π molecules are analyzed with LIF.