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.

The 2.5–12 μm spectrum of comet halley from the IKS-VEGA experiment

“…Indeed, several bands of the sequence ~v = 2 of OH were observed in the near-infrared spectrum of P/Halley . Laboratory measurements (Andresen et al 1984) have shown that the photodissociation of H20 at 157 nm yields more than 53% of OH in excited vibrational states, Assuming that the yield of v(l -0) OH emission subsequent to water dissociation is 50%, we infer an emission rate g = 6 x 10 -6 sec -1 per water molecule at 1 AU for this band, to be compared with g = 3.3 x 10 -4 sec I for the 2.7-p~m water bands in the optically thin case. Thus we expect an OH signal approximately one-fiftieth that of the water band (which is consistent with that observed by Tokunaga et al 1987), which can be seen against water emission in the IKS spectrum only with considerable difficulty.…”

A Comprehensive Study of Infrared OH Prompt Emission in Two Comets. II. Implications for Unimolecular Dissociation of H2O

“…A significant fraction of the population in v 0 ¼ 1 of OH is expected to be a direct (vs. cascade) product of the parent's photolysis. For dissociation at 157 nm ( FAB), Andresen et al (1984) found that the population ratio for vibrational levels 0, 1, and 2 is 1:1: 0.15. In addition to direct production from H 2 O dissociation, some decay from higher vibrational levels to OH Ã (X 2 Å; v 0 ¼ 1) is also possible.…”

“…Product OH Ã (X 2 Å ) distributions in different vibrational states have been measured in the laboratory, for water dissociation through both the first and the second absorption bands (hereafter referred to as FAB and SAB). In the FAB, the equivalent rotational temperatures of product OH Ã (X 2 Å ) vary from a few hundred to about 1000 K as the initial H 2 O rotational temperature is varied between $30 and $300 K (Andresen et al 1984;Häusler et al 1987). In the SAB, the OH Ã (X 2 Å) rotational distribution can be extremely ''hot,'' peaking at rotational quantum numbers N 0 $ 40Y45, equivalent to temperatures of tens of thousands of kelvin (Harich et al 2000).…”

“…For example, Andresen et al (1984) studied photodissociation at 157 nm; Häusler et al (1987) and Nizkorodov et al (2003) studied it at 193 nm. Works in the SAB, including those involving end products other than OH Ã (X 2 Å), have featured mostly dissociation at 121.6 nm ( Ly ) (Carrington 1964;Yamashita 1975;Harich et al 2000;Mordaunt et al 1994).…”

“…The latter problem has played a principal role in unimolecular dissociation studies, and it has received significant experimental and theoretical attention (e.g., Carrington 1964;Yamashita 1975;Andresen et al 1984;Häusler et al 1987;Engel et al 1988;Harich et al 2000;Nizkorodov et al 2003;Mordaunt et al 1994;Dixon 1995;Dixon et al 1999;van Harrevelt & van Hemert 2000). Our purpose is to augment the existing laboratory database for OH Ã rotational distributions with a set of measurements of emission from this photodissociation product of cometary H 2 O.…”