State to state photodissociation of H2O in the first absorption band

Abstract

The photodissociation of H2O in the first absorption band is studied from single rotational states of vibrationally excited water. A tunable IR laser is used to prepare single rotational states in the asymmetric stretch mode. The subsequent photodissociation at 193 nm favors product formation from these single prepared states. The formation of the OH product in different rotational, Λ-doublet, and spin states is analyzed for a series of initial rotational states of H2O. This is the first direct photodissociation studied on a state to state level. The product state distributions depend sensitively upon the prepared state in the parent molecule H2O and exhibit pronounced quantum structure. The experimental results are understood almost quantitatively in terms of theory. The photodissociation of water turns out to be a limiting case of a dissociation which is governed by transfer of parent motion to products. The experiment leads to a highly improved understanding for the selective population of Λ-doublet states.

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

“…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.…”

“…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).…”

“…The quantum state distribution of the dissociation product (OH Ã ) is governed by the exit channels from the electronically excited dissociative parent state (H 2 O Ã ), whose excitation in turn depends on the UV photon energy (h ) and the initial state population of H 2 O (see Häusler et al 1987). In comets, quiescent H 2 O occupies different rotational levels of its ground vibrational state, with the distribution governed mostly by collisions in the inner cometary atmosphere and described by the H 2 O rotational temperature (e.g., Bonev 2005;Dello Russo et al 2005;Mumma et al 1986).…”

“…The quantum state distribution of the dissociation product (OH Ã ) is governed by the exit channels from the electronically excited dissociative parent state (H 2 O Ã ), whose excitation in turn depends on the UV photon energy (h ) and the initial state population of H 2 O (see Häusler et al 1987 . 1986).…”

“…(1Y0) and (2Y1) vibrational bands were detected. Paper I discussed the principal excitation mechanism for these lines, namely, photolysis of the parent molecule ( H 2 O) producing OH fragments that are both vibrationally and rotationally excited:The quantum state distribution of the dissociation product (OH Ã ) is governed by the exit channels from the electronically excited dissociative parent state (H 2 O Ã ), whose excitation in turn depends on the UV photon energy (h ) and the initial state population of H 2 O (see Häusler et al 1987 . 1986).…”