Photodissociation of water in the first absorption band: a prototype for dissociation on a repulsive potential energy surface

Abstract

The photodissociation of water in the first absorption band, H20(X) + ftoi -* H20(Á'B1) -H(1 2S) + (2 ), is a prototype of fast and direct bond rupture in an excited electronic state. It has been investigated from several perspectives-absorption spectrum, final state distributions of the products, dissociation of vibrationally excited states, isotope effects, and emission spectroscopy. The availability of a calculated potential energy surface for the Á state, including all three internal degrees of freedom, allows comparison of all experimental data with the results of rigorous quantum mechanical calculations without any fitting parameters or simplifying model assumptions. As the result of the confluence of ab initio electronic structure theory, dynamical theory, and experiment, water is probably the best studied and best understood polyatomic photodissociation system. In this article we review the joint experimental and theoretical advances which make water a unique system for studying molecular dynamics in excited electronic states. We focus our attention especially on the interrelation between the various perspectives and the correlation with the characteristic features of the upper-state potential energy surface.

Excited states of the water molecule: Analysis of the valence and Rydberg character

“…This behavior is consistent with the photodissociation process into ground-state H· and OH· radicals following excitation of water in the energy range of the lowest absorption band. 24 A valence-Rydberg mixing is therefore also expected for those states ascribed to promotions from deeper orbitals to the 3s / 4a 1 orbital, as pointed out by Dutuit et al 23 and it occurs for the 2 1 A 1 ͑3a 1 → 3s / 4a 1 ͒ state considered here. Excitation to the 3p y / 2b 2 orbital also leads to excited states of valenceRydberg nature, as shown for the 1 1 A 2 ͑1b 1 → 3p y / 2b 2 ͒ state.…”

“…29,33,52 On the other hand, excitation to the second band has been established to lead also to photodissociation through a more complex mechanism 33,51,52 than in the lowest absorption band. 24 As shown in Table III, the MS-CASPT2 results predict the 2 1 A 1 state to be situated vertically at 9.86 eV above the ground state and support its assignment to the second band of the spectrum. Mota et al 4 have recently reported an experimental value of 9.991 eV for the origin of the Rydberg series arising from excitation from the 3a 1 orbital.…”

“…11,14,23 The band is broad ͑6.8-8.2 eV͒, with poorly defined vibrational progressions, and has its maximum absorption around 7.4 eV. 4 These features result from direct photodissociation of water into ground-state H· and OH· radicals, as shown experimentally 24,25 and theoretically. 26 There is a unanimous agreement in assigning the lowest-energy band to the 1 1 B 1 state, usually named à 1 B 1 state according to Herzberg's notation, 27 but not in the valence, Rydberg, or mixed valence-Rydberg nature of the state.…”

“…33 Some of the studies 31,34 relate the contribution of the antibonding valence 4a 1 orbital to the lowest singlet excited state, Ã 1 B 1 , to the dissociative nature of the state. 24,25 However, the character of the low-lying excited states as function of the O-H internuclear distance has not been analyzed and theoretical efforts 32,33 have been mainly focused on determining the low-lying potential energy surfaces of water related to photodissociation processes of the molecule. We report here a high-level ab initio study of the singlet and triplet excited states of water by using multiconfigurational second-order perturbation theory through the CASPT2 method.…”

Isotope effects in the photofragmentation of symmetric molecules: The branching ratio of OD∕OH in water

“…A simple example of a photoinduced reaction with chemically distinct products is the two-body dissociation of HOD in the first absorption band: 4 HOD → ͭ H + OD D + OH ͮ .…”