Photodissociation of water

The photodissociation of water has attracted considerable experimental and theoretical interest (Schinke, 1993 Lee, 2003) owing to the simplicity of its electronic structure and proposals that it played an important role in the evolution of the Earth s atmosphere, and other planetary atmospheres. Most of the laser-based studies have involved multiphoton excitation techniques, as the first absorption band lies in the VUV. However, a few studies have employed a VUV laser for the photodissociation step, and almost all recent 

Laser Chemistry Spectroscopy, Dynamics and Applications Helmut H. Telle, Angel Gonzalez Urena Robert J. Donovan 2007 John Wiley Sons, Ltd ISBN 978-0-471-48570-4 (HB) ISBN 978-0-471-48571-1 (PB) 

Selective bond breaking has been demonstrated with HOD by first exciting the fourth overtone (local mode) of the OH bond and then photodissociating the molecule via the A X transition. The A — X transition is red shifted (hot-band absorption) into the 240-270 nm region and the dissociation of the OH bond, relative to the OD bond, is enhanced by a factor of 15. This type of process is referred to as vibrationally mediated photodissociation and can be a very effective approach, provided the initial vibrational excitation remains localized in one chemical bond for a sufficient length of time to allow further excitation and dissociation. In the case of HOD it is clear that randomization of the vibrational energy is slower than the photodissociation step, and this further emphasizes the direct and impulsive nature of dissociation on the A Bi-state PES. 

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In order to calculate the steady-state concentration of ozone in the stratosphere, we need to balance the rate of production of odd oxygen with its rate of destruction. Chapman originally thought that the destruction was due to the reaction O + 03 —> 2O2, but we now know that this pathway is a minor sink compared to the catalytic destruction of 03 by the trace species OH, NO, and Cl. The former two of these are natural constituents of the atmosphere, formed primarily in the photodissociation of water or nitric oxide, respectively. The Cl atoms are produced as the result of manmade chlorofluorocarbons, which are photodissociated by sunlight in the stratosphere to produce free chlorine atoms. It was Rowland and Molina who proposed in 1974 that the reactions Cl + 03 —> CIO + O2 followed by CIO + O —> Cl + O2 could act to reduce the concentration of stratospheric ozone.10 The net result of ah of these catalytic reactions is 2O3 — 3O2. 

This means that the photoelectron is transferred to an electron acceptor concomitantly with trapping of the photohole by an electron donor (Fig. 10.1). Semiconductor materials have been tested as photocatalysts for the photodissociation of water. Fig. 10.4 shows the energetics in terms of standard redox potential of some semiconductors as compared to the standard redox potential of H2/H+ and H20/02 at pH 0. 

According to the energetics shown in Fig. 10.4, Fe203 cannot be used as a photocatalyst for the photodissociation of water. However, it has been reported that... 

Balint-Kurti, G.G. (1986). Dynamics of OH A-doublet production through photodissociation of water in its first absorption band. I. Formal theory, J. Chem. Phys. 84, 4443-4454. 

Vander Wal, R.L. and Crim, F.F. (1989). Controlling the pathways in molecular decomposition The vibrationally mediated photodissociation of water, J. Phys. Chem. 93, 5331-5333. 

Weide, K., Kiihl, K., and Schinke, R. (1989). Unstable periodic orbits, recurrences, and diffuse vibrational structures in the photodissociation of water near 128 nm, J. Chem. Phys. 91, 3999-4008. 

FIGURE 2 Vibrationally mediated photodissociation of water. (Provided by the courtesy of Fleming Crim.)... 

There are now a substantial number of nitrido complexes of osmium(VI) and osmium(IV) as well as the osmiamate ion [OsVII 03N]. In addition to the ammine and ethylenediamine complexes, much recent work has been carried out on the bipy, phen and terpy complexes, often in connection with research into the photodissociation of water. The nitrosyl chemistry of the element, though seemingly not as extensive as that of ruthenium, has received much attention, and there has been considerable work on the phosphine, arsine and stibine complexes. 

Evidence of Photodissociation of Water Vapor on Reduced SrTiCh (III) Surfaces in a High Vacuum Environment. First report of photodecomposition of water adsorbed from the gas phase in high vacuum conditions on metal-free, reduced single crystals. 396... 

An alternative way to achieve the photodissociation of water consists in the use of aqueous suspensions of powdered or colloidal semiconductors, in general loaded with noble-metal and/or noble-metal-oxide catalysts which act as short-circuited photoelectrolysis cells. Titanium dioxide was certainly (and is still being) the semiconductor most frequently employed in such systems. 

During the thermally driven differentiation of the Earth into core-mantle-crust, numerous reactions would have produced oxidized forms of iron, sulfur and carbon. These would have contributed to the redox chemistry in the early planet development. Volcanic and hydrothermal emission of sulfur dioxide, SO2, delivered oxidants to the oceans and atmosphere. Photodissociation of water vapor in the atmosphere have undoubtedly provided a small but significant source of molecular oxygen. Furthermore, UV-driven ferrous iron oxidation could have been coupled to the reduction of a variety of reactants, for instance, CO2 (Figure 16). 

At this time in the Earth s history the carbon dioxide abundance was higher than its present value since this gas accumulated in the absence of a biosphere. Rutten (1966) speculated that the atmospheric C02 level was 10 times PAL (present atmospheric level) about 3 x 109 years ago. On the other hand the presence of H20 in the atmosphere led, by photochemical dissociation, to the formation of free radicals and molecular oxygen. An estimate of the importance of these reactions is necessary to give some idea of the oxygen level in pre-biospheric times. The photodissociation of water vapour can be represented as follows (Suess, 1966) ... 

Thus it should be stated in spite of the fact that some free oxygen could be formed in the pre-biologic atmosphere by the photodissociation of water vapour, its present level cannot be explained by this process alone. 

Figure 4-40- Contributions of different spectral regions to the photodissociation of water vapor for an overhead sun.

Based on the dye-sensitized redox reactions, one can visualise two other cycles (designated C1 and C2, following Balzani) for the photodissociations of water. In C1 cycle, a visible light absorbing sensitizer S (a metal complex, for example) is photo-oxidised to a higher oxidation state which, in turn, oxidises water to 02 in the dark. The reducing equivalents are stored as a stable reduced acceptor QH or QH2. 

In this section we will focus our attention on experimental studies aimed at the photodissociation of water in homogeneous solutions. In the absence of detailed knowledge on... 

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