The Photochemistry of Ozone

Summary

Ozone plays a major role in the atmosphere, important chemical transformations in the troposphere, stratosphere, and mesosphere all being initiated by the absorption of radiation by ozone. This article surveys laboratory data concerning the photochemistry of ozone, and indicates the relevance of the laboratory studies to interpretations of atmospheric chemistry.

The material is introduced by a brief survey of the most important atmospheric processes. The structure, spectroscopy, and excited states of ozone ultimately control the photochemistry of the molecule, and they are discussed in this context. Photofragmentation itself is the subject of the main parts of the article. Emphasis is placed on the nature of the electronic states of the atomic and molecular oxygen fragments of photolysis, and the efficiencies with which the various species are formed. The primary quantum yield for O(¹D) formation is certainly less than unity for λ < 274 nm, and it may thus also be less than unity in the atmospherically critical region around λ = 300 nm. Similar considerations are likely to apply to the efficiency of formation of excited singlet molecular oxygen, O₂(¹Δ_g). On the other hand, O₂(¹Δ_g) seems to be formed with high efficiency at wavelengths longer than the λ ≃ 310 nm threshold for O(¹D) production. Calculations of atmospheric [O₃] that depend on measurement of the intensity of the O₂(¹Δ_g → ³Σ _g ⁻ ) Infrared Atmospheric Band may therefore be in error both if they assume a quantum yield of unity for O₂(¹Δ_g) production at λ < 310 nm and if they assume that the quantum yield is zero at longer wavelengths. The wavelength dependences of the quantum efficiencies are interpreted in terms of the spectroscopy of ozone; evidence for the breakdown of simple spin conservation arguments is presented, and some explanations for the behaviour are suggested. Consideration is given to the occurrence of isotope-selective chemistry in the photolysis of ozone that might give rise to the observed enrichment of ⁵⁰O₃ in the atmosphere.

Vibrationally-excited ozone is implicated in both atmospheric processes and in laboratory experiments, and the vibrational photochemistry of ozone is explored here. Optical emission may arise from recombining O+O₂, and the energy-rich ozone formed may show enhanced chemical and photolytic activity.

An understanding of the details of the photodissociation of ozone, and thus the development of reliable predictive models, depends on a knowledge of the dynamics of photodissociation. Photofragment energy analysis and coherent Raman studies have provided information about the nascent vibrational and rotational product distributions that supplements the data about electronic states obtained from more conventional techniques. Photofragmentation appears to be rotationally impulsive and vibrationally adiabatic. A propensity for even-J rotational states in the O₂(¹Δ_g) product for photolysis by ultraviolet radiation is shown to result from radiationless transition to a surface that yields O(³P)+O₂(³Σ _g ⁻ ) for ca. 10% of the reaction, but only for odd-J levels in O₂. This result is entirely consistent with the measurements of the quantum yield for O(¹D) formation. Fluorescence techniques have been used to probe the ozone molecule as it is falling apart on the femtosecond time scale. The vibrational intensities reveal the nature of the upper electronic surface on which dissociation occurs and of the earliest stages of the dissociation itself. Results from some of these experiments are used to show how further information needed for atmospheric studies may eventually be won.