Photodissociation dynamics and atmospheric chemistry
Authors: Wayne R. P. 
Published: February 2008

Chemistry in most atmospheres is ultimately driven by photochemical processes. In general, the knowledge taken over from laboratory systems for the interpretation of atmospheric phenomena has been about the chemical channels available in photodissociation processes, the quantum efficiencies for those channels, and how the quantum efficiencies change with wavelength of photolysis. It is, however, evident that the energy disposal in the fragments as well as their chemical identity may have a profound influence on many aspects of atmospheric chemistry. Electronic excitation clearly affects chemical reactivity, and emission from electronically excited states contributes to the atmospheric airglow of many planetary atmospheres, which often provides deep insight into the nature of the chemistry occurring. Vibrational excitation is now seen increasingly as of importance in various kinds of atmospheric chemistry where vibrational equilibration is relatively slow. An understanding of the nascent energy distributions in the products of photodissociation is thus turning out to be rather important for the detailed interpretation of atmospheric processes. The newer kinds of laboratory study on photodissociation focus on the dynamics of the processes. Although at first sight they do not seem of relevance to atmospheric studies, the detailed information that they provide is of considerable significance. In this paper, attention is given to the relationship between the modern laboratory studies and the needs of atmospheric chemistry. As a main illustration, the molecule ozone is chosen. Some of the most detailed and fascinating of the laboratory studies have been conducted on it. A triatomic molecule such as O3 provides the simplest example where fragmentation yields a molecule as well as an atom. Furthermore, O3 is important not only in Earth's atmosphere, but also in that of Mars. Its dissociation to O(1D) contributes to some of the odd hydrogen chemistry, and the O2(a1Δg) molecular fragment contributes to the Martian airglow. Photofragment energy analysis and coherent Raman studies have provided information about the nascent vibrational and rotational states that supplements the data about electronic states obtained by more conventional techniques. The results give enhanced insight into the dissociation process. Not only does the increased understanding of the dynamics of dissociation help to interpret the factors that control the efficiency of photolysis, but the data about vibrational and rotational energy releases may themselves help interpret some features of atmospheric behaviour. The latest studies probe the ozone molecule as it is in the act of falling apart on the femtosecond time scale, and reveal the nature of the upper electronic surface on which dissociation occurs and other details of the dissociation. There are apparently some similarities in the dissociation of CO2, which is, of course, of key importance on Mars and Venus. Experiments probing the translational energies of the O and the vibrational and rotational distributions in the CO suggest that a spin‐forbidden channel operates as it does in ozone photolysis. Once again, the data suggest a relationship between the structure of the parent molecule and the dynamics of dissociation. Molecular species present in the atmospheres of the outer planets have also been investigated by the more modem techniques. Even photolysis of CH4 seems to yield fragments (CH3 + H) other than those hitherto thought to be the major ones (CH2 + H2 or 2H). Some discussion will also be presented of the photodissociation of NH3 and of C2H2. In the former case, both translational energies in the H fragment and the internal excitation in NH2 have been used to infer how the molecule dissociates. Most of the work on C2H2 has looked at the H atom, but some recent experiments purport to have discovered laser induced fluorescence from C2H. Results from some of these experiments are used to show how further information needed for atmospheric studies may eventually be won.