Photoionization Studies of Reactive Intermediates of Importance in the Atmosphere

Abstract

Studying a reactive intermediate in the gas-phase with photoionization provides an important way of obtaining information on neutral and ionic bond energies and cationic vibrational constants, as well as more generally of providing insight into its electronic structure and reactivity.

This presentation provides an overview of the information that can be obtained by taking recent examples of research investigations from the Southampton PES group, notably

• (i) Preparation of reactive intermediates for study using gas-phase reactions, taking the ClO and BrO radicals as examples. A review is given of the use of atom-molecule reactions to prepare reactive intermediates for study by PES, the experimental steps which are necessary to establish the presence of a reactive intermediate, and the importance of electronic structure and Franck-Condon factor calculations in assigning the PE spectral bands to a particular reactive intermediate.

• (ii) Measurement of reaction rate coefficients, using the atmospherically important reaction Cl₂ + DMS as an example. A summary is given of how a flow-tube can be interfaced to a photoelectron spectrometer to obtain rate coefficients of bimolecular reactions. This is exemplified by considering the reaction Cl₂ + DMS. This reaction proceeds via a reactive intermediate, Me₂SCl₂, and the final products are CH₃SCH₂Cl + HCl. The reaction rate coefficient has been determined at room temperature and the implications of the value obtained to atmospheric chemistry are considered. Also, the structure of the reactive intermediate has been established. It can be thought of as a trigonal bipyramidal structure with the Cl atoms in axial positions, and the methyl groups and the S lone pair in equatorial positions.

• (iii) Study of reactive intermediates of atmospheric importance with synchrotron radiation, using atomic nitrogen as an example. The advantages of using synchrotron radiation to study reactive intermediates compared to using a fixed frequency photon source from an inert gas discharge are summarised. The extra information to be obtained from angularly resolved constant-ionic-state (CIS) and threshold photoelectron spectroscopy are illustrated by considering recent studies on atomic nitrogen, a reactive intermediate which is important in the upper atmosphere.