Charge Transfer in Collisions and in Complexes Involving Excited Atoms

Abstract

In the understanding of chemical reactions the transfer of energy or charge between atoms or molecules plays an important role. In general, atoms or molecules must be in excited states for a transfer reaction to take place. Also the orientation and alignment of the electronic charge clouds will influence the probability for such processes. Initial electronic excitation is most easily achieved by photon absorption, using lasers or synchrotron radiation sources. This leads us to a second approach: Photophysical reactions, i.e. processes initiated by photon absorption, are fundamental in every living substance. The detailed dynamics of such reactions are far from being understood. Basic questions that have to be asked are: Where does the energy go? How is it transferred? Are photons reemitted? Are free charges generated? Are electrons or holes or protons exchanged? Which are the reactants? Which steps are involved? What is the timescale? etc. The path to follow in order to eventually reach a “detailed understanding” is to study a few representative cases on simple systems like isolated molecules or atoms. In experiments, we can fairly well define internal states and the relative kinetic energy of the reactants and products by combining modern techniques. To illustrate the state of the art we would like to discuss three representative processes: Associative Ionisation, Resonant Charge Exchange and Intramolecular Penning Ionisation. However, the discussion on associative ionisation is deferred in this contribution and the reader is referred to the paper by J.Weiner /1/ on this topic.