Photoionization Dynamics Studied by Coincidence Spectroscopy

Abstract

Photoionization studies may be subdivided into two fields of interest, structure and dynamics. The first sub field is the domain of all kinds of spectroscopic investigations, in particular, high resolution ion yield studies of resonances, whereas the second sub field is concerned with electron spectrometry at different levels of differentiation. However, both field are closely linked to each other because resonance behavior in the photoionization continuum is directly affected by the dynamics of the continuum electron due to the interference between bound and free electrons above the first ionization threshold. Nevertheless, the two fields concentrate on different aspects of the photoionization process. Whereas structural studies have the ambition to achieve the best resolution available, dynamical studies depend more on the photon flux in a certain energy range and on a high degree of linear polarization of the ionizing radiation. This is because dynamical studies require the determination of more parameters than just the photoionization cross section. In order to derive these parameters unambiguously, the target and/or the products of the photoionization process have to be prepared and/or analyzed in more detail. An feasible way to do this is the spin-sensitive detection of the emitted photoelectron, a method which has been pioneered since more than a decade by Ulrich Heinzmann and is, in fact, the method which provided most of the presently known dynamical photoionization data beyond the photoelectron angular distributions [1]. However, there are other methods for such dynamical studies, e. g. the electron spectroscopy of polarized atoms [2] and oriented molecules [3]. Both are just starting to become more routinely used methods as spectroscopic tools for a variety of applications in dynamical studies. A third method suitable for the investigation of dynamical properties is coincidence spectroscopy, in particular angle-resolved electron-electron coincidence spectroscopy [4].