Ionization Processes and Ion Dynamics

BY I. POWIS

1 Introduction

Research into the basic processes in which ions are produced and interact continues to be an active and expanding area. Traditional methods for the generation and monitoring of ions are being extended and complemented, most notably by the introduction of new sources of ultraviolet radiation and molecular-beam technology. With such experimental advances new questions can be addressed and old problems tackled in fresh ways.

For the reviewer, however, the old problem of presenting a critical account of developments that are embodied in a large number of publications remains to be tackled, as before, by a selective citation of the literature. My objective, therefore, has been to identify work in which fundamental aspects of the behaviour of molecular ions are featured. Whatever criteria one employs, difficult choices remain to be made, and it is reasonable to acknowledge that in such circumstances the ultimate arbiter is perhaps a personal preference.

2 Ionization Processes

Molecular Photo ionization. — Of all the processes leading to positive-ion formation, photoionization is the best able to be characterized at a fundamental level in a general manner applicable to all systems. The pursuance of this aim is a vigorous, developing field at present, as regards both theoretical and experimental advances. Apart from the immediate advantage of being able to model photoionization processes such as are encountered in a wide variety of situations, photoionization represents the simplest electron-scattering process and is of considerable intrinsic interest. Studies of photoionization provide an important means for the investigation of molecular electronic structure and dynamics.

An idealized experimental objective would be the measurement of partial cross-sections for the production of specified quantum states of the molecularion products and the electron, as a function of photon energy and relative orientations. Indeed, most of these requirements are currently feasible and have been attained, at least for some specific systems. A full description of the photoelectron includes its spin polarization, and whilst this has previously been considered in atomic systems it is just now receiving attention in connection with molecular systems including, recently, the case of chiral molecules. Most attention, however, focuses upon the various resonances that appear with changing photon wavelength in measured cross-sections and, often even more markedly, in the photoelectron angular distributions. Although pseudo-photon dipole (e, 2e) experiments have been used to establish a substantial body of absolute photoionization cross-section data and continue to be used for systems such as HF and HCl, the bulk of recently published experimental data has been obtained using tunable synchrotron radiation sources. It is the consequent ability to scan continuously from below threshold up to photon energies of ~100eV with high intensity which allows the rich dynamical structure, apparent with highly differentiated cross-section measurements, to be uncovered.

Concomitant with the growth in quantity and quality of experimental data is the development of theoretical treatments for the understanding of such processes. The relationship between theory and experiment here appears to be particularly close, with approximately simultaneous treatments for a given system being quite common. Naturally enough, first-row diatomics such as HCl, N2, and O2 and triatomics such as N2O, CS2, and COS receive attention as theoretically tractable systems, but increasingly larger polyatomics can be tackled in this manner: examples include acetylene, cyanogen, ethylene, and even benzenoid compounds.

Photoionization resonances and structures are conveniently considered as either one-electron phenomena, such as shape resonances (i.e. quasi-bound states where an electron is temporarily trapped by a potential barrier), or two-electron phenomena such as autoionization (where photoionization takes place via excitation into a discrete bound state lying above an ionization threshold followed by decay into the ionization continuum).

As with other photoabsorption processes, photoionization cross-sections depend upon dipole transition amplitudes between initial and final states. The final-state wavefunction describes the continuum electron orbital, and herein lies the computational challenge. Although in principle obtainable from the solution of the Schrödinger equation using a static-exchange potential (i.e. at the Hartree–Fock level of approximation, appropriate for the description of one-electron phenomena), significant difficulties arise in the calculation in this manner of the continuum function. This is due primarily to the non-central nature of the potential in a non-spherical molecule and the inapplicability of the algebraic variational methodology commonly applied to bound-state problems.

One of the more generally successful approaches has been the multiple-scattering method (MSM), in which these difficulties are circumvented by representing the molecular potential as a cluster of spherical potentials positioned on the atomic sites. The MSM also often employs a local exchange approximation. A recent extension to this approach covers the evaluation of the interstitial integrals encountered with such a model potential. The MSM potential is clearly very approximated; nevertheless the MSM formalism is routinely capable of producing qualitatively correct predictions of such features as shape resonances. It thus fulfils its aim of providing a valuable tool for exploratory investigation of these phenomena. A particular advantage of MSM at the present time is its applicability to large molecules such as benzene. In this particular example calculated asymmetry parameters were found to agree with experiment for the first two ionizations, (1e1g)-1 and (3e2g)-1. A disagreement for the third (la) channel was ascribed to vibronic mixing effects, which are not reproduced in the calculations.

Thiel has demonstrated how shape resonances may be visualized by means of radial-density plots derived from MSM continuum...