Ionization Ion-Atom Collisions: Recoil-Ion Momentum Spectroscopy and Ejected Electron Spectroscopy

Abstract

In the last few years there has been a very intense effort to understand the process of single ionization in ion-atom collisions. Experiments in this field have followed largely along two main lines; recoil-ion momentum spectroscopy and ejected electron spectroscopy. Both techniques can address distinct features characterizing ionization processes in ion-atom collisions. At this stage it is essential to understand the complementary nature of both recoil-ion momentum spectroscopy and ejected electron spectroscopy. Each technique provides detailed information on the ionization mechanism which can serve as a most stringent test for theory. The aim of this paper is to discuss the non-perturbative quantum mechanical models used most commonly in the description of ion-atom ionization collisions. From the theoretical point of view the main difficulty is the representation of the final electronic state, where the ionized electron travels in the presence of two Coulomb potentials (target and projectile). Due to the long-range ionic tail of the Coulomb potential the ”free particle” cannot be represented by a plane wave. So far an exact solution to this problem has not been found. However an exact asymptotic form can be obtained exactly1,2 and the main objective of this paper is to review the continuum-distorted-wave3,4 (CDW) and continuum-distorted-wave eikonal-initial-state1,2 (CDW-EIS) models which at least satisfy the exact asymptotic conditions in the initial and final states. Both the approximations are based on distorted wave perturbation theory. The difference between CDW and CDW-EIS theory lies in the distortion of the initial state, which is a Coulomb wave in the former and an eikonal phase in the latter. First we will discuss the applicability of these models for kinematically complete experiments on target single ionization in ion-atom collisons which have been performed using the tecnique of recoil-ion momentum spectroscopy. The examples illustrated will include the pioneering experiments5,6,7 of 3.6 MeV/u Ni24+, Se28+, Au24+ and Au53+ ions on helium target atoms. Secondly we are interested in the electron spectroscopy method which allows investigations on the two-centre effects that influence electron emission. The strength of this particular experimental technique lies in its ability to measure the doubly differential cross sections (DDCS) as a function of the electron emission angle and energy. There are two distinct characteristics based on two-centre electron emission which can be easily identified in this spectrum. The first is electron capture into the projectile continuum (ECC mechanism). Here the DDCS shows a cusp at an electron velocity which matches the projectile velocity. The second is the binary encounter (BE) mechanism which can be identified as a peak, centred around an electron velocity which is twice the projectile velocity. It has also been suggested that another signature, called the saddle point emission mechanism8 arises from the possibility that the emitted electron is stranded on the the saddle point of the two-centre potential between the target ion and receding projectile. This mechanism leads to an electron distribution centred at a velocity close to half the projectile velocity. We consider the applicability of the CDW and CDW-EIS to predict these ionization features in the DDCS. The examples illustrated include the measurements from Lee et al9 of 1.5 MeV/u H+ and F9+ on helium and measurements from Shah et al10 of 50 keV H+ on H2 and 75 keV H+ on helium. We begin section 2 with a brief description of the essential equations for the various ionization cross sections. In section 3 we compare the theoretical calculations with the recoil-ion momentum spectroscopy and electron spectroscopy techniques. Finally we summarize our results in section 4.