Electron and X-ray spectroscopy of electron-atom collisions

Abstract

Electron-ion and x-ray-ion coincidence techniques have been used to measure the relative values of double-differential cross- sections for n-fold ionization DDCS(n+) , for helium, arqon, krypton and xenon atoms. In these experiments a focussed beam of energetic electrons is crossed with a dilute beam of thermal gas atoms. The electron- atom interaction can produce ionization of the atom when the incident electron energy is greater than its ionization potential. In electron-ion coincidence experiments the electrons ejected from an atom as a result of the ionization are energy analysed in a 30o parallel plate electrostatic analyser and are detected in coincidence with the product ions which are also analysed with respect to charge by a time-of-flight (TOP) type analyser. The delay time of the ions with respect to the detected electron gives information about the charge state of the ions. From these delay time spectra true coincidences are measured for every charge state n up to n = 9 to determine (Chaudhry et al. 1906; Hippler et al. 1984b) relative values of DDCS(n+) as a function of the detected electron energy and the incident electron energy. These values have been compared with other experimental data as well as theoretical values from literature where possible. In x-ray-ion coincidence experiments with xenon atoms, x-rays produced as a result of the de-excitation of the ionized atoms are detected with a liquid nitrogen cooled hyperpure germanium (HPGe) detector, in coincidence with the product ions which are analysed by a TOF type analyser. The time delay of the detected ions with respect to the detected x-ray gives information about the charge states of these ions. A Bragg type crystal x-ray spectrometer (Jitschin et al, 1984) has also been set up for high resolution x-ray spectroscopy. In this instrument collimated x-rays are specularly reflected from a plane crystal which is rotated by a micro-computer -controlled stepping motor. A constant gas flow type proportional counter with a large thin window monitors the reflected x-rays. Pulses from the proportional counter are fed to an MCA in HCS mode which is also controlled by the same micro-computer. An x-ray spectrum can be built up in the MCA giving about 10-15 times better resolution than the HPGe detector in the region of the characteristic x-rays emitted by the ionized rare gas atoms.