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Electron impact ionisation and excitation of Laser prepared atomic targets

John Chidowerem Agomuo, I. Williams, H. W. van der Hart, J F McCann, Derrick S. F. Crothers

Published Nov 5, 2012• 0 citations
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Abstract

A review of current work at Manchester is given, with emphasis on low energy electron impact ionization & excitation experiments from laser-prepared targets. Three different methods are considered: super-elastic scattering from targets prepared by laser radiation in an optical enhancement cavity, (e,2e) studies of laser-aligned atoms, and electron impact ionization studies from laser cooled atoms in a new type of atom trap - the AC-MOT. The status of each experimental programme is detailed, and new data presented. Future directions and techniques are then discussed.

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A review of current work at Manchester is given, with emphasis on low energy electron impact ionization & excitation experiments from laser-prepared targets. Three different methods are considered: super-elastic scattering from targets prepared by laser radiation in an optical enhancement cavity, (e,2e) studies of laser-aligned atoms, and electron impact ionization studies from laser cooled atoms in a new type of atom trap - the AC-MOT. The status of each experimental programme is detailed, and new data presented. Future directions and techniques are then discussed.
A review of current work at Manchester is given, with emphasis on low energy electron impact ionization & excitation experiments from laser-prepared targets. Three different methods are considered: super-elastic scattering from targets prepared by laser radiation in an optical enhancement cavity, (e,2e) studies of laser-aligned atoms, and electron impact ionization studies from laser cooled atoms in a new type of atom trap - the AC-MOT. The status of each experimental programme is detailed, and new data presented. Future directions and techniques are then discussed.
In this brief review a number of different experimental techniques that combine the advantages of laser and electron interactions with atoms and molecules will be described. These processes range from super-elastic electron scattering from laser excited atoms so as to ascertain the electron collision parameters over all scattering geometries, through new electron interactions with laser cooled and trapped atoms, to the process of electron impact ionization of laser-aligned targets. Examples from work carried out in Manchester are given, and new types of measurements that may be possible in the future are described. © 2010 IOP Publishing Ltd.
The key features of the cross sections for electron-impact excitation out of the ground levels of the rare-gas atoms are summarized and application of these cross section data is shown. This is followed by discussions of recent experiments on determination of electron-impact cross sections for excitation out of the metastable levels of He and Ar. The metastiable atom targets are produced by two different methods, i.e., hollow-cathode discharges and charge-exchange collisions of rare-gas ions with Cs atoms. The former method is simple but is limited to low electron energies because of the very small metastable concentration, whereas the latter method yields a fast target beam with predominantly metastable atoms allowing measurements up to the keV range. The measured cross sections show features very different from those of excitation out of the ground levels. Another new development in electron-atom collision is the use of atom traps for measuring ionization cross sections. The experiment is conducted by passing an electron beam pulse through magneto-optically trapped Rb atoms while the trap is momentarily turned off. Turning the trap back on immediately after the electron beam pulse recaptures the unionized atoms allowing only the ions to escape. The ionization cross section is determined bymore » measuring the electron beam current density at the trapped atom region and the fractional loss of trapped atoms due to the electron beam. This method avoids measurements of the absolute number of the target atoms and the overlap between the atomic and electron beams and therefore eliminates the major sources of uncertainty associated with the crossed-beam method for measuring ionization cross section.« less
Recent data from two methods in which high resolution laser radiation is used to assist in determining electron-atom collision parameters are presented. The electron superelastic method has yielded the first measurement of Stokes parameters for electron de-excitation of the 32D5/2–32P3/2,1/2 transition of atomic Na, the upper level having been optically prepared by resonant, stepwise excitation from the 32S1/2 ground level via the 32P3/2 level using two single mode lasers. As well, we report on the development of a model to determine the optical pumping parameters for superelastic scattering from the 32P3/2 level when it is prepared by two lasers exciting from the F = 1 and F = 2 states respectively of the 32S1/ 2 ground level. Data are also presented for collision parameters for the excitation of the 61So–61 PI transition of the I = 0 isotope of Hg by electrons of 50 eV incident energy. The technique employed for these measurements is the stepwise electron–laser excitation coincidence method, in which the electron excited atom is further excited by resonant laser radiation, and fluorescence photons emitted by relaxation from the laser excited state are detected in coincidence with the scattered electron.
An experiment is described that studies electron collisions with laser-excited atoms inside a resonant optical enhancement cavity. A confocal cavity is used to significantly increase the laser power at the interaction region, so that experiments become possible with low-power continuous-wave lasers, particularly at blue and UV wavelengths. The increased power inside the cavity allows previously inaccessible targets and states to be investigated. The method is tested using superelastic scattering experiments from laser-excited calcium at 423 nm for an electron energy of 45 eV, and data are presented at 100 eV using the enhancement cavity.