J. P. Sullivan, D. S. Newman, P. D. Burrow*, J. A. Michejda*
and S. J. Buckman

Atomic and Molecular Physics Laboratories, Research School of Physical Sciences and Engineering
Australian National University, Canberra, ACT, 0200
stephen.buckman@anu.edu.au

*Department of Physics and Astronomy, University of Nebraska
Lincoln, Nebraska 68588, USA

Temporary negative ion states, or resonances, occur in electron-atom scattering when the incident electron attached to the target atom for a time which is long compared to the direct scattering time. They subsequently decay by electron ejection, leaving the neutral atom in the ground or excited state. Resonances have been extensively studied in the scattering cross sections of the rare gases and a few other elements spread throughout the periodic table. Various schemes have been proposed for their classification (see Buckman and Clark 1994 for a comprehensive review). The present paper discussed a collaborative study of resonances observed in electron-metal vapour (Cd, Zn) scattering using results obtained from electron transmission and crossed beam experiments at low impact energies.

Electron transmission experiments carried out at the University of Nebraska measured the derivative of the transmitted current as a function of electron energy when a high resolution electron beam passes through a chamber containing the metal vapour. This technique derives a signal related to the total electron scattering cross section and provides, with high sensitivity, a measurement of the positions and profiles of resonances. However, no information regarding the decay channels cam be extracted from the spectrum. The crossed beam apparatus at te ANU employed a high resolution, low energy electron beam crossed at right angles with a metal vapour emanating from a high temperature oven. Three types of decay channels were observed: elastically scattered electrons over the angular range of 24 - 122 degrees, metastable atom excitation functions and various optical excitation functions for neutral atomic excited states.

Classifications of some resonances observed below the 2S1/2 ionisation potentials of both atoms are proposed from an analysis of the experimental data utilising extrapolations of isoelectric series, comparison with analogous electron configurations in the extensively studied Hg- system, and predictions of energy levels from modified Rydberg formulae.

References
Buckman, S.J. and Clark, C.W. 1994 Rev. Mod. Phys., 66, 539-655.