Absolute experimental cross sections for the excitation of barium ions by electron impact

The crossed-beam technique has been used to measure absolute emission cross sections for the excitation of electric dipole transitions in the Ba⁺ ion by electron impact at energies near and below threshold. The basic experimental method involves intersecting modulated ion and electron beams at right angles in a well-defined collision volume. A portion of the photon flux radiated from the collision volume by excited ions is detected by an optical system viewing the collision volume along the axis perpendicular to both beams. The total emission is calculated from the portion detected using the experimentally determined response of the detection system to isotropic radiation and a correction for anisotropy, obtained from the polarization of detected emission. The emission cross section is then calculated from the total emission, the two beam currents and their spatial distributions, and the energies. The cross sections measured are absolute in the strict sense that the optical detection system was calibrated with a light standard by a technique which takes into account all spatial and spectral variations. A stable point source of light has been devised to determine the variation of the detection-system response with the position in the collision volume where the photons are produced. Another small light source has been developed for transferring to the detection system the calibration of a tungsten-ribbon standard of spectral radiance traceable to blackbody standards. Techniques of careful spectral comparison of the sources are used for this purpose. An electron source has been developed with the energy resolution and current necessary for crossed-beam measurements in the 3- to 20-eV range. This tool made possible the examination of the range of impact energies near and below the excitation threshold.