Relative Experimental Cross Sections for Excitation ofBa+Ions by Electron Impact (8.0-98 eV)

Abstract

A crossed beam technique has been developed for measuring the emission cross section for the excitation of electric dipole transitions in ions by electron impact. The experimental method involves crossing modulated ion and electron beams in a well-defined collision volume. A portion of the photon flux radiated from the excited ions is detected at an angle of 90\ifmmode^\circ\else\textdegree\fi{} to the plane of the two beams by direct observation of the collision volume with a photomultiplier tube. The selection of a single emission line is accomplished with an interference filter. The total flux of radiation from the collision volume for a particular transition can be determined with a knowledge of the photon detection probability of the apparatus. The possibility of an anisotropic angular distribution of radiation from the collision volume is evaluated by measuring the polarization fraction. The emission cross section can be determined from the ion and electron beam currents, current density distributions and energies, and the total radiation flux.This technique has been used to measure the relative emission cross sections for excitation of the resonance transitions in ${\mathrm{Ba}}^{+}$ ions by electron impact. The resonance transitions, between the excited $6^{2}P_{\frac{1}{2}}^{ \ensuremath{\circ}}$ and $6^{2}P_{\frac{3}{2}}^{ \ensuremath{\circ}}$ levels and the $6^{2}S_{\frac{1}{2}}$ ground state, produce photons with wavelengths of 4934 and 4554 \AA{}. The thresholds for exciting the $6^{2}P_{\frac{1}{2}}^{ \ensuremath{\circ}}$ and $6^{2}P_{\frac{3}{2}}^{ \ensuremath{\circ}}$ levels are 2.5 and 2.7 eV, respectively. Over the energy range of the experiment, from 8 to 98 eV, the relative cross sections differ by approximately a factor of two, the ratio of the statistical weights of the $6P$ levels. The relative data also exhibit the high energy dependence predicted by the Bethe-Born approximation. From a best estimate of the photon detection probability of the apparatus, estimated absolute cross sections are obtained which agree remarkably well in magnitude and shape with the theoretical predictions. Checks on the data were performed to evaluate the possible effects of such parameters as the beam intensities, beam modulation frequency, ion beam composition, ion beam energy, electron beam energy distribution, beam profiles, and signal-to-noise ratio.