Excitation of the Ba andBa+resonance lines by electron impact on Ba atoms

The relative optical excitation functions and polarizations of the Ba resonance line (5535 Å) and of the ${\mathrm{Ba}}^{+}$ resonance lines (4554 and 4934 Å) have been measured, using crossed electron and barium-atom beams. The $\mathrm{Ba} 6{}_{}{}^1{}_{}{}^{}P$ excitation function has been normalized to the Bethe theory in the high-energy limit, and the excitation functions of the ionic lines have been measured relative to that of the atomic resonance line. Using spontaneous-emission branching ratios for the ${\mathrm{Ba}}^{+}\left(6\mathrm{}{}_{}{}^2{}_{}{}^{}P\right)$ levels, we have also obtained normalized cross sections for these ionizing excitation levels. From a few electron volts above threshold, the ${\mathrm{Ba}}^{+}\left(6\mathrm{}{}_{}{}^2{}_{}{}^{}P\right)$ ionizing-excitation cross section has nearly the same energy dependence as has been reported for the total ${\mathrm{Ba}}^{+}$ ionization cross section, but about 1/6 the magnitude. In particular, a broad maximum at ∼22 eV, attributed to core-excited autoionizing levels, apparently contributes an equal fraction to both cross sections. This relationship holds also for the ionizing excitations versus total ionization of Sr. The cross sections for exciting the resonance levels of He, Mg, Ca, Sr, and Ba are also intercompared in reduced units and found to be strikingly similar at collision energies above ∼3 times the threshold energies.