Cross sections for electron excitation of the ${2\mathrm{t}}^3$S metastable level of He into higher triplet levels

Cross sections for electron excitation out of the 2${\mathrm{}}^3$S metastable level of He into the 2${\mathrm{}}^3$P, 3${\mathrm{}}^3$S, 3${\mathrm{}}^3$P, 3${\mathrm{}}^3$D, 4${\mathrm{}}^3$S, 4${\mathrm{}}^3$P, 4${\mathrm{}}^3$D, 5${\mathrm{}}^3$S, and 5${\mathrm{}}^3$D levels have been obtained for energies up to 18 eV. We have observed a broad excitation function for the 2${\mathrm{}}^3$P level with a peak apparent cross section of 1.2×${10}^{\mathrm{-}14}$${\mathrm{ncm}}^2$. For the n=3, 4, and 5 levels, the excitation functions show a pattern of sharp peaks for excitation into the n${\mathrm{}}^3$S levels, slightly less sharp peaks for excitation into the n${\mathrm{}}^3$P levels, and relatively broad peaks for excitation into the n${\mathrm{}}^3$D levels. Absolute cross sections have been obtained for all the above mentioned levels using a laser-induced fluorescence technique and the results agree well with experimental values reported by Lagus et al. [Phys. Rev. A 53, 1505 (1996)] The cross sections for the 2${\mathrm{}}^3$S→n${\mathrm{}}^3$P excitations which correspond to dipole-allowed optical transitions are smaller than the corresponding 2${\mathrm{}}^3$S→n${\mathrm{}}^3$S and 2${\mathrm{}}^3$S→n${\mathrm{}}^3$D excitation cross sections, in contrast to the trends observed for excitations out of the ground level. This reversal behavior is discussed in terms of the dipole matrix element sum rule. Our cross-section data are compared with those of the alkali-metal atoms.