Electron excitation out of the 23S metastable level of He into high-n triplet levels

We report cross section results for electron excitation out of the 2(3)S metastable level of He into the 5(3)P, 6(3)S, 6(3)P, 6(3)D, 7(3)S, 7(3)P, 7(3)D, 8(3)S, 8(3)P and 8(3)D levels for energies from onset to 18 eV. For each level, we use the optical method to measure the apparent cross section, which equals the sum of the direct excitation cross section and the cascade cross section. The direct cross section is then determined for each level by subtracting the measured cascade cross section from the apparent cross section. The shape of the excitation function is Similar for levels of the same L and different n, but different for the S, P, and D families. The characteristic shapes for these three families closely resemble the corresponding ones for excitation out of the ground state of the Na atom. Trends of cross section versus principal quantum number, n, are also analysed and compared with theoretical predictions.