We present convergent close-coupling (CCC) calculations and measurements of 26.5 eV electron-impact excitation of the ground state of helium. The present measurements extend the experimental information at this energy to include $3^{3}D$ excitation. The CCC theory is compared with all of the available $2^1{P,3\mathrm{}}^1{P,3\mathrm{}}^1{D,3\mathrm{}}^{3}D$ measurements, the agreement with which is very satisfactory and comparable to that at higher impact energies.

• Received 23 May 1997

DOI:https://doi.org/10.1103/PhysRevA.56.4606