Electron impact differential and integral cross sections for excitation of the n = 2 states of helium at 29.2 eV, 39.2 eV and 48.2 eV

A high resolution electron impact spectrometer using electrostatic monochromators and the cross-beam technique is described. Considerations related to its construction and use in measuring cross-sections are discussed. This instrument has been used to obtain absolute values for differential and integral excitation cross-sections of the n = 2 manifold states of helium at 29.2 eV, 39.2 eV and 48.2 eV. The observations are presented and compared with currently available theoretical calculations.