Vacuum ultraviolet measurements of the electron impact excitation of helium

Total cross sections for the excitation of helium from its ground state to the 2 1 P, 3 1 P and 4 1 P levels by electron impact have been measured in a polarization free manner using a crossed beam apparatus. The apparent excitation functions have been cascade corrected and made absolute using the optical oscillator strengths. The excitation functions exhibit shapes which differ from some other recent measurements particularly around the peak of the cross section curves. Agreement at high energies between the measured total cross section values and the Bethe approximation is verified within experimental error.