Absolute electron impact emission cross section of the He+ 22P→12S line at 304A produced by simultaneous ionisation-excitation

A simple and straightforward approach to an absolute emission cross section measurement of the He+ 22P→12S line at 304 A following the simultaneous ionisation-excitation of He by electron impact is presented. A 304 A emission cross section of (6.2 +- 1.7) x 10-19 cm2 at 200 eV impact energy is extracted utilising the known He cross sections, the measured He/He+ ratio and the wavelength-dependent transmission of the XUV filter and quantum yield of the photon detector. In the regime of high impact energies the emission cross section is found to be inversely proportional to the incident electron energy as expected for the optically forbidden two-electron simultaneous ionisation-excitation process. (author)