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

A simple and straightforward approach to an absolute emission cross section measurement of the He⁺ 2²P to 1²S line at 304 AA following the simultaneous ionisation-excitation of He by electron impact is presented. The undispersed He and He⁺ impact radiation in the extreme ultraviolet (XUV) region (lambda ≤600 AA) is detected by a channel electron multiplier after passing through an XUV filter. A 304 AA emission cross section of (6.2±1.7)*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.