Absolute emission cross sections at 30.4 nm for e-He collisions and at 20.8 nm for Ne4+-He collisions

Absolute emission cross sections have been determined in the extreme vacuum-ultraviolet region at 30.4 nm for e-He collisions at 200 eV and at 20.8 nm for Ne 4+ -He collisions at 200 keV using a photoionisation chamber as an absolute radiation detector. The measured cross sections are (5.6+or-1.4)*10 -19 cm 2 and (3.4+or-1.4)*10 -16 cm 2 respectively. The detector used was originally introduced by Samson (1964) to measure absolute intensities of monochromatic radiation below 102.2 nm. Because the emitted radiation is not monochromatic for our collision systems the ionisation chamber as used by Samson had to be modified.