Measurement of absolute emission cross sections for extreme ultraviolet (EUV) photons in thermal collisions of metastable helium (2 sup 1 S) with neon and helium

Under single-collision conditions EUV photons emitted in the collision of metastable He with Ne and He-atoms were directly observed in an atomic beam experiment. The EUV emission was measured as a function of collision energy. The emission cross sections were calibrated in absolute units. The method uses the natural decay photons of the free metastable He. Stueckelberg oscillations are resolved in the total cross section with Ne; Close-coupling calculations were performed for the dominant excitation process in He. A comparison between theory and measurements shows three onset thresholds for several photon groups, and the collision dynamics picture is clarified. In the collision system metastable He-He the deexcitation is determined as energy-dependent EUV emission.