Selective detection of metastable helium atoms by elastic scattering collisions

A method for efficient and selective detection of 2 S-1 and 2 S-3 metastable He atoms is presented. Elastic collisions with Ar atoms in a diffuse gas spatially separate these 2S atoms from the copropagating UV photons that are inherently present in metastable beams. The deflected 2S atoms are detected on a large-solid-angle Faraday cup. A He discharge lamp can selectively quench 2 S-1 atoms, and optical pumping together with a Stern-Gerlach magnet can selectively remove 2 S-3 atoms. As a result, we are able to efficiently detect either 2 S-1 or 2 S-3 atoms (or both) without a UV background.