A Versatile Molecular Beam Scattering Apparatus Utilizing Electron Bombardment Detection

Abstract

A versatile crossed molecular beam apparatus has been designed, fabricated, and tested. The instrument consists of a large cylindrical stainless steel ``scattering chamber'' which contains differentially pumped nozzle-skimmer and detector chambers. The crossed beams intersect at the cylinder axis. The detector assembly can be rotated about the scattering center from −43 to +133° with respect to the primary beam direction. The scattered intensity is measured using a modified Weiss electron bombardment ionizer with quadrupole mass filter. Over-all detection efficiency of the system for a thermal argon beam is 0.25%. Background discrimination is effected by means of double modulation (synchronous chopping of both crossed beams with beat frequency lock-in detection). The apparatus has been employed in studies of the rainbow scattering of Ar by various gases.