Angular Dependence of the Scattering of Metastable Helium Atoms in Helium and Neon

Abstract

A molecular beam method has been used to perform scattering experiments with an atomic beam of metastable helium atoms in helium and neon. The atomic beam, in which some of the atoms had been excited by electron impact, was passed through a scattering chamber containing gas at low pressures and subsequently detected by electron ejection from gold surfaces. The 21S to 23S helium atom ratio in the beam was determined using an inhomogeneous magnetic field. Total collision cross sections for the scattering of both metastable helium atoms in helium and neon have been measured as a function of the angular resolution of the apparatus. Angular scattering coefficients in the angular range 7°—25° have been calculated. In both gases the scattering at these angles of 23S helium was larger than 21S helium. These differences can only be caused by differences in the interaction forces between the colliding particles. The results are consistent with the higher interaction of triplet metastable atoms with ground-state atoms calculated by Buckingham and Dalgarno. Separate measurements of the backward scattering allowed a lower limit of 2×10—16 cm2 to be set for the He*–He energy-transfer cross section.