A novel electron scattering apparatus combining a laser photoelectron source and a triply differentially pumped supersonic beam target: characterization and results for the He~- (1s2s~2) resonance

摘要

A novel electron scattering apparatus for high resolution studies of angle-differential elastic and inelastic electron scattering from atoms and molecules in the gas phase is described and its performance characterized. It combines a laser photoelectron source, a triply differentially pumped collimated supersonic beam target (half angle 0.015 rad, background to beam density ratio <0.01), and several electron multipliers for simultaneous detection of elastically scattered electrons and metastable atoms (or molecules) due to inelastic scattering. In detailed test measurements of the yield for the production of metastable He~*(2~3S_1) atoms around its threshold, the dependence of the overall energy width on various experimental parameters has been investigated. So far a resolution down to 7 meV (FWHM) has been obtained. Under such conditions we have investigated the profile of the He~- (1s 2s~2 ~2S_(1/2)) resonance at the scattering angles 22°, 45°, and 90°. From a consistent fit of the measured profiles by resonant scattering theory we determine a new value for the resonance energy (E_r = 19.365(1) eV) and an accurate resonance width (Γ = 11.2(5) meV). These results are consistent with the previously recommended values.