Excitation of the 2 3S state of helium by electron impact from threshold to 24 eV: measurements with the `magnetic angle changer'

Abstract

The magnetic angle changer recently invented by Read and co-workers (Read F H and Channing J M 1996 Rev. Sci. Instrum. 67 2373) has been incorporated into an existing electron impact spectrometer. Both the inner and the outer solenoids consist of a single layer of self-supporting thin copper tubing, cooled by circulating water. This construction makes the solenoid nearly optically transparent, reducing the interference of the new device with the flow of sample gas - a feature important for measurements of absolute cross sections and for the operation of a supersonic free jet. The properties of the device are illustrated with measurements of the differential cross sections for the excitation of the 2 3S state of helium by impact of electrons from threshold to 24 eV. Spectra are presented at the scattering angles of 0°, 45°, 90°, 135° and 180°, important for deriving integral cross sections, and 145°, 155° and 165°, revealing interesting details of the interference of narrow ridge resonances with background scattering at angles surrounding the deep minimum of the angular distribution at 22 eV.