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

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 how 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 Cif the 2 S-3 state of helium by impact of electrons from threshold to 24 eV. Spectra are presented at the scattering angles of 0 degrees, 45 degrees, 90 degrees, 135 degrees and 180 degrees, important for deriving integral cross sections,and 145 degrees, 155 degrees and 165 degrees, 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.