MEASUREMENT OF ABSOLUTE DIFFERENTIAL CROSS-SECTIONS FOR THE EXCITATION OF THE N=2 STATES OF HELIUM AT 0-DEGREES AND 180-DEGREES

A novel scheme for the measurement of inelastic differential cross sec tions (DCS) at 180 degrees is presented. It uses a magnetically collim ated electron-impact spectrometer where the backward-scattered electro ns are reflected into the forward direction by a potential barrier, an d enter the energy analyser located at 0 degrees. Pulsed incident elec tron beam and time-of-flight detection are used to differentiate the b ackward- and forward-scattered electrons. The instrument yields the ra tio of the forward and backward DCS. This data is then normalized to t he absolute DCS at 0 degrees, measured with an instrument using electr ostatic hemispherical analysers, to yield absolute DCS at 180 degrees. The combined results From both instruments are absolute DCSs as a fun ction of scattering angle continuously from 0 degrees to 136 degrees p lus one more data point at 180 degrees. Absolute DCS is also obtained as a function of incident energy at OD and at 180 degrees. The techniq ue is applied to measure the DCSs for the excitation of the four n = 2 states of helium as a function of scattering angle at 22, 25, and 29. 6 eV, and as a function of energy from threshold to 28 eV at 0 degrees and 180 degrees. The results are compared with recent calculations.