Determination of optical sums from the small angle electron impact spectra of argon

Abstract

A method for obtaining optical sums directly from high energy (25 keV) electron impact spectra in the forward direction is suggested. The electron energy loss spectrum of argon at an energy resolution of ±600 meV was obtained for energy losses from zero to 50 eV and 200 to 300 eV at an energy resolution of 5 eV averaged over an angular range of ±5 × 10 −4 radians. The data were corrected for finite detector size, gas and electron beam spatial distributions to obtain an estimate of zero angle scattering spectrum. A hydrogenic contribution was assumed for the 3p, 3s continuum for energy loss greater than 5- eV and for the 2p continuum for energy loss greater than 280 eV. Various moments of the spectrum were computed and related to optical sums by use of the measured experimental moments of the energy distribution of the incident beam and the scale factor needed to place the data on an absolute scale. The scale factor was obtained by use of the Thomas-Reiche-Kuhn sum rule. The results are in excellent agreement with previous estimates of the various sum rules and the present approach shows promise for providing useful data for a large number of atomic and molecular targets.