Effect of a finite angular resolution on the measurement of polarization correlation parameters in electron-heavy-noble-gas collisions.

Abstract

We present the results of a simple model calculation that highlights the effect of the finite (i.e., nonzero) angular resolution of electrostatic energy analyzers on the measurement of polarization correlation parameters in electron-photon coincidence experiments. It is demonstrated that measurements can yield drastically reduced ``apparent'' parameters in cases where the coincidence parameters vary rapidly as a function of electron scattering angle. For example, a measurement of the ${P}_{1}$ parameter for Ar${(}^{1}$P) excited by 100-eV electrons which are scattered in the forward direction and detected by an analyzer with an angular resolution (full width at half maximum) of 3\ifmmode^\circ\else\textdegree\fi{} will yield an apparent ${P}_{1}$ value of 0.83 compared to the nominal value of 1.00. Since the measurement of ${P}_{1}$ in the forward direction reveals important information about the relative contributions of direct excitation and exchange excitation, it is imperative that an experiment can distinguish a ${P}_{1}$ value truly smaller than unity from an ``apparent'' value which is reduced merely by instrumental effects.