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

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 P1 parameter for Ar(1P) 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°will yield an apparent P1 value of 0.83 compared to the nominal value of 1.00. Since the measurement of P1 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 P1 value truly smaller than unity from an apparent value which is reduced merely by instrumental effects.