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 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 3⁰ 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.