Photon Vector Polarization and Coherence Parameters in an Electron-Photon Coincidence Experiment on Helium

We present linear and circular polarization measurements of $3{}_{}{}^1{}_{}{}^{}P-2\mathrm{}{}_{}{}^1{}_{}{}^{}S$ (501.6 nm) photons detected in delayed coincidence with electrons inelastically scattered from the $3{}_{}{}^1{}_{}{}^{}P$ level of helium. Vector polarization and coherence parameters of the coincident radiation provide direct experimental evidence of the coherent nature of the excitation process. Atomic orientation is obtained directly from the circular polarization measurements. Analysis of the data also yields $\lambda$ and $\chi$ parameters for $3{}_{}{}^1{}_{}{}^{}P$ excitations, which are compared with previous data and theory.