Electron-Polarized Photon Coincidence Study of the Heavy Noble Gases.

An electron-polarized photon coincidence apparatus has been constructed to measure polarization correlation parameters for heavy noble gas excitation by electron impact. Polarization correlation parameters are linear combinations of the off -diagonal elements of the density matrix which describes the collisionally excited state. Their measurement provides insight into the role of spin-dependent interactions in the excitation process and allows a comparison between experiment and theory at the most fundamental level. The polarization correlation parameter P _1 measured in the forward direction for excitation of the (^2P_ {1/2}) ns^'(1/2) _1 state in Neon, Argon, and Krypton at electron energies from 35-100eV indicates that spin effects in heavy noble gas excitation do not play a significant role during the electron impact excitation of these states at very large impact parameters. It was found that significant corrections of the measured values were necessary to account for the finite angular acceptance of the electron analyzer in order to avoid erroneous conclusions. Measurements of both linear polarization correlation parameters perpendicular to the scattering plane (P _1 and P_2) have been carried out at 50eV and small scattering angles (0-20 ^circ) for the (^2P _{3/2})ns(3/2) _1 state in Krypton. A comparison was made with the results of previous angular correlation studies and with the theoretical predictions. The agreement between the present results, the angular correlation results, and with theory is overall satisfactory. The coherence parameters expressed in terms of the natural parameters reflect the shape and dynamics of the angular part of the collisionally induced excited state charge distribution. Two parameters, the charge cloud alignment angle and the linear polarization of the charge cloud, can be extracted from the P _1 and P_2 measurements. The agreement between the experimentally derived alignment angles and the calculated values is excellent. The comparison between the experimental and theoretical linear polarization is less satisfactory. The existing theories appear to be quite capable in predicting the orientation of the charge cloud in the scattering plane (gamma), while the exact shape of the charge cloud (P_ {rm lin}) seems to be influenced by details of the collision process that are not as well reproduced by these theories.