Angular Distribution of Fluorescence from Helium II (n = 2) Produced by Photoionization.

Helium is the simplest atom in which electron -electron correlations can be investigated. These correlation effects are highly pronounced in the region of the spectrum where photoionization leaves the ion in the n = 2 excited state (E >=q 65.4 eV). Experimental results are available for the subshell branching ratio R = sigma_{rm 2s} /sigma_{rm 2p} and the photoelectron asymmetry parameter beta. In this thesis the first experimental results for the alignment A_0, a parameter complementary to beta, are presented. The alignment of the 2p state of helium II produced by photoionization was determined by measuring the angular distribution of the subsequent fluorescence. The experiment took place at the Synchrotron Ultraviolet Radiation Facility in the National Bureau of Standards. Monochromatized synchrotron radiation was used to ionize a pseudo beam of helium, leaving a fraction of the ions in the n = 2 state. The angular distribution of the fluorescence was measured by rotating a proportional counter about an axis lying along the direction of incidence of the partially polarized ionizing radiation. The quantization axis was taken parallel to the plane of polarization of the synchrotron radiation. The measured values of the alignment are A _0 = -0.60 +/- .04 and A_0 = -0.62 +/-.03 at energies of 65.5 and 66.5 eV respectively. The alignment is directly related to the amount of d-wave present in the total scattered electron wave through the ratio R_{rm p} = sigma(2pkd)/(sigma (2pks) + sigma(2pkd)). The measured values of the alignment translate into ratios R_{rm p} = 0.25 +/-.04 at 65.5 eV, and R_ {rm p} = 0.25 +/-.03 at 66.5 eV. By combining the results for R_ {rm p} with those for R and beta, the n = 2 subshell cross sections sigma(2skp), sigma(2pks), sigma(2pkd), and the relative phase between the (2pks) and (2pkd) channels are calculated. Therefore, four of the five real parameters needed to describe the continuum density matrix are obtained for two energies above the n = 2 threshold. Comparison with theoretical results is made, and excellent agreement with close-coupling calculations is found.