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Coherence parameters and excitation cross -section for electron impact excitation of the 3(1)P and 3(1)D states of helium
Abstract
This dissertation describes two experiments which measured: (1) the relative differential cross section for the excitation of the 3$\sp1$P($m\sb{\ell}$ = 0) state in helium by electrons, and (2) the coherence parameters for electron impact excitation of the 3$\sp1$D state of helium using an angular correlation experiment. Both of these experiments utilize the electron-photon coincidence technique which yields much more detailed information on the scattering process than is provided in more traditional electron atom scattering experiments. In the 3$\sp1$P state of helium, the magnetic substates are degenerate with energy, and therefore, it is not possible to measure these magnetic sublevel differential cross sections with the traditional energy loss experiment. However, it was possible to measure the relative differential cross section for the $m\sb{\ell}$ = 0 substate of helium by detecting the 3$\sp1$P $\to$ 2$\sp1$S decay photons (501.6 nm) in the scattering plane and perpendicular to the incident electron beam. These photons were detected in coincidence with the inelastically scattered electrons. The results are compared with previous experimental results and different theoretical model calculations. The 3$\sp1$D excited state of helium is fully characterized by four coherence parameters. The behavior of these coherence parameters as a function of energy and scattering angle gives valuable information about the dynamics of the excitation process. An angular correlation experiment for the 3$\sp1$D state of helium was performed for the first time by measuring the angular distribution of the 3$\sp1$D $\to$ 2$\sp1$P (667.8 nm) radiation emitted in the scattering plane in coincidence with the inelastically scattered electrons. With a measurement of intensity perpendicular to the scattering plane, this experiment yielded three of four coherence parameters, including one parameter that has never been measured before. These data are in good agreement with other published data obtained from polarization correlation experiments which measured linear and circular polarization of the 3$\sp1$D $\to$ 2$\sp1$P decay radiation in coincidence with the scattered electrons. Results from the present experiment are compared with a number of recent theoretical models.
Subject Area
Atoms & subatomic particles
Recommended Citation
Perera, N. W. P. H, "Coherence parameters and excitation cross -section for electron impact excitation of the 3(1)P and 3(1)D states of helium" (1990). ETD collection for University of Nebraska-Lincoln. AAI9034285.
https://digitalcommons.unl.edu/dissertations/AAI9034285