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An electron scattering apparatus for studies of temporary negative ion formation in complex molecules
Abstract
The angular distributions of scattered electrons and the energy dependence of vibrational excitation cross sections are widely used to examine temporary negative ion formation in small molecules. In this thesis we examine the extent to which these methods are applicable to a more complex molecule, 1,4-cyclohexadiene, chosen because of its high degree of symmetry and well-characterized vibrational levels. The design and construction of the electron scattering apparatus is documented and its performance illustrated in the well-known $\sp2\Pi\sb{\rm g}$ resonance in N$\sb2$. The electron spectrometer is capable of angular studies from $-$10 to 110 degrees in the elastic and inelastic channels. The typical system resolution is 15-20 meV, at full-width half-maximum. The apparatus is interfaced to a laboratory micro-computer capable of controlling the energy and electron optics of the electron spectrometer. The computer is also used to store and display the data. 1,4-cyclohexadiene has two low-lying unfilled $\pi$* orbitals which are manifested as electron scattering resonances. From simple molecular orbital considerations, the symmetries of these two anion states are $\sp2$A$\sb{\rm u}$ and $\sp2$B$\sb{3\rm g}$. Attachment of the impinging electron to form these states occurs through different combinations of partial waves. These combinations are reflected in the angular distributions of electrons that excite totally symmetric vibrational modes of the neutral molecule. Such a mode, v$\sb{8\rm a}$, associated primarily with C=C motion, is found to be resonantly excited. The angular scattering dependence of this inelastic mode reveals that the $\sp2$A$\sb{\rm u}$ state of the anion lies energetically below the $\sp2$B$\sb{3\rm g}$ state, in agreement with the assignments arrived at by less direct methods. We also present supporting data in this compound. These include the angular scattering dependence of elastically scattered electrons at energies encompassing the resonances, excitation functions of other vibrational modes and electron energy loss data.
Subject Area
Molecules
Recommended Citation
Stephen, Thomas M, "An electron scattering apparatus for studies of temporary negative ion formation in complex molecules" (1988). ETD collection for University of Nebraska-Lincoln. AAI8904513.
https://digitalcommons.unl.edu/dissertations/AAI8904513