Non-Dipole and Dipole Core Electronic Excitation

Abstract

<p>In contrast to photoabsorption techniques, which are subject to electric dipole selection rules, electron energy loss spectroscopy (EELS) can provide a more complete investigation of atomic and molecular electronic structure. This is due to the added capability of accessing dipole and/or spin forbidden electronic transitions under conditions of significant momentum transfer.</p> <p>This work documents the design, construction and performance of a new, high resolution, variable scattering angle, variable impact energy electron spectrometer (McVAHRES) which has been used to investigate electronic transitions in gas phase atoms and molecules under both dipole and non-dipole conditions. The home-built instrument features a complex electron optics system and a sophisticated, computer-interfaced electronics system which allows a high degree of flexibility with regards to control and acquisition. This permits a wide range of experimental conditions.</p> <p>New spectroscopic studies include the observation of spin-forbidden, C is core excited triplet states in C₂H₄, C₂H₂, and C₆H₆. The momentum transfer dependence of the (C 1sˉ¹, π*) ³π state of CO was also investigated. These results are compared to the results of theoretical calculations. Generalised Oscillator Strengths (GOS) as a function of momentum transfer were derived for the S 2p edge of SF6. This work greatly expands previous results reported in the literature. Finally, an interesting feature was observed in the S 2p spectrum of SF₆, which displays quadrupolar momentum transfer behaviour.</p>