An investigation of electron helicity density in bromocamphor and dibromocamphor as a source of electron circular dichroism

Using electron transmission spectroscopy (ETS), we have measured the vertical attachment energies (VAEs) of bromocamphor and dibromocamphor. We have also performed quantum chemical calculations to obtain the virtual orbital energies (VOEs) of camphor, bromocamphor and dibromocamphor. With scaling based on the trend between VAEs and VOEs of previously studied chloroalkanes, we have assigned the resonances observed in ETS to temporary occupation of specific normally empty orbitals. Further calculations were conducted to determine the helicity densities of bromocamphor and dibromocamphor. We find large helicity densities in the regions of large wave function amplitude of the normally unoccupied orbitals responsible for resonances in the scattering cross sections. Our ETS assignments and helicity density results are compared with the chiral asymmetry data observed in electron-circular dichroism experiments by the M\"{u}nster group [1]. Evidence is seen to support helicity density as a source for chiral asymmetry at certain resonance positions in bromocamphor and dibromocamphor. [1] C. Nolting, S. Mayer, and J. Kessler, J. Phys. B \textbf{30}, 5491 (1997).