Dipole-forbidden excitations in electron-energy-loss spectroscopy.

Abstract

For momentum transfers of order 6 A${\r{}}^{\mathrm{\ensuremath{-}}1}$ nondipole transitions can make the same contribution to inner-shell energy-loss spectra as dipole transitions. Nondipole transitions are particularly significant when there is a high density of dipole-forbidden states in the near-threshold region. We have studied the near-threshold region in the excitation of molybdenum 3d and the excitations of 2p electrons in ${\mathrm{TiO}}_{2}$ and SiC as a function of scattering wave vector q. At large q dipole-forbidden monopole transitions become visible as a sharp peak on the ${M}_{4}$,5 threshold for molybdenum corresponding to 3d\ensuremath{\rightarrow}4d transitions or as a change in the ${L}_{2}$,3 peaks intensity ratio in the cases of titanium and silicon. Experimental results are confirmed by calculations.