Dipole-forbidden excitations in electron-energy-loss spectroscopy

For momentum transfers of order 6 A${̊}^{\mathrm{-}1}$ 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→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.