Dipole-forbidden excitations in electron-energy-loss spectroscopy

For momentum transfers of order 6 A-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 TiO2 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 M4,5 threshold for molybdenum corresponding to 3d4d transitions or as a change in the L2,3 peaks intensity ratio in the cases of titanium and silicon. Experimental results are confirmed by calculations.