Validity of the dipole-selection rule for the Al-L2,3 edge of α-Al2O3 under channeling conditions

The validity of the dipole-selection rule for the Al-L2,3 electron energy-loss edge of α-Al2O3 is investigated. Dipole forbidden transitions can be observed in a transmission electron microscope operated with large collection apertures. In addition, it is shown that channeling along a highly symmetrical zone axis in α-Al2O3 can lead to the detection of dipole forbidden transitions even with small collection apertures. For an incident electron direction parallel to the 〈0 0 0 1〉 orientation it is observed experimentally that the fine structure of the Al-L2,3 edge shows additional features compared to measurements with the electron beam parallel to 〈1 1̄ 0 0〉. This effect is due to the occurrence of channeling conditions, indicated by the dependence of the additional dipole forbidden features on the sample thickness. These additional features disappear when tilting the crystal by 1.8° (0.84 Å−1) or even into the less-symmetrical 〈1 1̄ 0 0〉 zone axis. It is suggested that these observations are explained by differences in local symmetry at the excited center with respect to the incident beam directions.