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3. W. Rüihle, V. Marrello and A. Onton, J. Luminescence18/19, 729 (1979).
4. R. H. Geiss, W. Parrish and V. Marrello, unpublished data.
5. P. M. Alt and W. E. Howard. We thank PMA and WEH for sharing their unpublished results on EL excitation.