IN SITU PROBING OF ELECTRONIC SURFACE STRUCTURE IN IONIC CRYSTALS BY RESONANT SECOND HARMONIC GENERATION J. REIF, P. TEPPER, O. SEMMLER et E. MATTHIAS J. Phys. Colloques, 48 C7 (1987) C7-733-C7-735 DOI: 10.1051/jphyscol:19877180