REFRACTION OF DIRECTED SHOCK ELECTRONS AT PLANAR SOLID SURFACES H. ROTHARD, K. KRONEBERGER, M. BURKHARD, C. BIEDERMANN, J. KEMMLER, O. HEIL and K.O. GROENEVELD J. Phys. Colloques, 50 C2 (1989) C2-105-C2-110 DOI: 10.1051/jphyscol:1989219