Impact ionization and interface transmission, two important processes in hot-electron transport in semiconductor structures, have been studied by ballistic electron emission microscopy (BEEM) at interfaces of NiSi2/n-Si(111)7 X 7. The analysis of BEEM spectra taken in pinholes of thin NiSi2 films allows a direct determination of the quantum yield of impact ionization in Si over a wide energy range (>7 eV). BEEM images and spectra recorded on NiSi2 terraces show evidence for the conservation of transverse momentum and a small probability for normal interface transmission up to high energies (almost-equal-to 4 eV above the Fermi energy), in accordance with dynamical calculations of the transmission probabilities.