A shift of the band edge in CdS has been brought about by application of a high electric field to CdS crystals. High fields are produced at the rectifying junction between a conducting CdS crystal and an electrolyte solution. A shift of the band edge of 70 A was observed for an applied emf of 120 volts. The effect is reversible and the absorption spectrum of the crystal after removing the field is identical with that before the field was applied. The absorption parallel to the crystal $c$ axis and the absorption perpendicular to the $c$ axis are affected in the same way by the applied field. The enhanced absorption at the band edge is believed to be the result of photon-assisted tunneling of electrons from valence band to conduction band in the high-field region within the crystal.

• Received 13 October 1959

DOI:https://doi.org/10.1103/PhysRev.117.1487