An optical spectroscopy, based upon the determination of the difference between the reflectivity forp-polarized light (Rp) and the squared reflectivity fors-polarized light (Rs2) with an angle of incidence of 45°, is proposed to study the interaction of excitons with the surface potential in semiconductors. It is found thatRp−Rs2as a function of the wave frequency is quite sensitive to the form of the surface potential. In addition, the generation of longitudinal modes such as the quantized polarization waves of excitons produces resonant dips well-identified in the new spectrum. On the other hand, the transverse resonances present inRpandRsdisappear in the differenceRp−Rs2.©1997 American Institute of Physics.