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A method is developed to obtain Wannier-Mott --- exciton wave functions in semiinfinite crystals in the framework of the effective-mass approximation. An analytical approximation is shown to agree well with numerical wave functions and is used to compute exciton nonlocal polarizability (in closed form) and s-wave reflectivity. The results are compared with normal-incidence reflectivity experiments in CdS, ZnSe, GaAs, and InP, and the experimental line shapes are well reproduced. The existence of an intrinsic dead layer is confirmed and a new additional boundary condition is derived. It is shown that the spike, frequently observed at the longitudinal-exciton frequency, is due to extrinsic dead layers.
D’Andrea et al. (Mon,) studied this question.