  Effect of short-range disorder on the excited states of the exciton is
studied. Disorder causes an obvious effect of broadening. Microscopically, an
exciton, as an entity, is scattered by the large-scale disorder fluctuations.
Much less trivial is that short-scale fluctuations, with a period of the order
of the Bohr radius, cause a well-defined down-shift of the exciton levels. We
demonstrate that this shift exceeds the broadening parametrically and study the
dependence of this shift on the orbital number. Difference of the shifts for
neighboring levels leads to effective renormalization of the Bohr energy. Most
remarkable effect is the disorder-induced splitting of S and P exciton levels.
The splitting originates from the fact that disorder lifts the accidental
degeneracy of the hydrogen-like levels. The draw an analogy between this
splitting and the Lamb shift in quantum electrodynamics.
