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We have investigated high-excitation photoluminescence properties of CuI thin films with a thickness of 100 nm grown on a NaCl substrate by vacuum deposition. The energies of the heavy-hole and light-hole excitons in the CuI thin film are split at low temperatures by the thermal strain resulting from the difference in the thermal expansion coefficients of CuI and NaCl: the heavy-hole exciton is lower in energy than the light-hole exciton. Under resonant excitation of the heavy-hole exciton, a new photoluminescence band appears on the low-energy side of the heavy-hole exciton. The energy spacing between the new photoluminescence band and the heavy-hole exciton is almost equal to the splitting energy of the heavy-hole and light-hole excitons. The photoluminescence intensity exhibits an almost quadratic dependence on the excitation power. These results indicate that the new photoluminescence band originates from an inelastic scattering process from the heavy-hole-exciton state to the light-hole-exciton state. We have confirmed the existence of optical gain in the energy region of the new photoluminescence band by using a variable-stripe-length method. This fact demonstrates that the inelastic scattering process from the heavy-hole-exciton state to the light-hole-exciton state may lead to stimulated emission. © 2002 American Institute of Physics.