The energy states of a particle confined in a narrow space are discrete and lined up in the order of $n=1,2,3,\dots$. However, if the particle interacts with a radiation field, modification of the energy, referred to radiative correction, will occur and quantum states are expected to interchange. We investigated the center-of-mass confinement of excitons in CuCl films by a new method based on “nondegenerate two-photon excitation scattering.” The energies of confined excitons in a 19.3 nm thick film are found to be lined up in the order of $n=1,3,5$, because the radiative correction is very weak. On the other hand, in a 35.3 nm thick film, in which the radiative correction becomes large, the energies of quantum states are ordered $n=2,3,4,1,5,7$. This interchange is confirmed by comparing the calculated scattering spectra, in which radiative correction is taken into account, with the measured ones.

• Received 21 April 2003

DOI:https://doi.org/10.1103/PhysRevLett.92.257401