LO-Phonon-Assisted Transitions in the Two-Photon Absorption Spectrum of KI

The two-photon absorption spectrum of KI is measured experimentally with high resolution in the exciton region at approximately 6 ^∘{}K. Fine structure is resolved in which the $2P$ exciton is seen at 6.263 eV and a peak at 6.285 eV is identified as a LO-phonon-assisted transition. The exciton-phonon interaction is calculated using third-order time-dependent perturbation theory. The two-band model is used, where the two intermediate states are virtual excitons excited by the photons and the final state is composed of an exciton and LO-phonon. The theoretical prediction of the strength and location of the phonon-assisted peak agrees favorably with the experiment. Calculations are performed with several values for the hole mass; the value 3.0 is chosen as representing a good average for the fourfold-degenerate hole.