LO-Phonon—Assisted Two-Phonon Absorption in KI

The two-photon absorption constant is measured experimentally with high resolution in the exciton region of KI. Fine structure is resolved in which the $2P$ exciton is seen at 6.263 eV. The peak at 6.285 eV is identified as a LO-phonon—assisted transition. The latter assertion is verified theoretically by invoking third-order time-dependent perturbation theory.