The three-photon absorption (3PA) spectra of KI and RbI have been measured at 80 K in a wide excitation-energy range by monitoring the resulting self-trapped exciton luminescence. The experimental results have shown that the spectral behavior of the 3PA coefficient is fully described by a parametric formula containing four different terms, their energy dependences being predicted on the grounds of 3PA selection rules. A qualitative and quantitative comparison between the experimental and the various theoretical 3PA line shapes has been carried out.

• Received 5 January 1988

DOI:https://doi.org/10.1103/PhysRevB.38.3438