TWO-QUANTUM ABSORPTION SPECTRUM OF KI AND CsI

The measurement of the two-quantum fundamental absorption edge in KI and CsI at 300, 77, and 4\ifmmode^\circ\else\textdegree\fi{}K is described. Selection rules, exciton models, phonon coupling, and the theory of two-quantum absorption are briefly discussed. Comparison of the present results with theory and the one-quantum absorption leads to the quantitative understanding of the shape and magnitude of the two-quantum spectrum. It is shown that one-electron band structure and Wannier excitons, rather than excitation models, provide the best understanding of the low-energy part of the fundamental absorption edges in the materials studied.