The pressure shifts of the optical absorption edges in four thallium and silver halides at 10°K were determined to be, in units of ${10}^{-6\mathrm{}}$ eV/atm, -1.5 (AgCl), -1.9 (AgBr), -18 (TlCl), and -16 (TlBr). The pressure shifts of the first exciton peaks in thin evaporated films of these salts were measured at 80°K. In the same units these are 6.2 (AgCl), 6.4 and 5.6 (AgBr doublet), -13.9 (TlCl), and -13.4 (TlBr). The similarity of the pressure coefficients of the absorption edge in the two silver halides is evidence that the same indirect transition is responsible for the edge in both salts, presumably $L_{3^{\prime }}\to {\Gamma }_1$. The deformation potentials for the first direct exciton in the silver halides are approximately the same as those for the corresponding transitions in KBr and KI. The similarity of the pressure coefficients of the absorption edge and exciton peaks in the thallium salts is evidence that the absorption edge in these salts is the tail of the first direct exciton peak. The "anomalous" red shift upon cooling of the absorption edge and exciton peak in the thallium halides is shown to be the effect of thermal contraction.

• Received 16 May 1968

DOI:https://doi.org/10.1103/PhysRev.180.911