Hydrostatic pressure effects on the free and self-trapped exciton states in CsI

The s-like (Gamma(8)(-),Gamma(6)(+)) and d-like (Gamma(8)(-),Gamma(8)(+)) free exciton (FE) states and the corresponding self-trapped exciton (STE) states in CsI have been studied under hydrostatic pressure up to 1.3 Gpa. As the pressure is increased, the lowest-energy FE state is changed from the s-like (Gamma(8)(-),Gamma(6)(+)) state to the d-like (Gamma(8)(-),Gamma(8)(+)) state, and consequently the lowest-energy STE state is changed from the s-like state to the d-like state: The latter is evidenced by the replacement of the s-like STE luminescence (3.7-eV band) by the d-like STE luminescence (4.7-eV band). At high pressure the s-like and d-like STE's fall into a quasithermal equilibrium distribution. Based on the results, we discuss the pressure-induced changes of the adiabatic potential energy surfaces of the STE states in CsI.