Effects of hydrostatic pressure on free and bound exciton states in silver bromide

Abstract

In AgBr at low temperature, resonant Raman scattering is employed to investigate the Gamma 6+(X)L4.5- indirect free exciton state under hydrostatic pressure up to 0.7 GPa. From the pressure dependence of resonance behaviour and Raman energy shifts, the deformation potential constant of the exciton edge and mode Gruneisen parameters of various phonons involved in exciton relaxation are obtained simultaneously. Also, the effect of pressure on several bound exciton states is studied by luminescence. Donor-bound excitons of type (D0, X) with localisation energies E1=6 to 8 meV show a negative pressure shift (dE/dP=-15.1 meV GPa-1) completely parallel to the free exciton gap proving their effective-mass nature with wavefunctions identical to those of the free exciton. Similarly, the iodine-bound exciton state (E1=43 meV) is found to derive mainly from the L-point valence band states with only minor contributions from Gamma . Different contributions to the pressure-induced change in lattice relaxation occurring in this case are determined and discussed.