Temperature and hydrostatic pressure effects on the exciton spectra of some lead and cadmium halides

The temperature and pressure shifts of the exciton peaks and positions on the optical absorption edges in thin films of ${\mathrm{PbBr}}_2$, ${\mathrm{PdCl}}_2$, ${\mathrm{PbF}}_2$, ${\mathrm{CdBr}}_2$, and ${\mathrm{CdI}}_2$ in the region from 3.5 to 6.0 eV were measured in the temperature range -150-0°C and with hydrostatic pressures to 3500 atm. The electron-phonon contributions of the observed temperature shifts were calculated and found to be larger than the shift due to changing lattice parameters in all cases except ${\mathrm{CdI}}_2$. Similarities in the results for ${\mathrm{PbBr}}_2$ and ${\mathrm{PbCl}}_2$ suggest an $s$-like valence band, as is the case for ${\mathrm{PbI}}_2$. The cadmium-halide results suggest these two compounds have spectra arising from transitions of a closely related nature. The results indicate that the cationic exciton model is appropriate for these halides.