Spatial dispersion and exciton spectrum of layered bismuth triiodide single crystals

A theory of spatial dispersion and the framework of Pekar's additional boundary conditions are used to analyze for the first time the exciton reflection spectra of layered (BiI 3 ) crystals with strong damping of exciton waves. The most satisfactory description of the experimental curve by a theoretical one is obtained for large masses (M⊥ = 100m 0 ) and strong damping (ħγ = 33 meV) of excitons. The observed difference between the experimental and calculated spectra on the short-wavelength side of the main reflection minimum is attributed to lack of allowance for quasisurface exciton states. The spatial dispersion effects in bismuth triiodide are negligible since the inequality ħγ/ħγ cr > 1 is satisfied by a margin amounting to a factor of about 1000