Quasiparticle spectra, absorption spectra, and excitonic properties of NaI and SrI2 from many-body perturbation theory

We investigate the basic quantum-mechanical processes behind the nonproportional response of scintillators to incident radiation responsible for reduced resolution. For this purpose, we conduct a comparative first-principles study of quasiparticle spectra on the basis of the G₀W₀ approximation as well as absorption spectra and excitonic properties by solving the Bethe-Salpeter equation for two important systems, NaI and SrI₂. The former is a standard scintillator material with well-documented nonproportionality, while the latter has recently been found to exhibit a very proportional response. We predict band gaps for NaI and SrI₂ of 5.5 and 5.2 eV, respectively, in good agreement with experiment. Furthermore, we obtain binding energies for the ground state excitons of 216 meV for NaI and 195-225meV for SrI₂. We analyze the degree of exciton anisotropy and spatial extent by means of a coarse-grained electron-hole pair-correlation function. Thereby, it is shown that the excitons in NaI differ strongly from those in SrI₂ in terms of structure and symmetry, even if their binding energies are similar. Furthermore, we show that quite unexpectedly the spatial extents of the highly-anisotropic low-energy excitons in SrI₂ in fact exceed those in NaI by a factor of two to three in terms of the full width at half maxima of the electron-hole pair-correlation function.