Measurement of exciton masses by electron energy loss spectroscopy

A 300-keV transmission electron-energy-loss spectrometer was used to measure the energy-loss function of thin films of CuCl, LiF, NaF, KR, KCl, and NaCl in the energy range 0 to 100 eV. The data were corrected for multiple scattering and kinematic effects and the optical properties and dielectric function were obtained using a Kramers-Kronig analysis. A comparison was made with spectra obtained in previous optical and energy loss experiments. The momentum-transfer capability of the instrument was used to measure the dispersions of the fundamental exciton and the band gap for the materials investigated. From the measured dispersions the translational mass of the exciton, M/sup */, and the sum of the effective electron and hole masses, m/sub e//sup */ + m/sub h//sup */, are obtained. In all cases investigated, the exciton mass is greater than the sum of the effective electron and hole masses. This result is consistent with a recent prediction by Mattis and Gallinar. In addition, we obtain values for the reduced mass and radius of the exciton, the individual electron and hole masses and an average value for the sum of the widths of the valence and conduction bands. The values obtained for these parameters are compared with those obtained from previous experiments and theoretical calculations