Effect of Particle Size on the Colloid Band in CsBr

The effect of particle size on the half-width of the colloid band in CsBr is discussed. Boundary scattering of electrons predominated for small particles and radiation damping for large particles, yielding broad half-widths. For intermediate-sized particles, bulk relaxation effects predominated and the half-width was a minimum. Variations in the minimum half-width in different specimens and the lack of the expected temperature dependence indicated the presence of impurities in the colloidal particles. Electron-microscope observations revealed that the particles were spherical. The colloid band occurred at 1050 nm as compared with the predicted position at 960 nm. Comparison of the observed colloid-band positions in the alkali halides with the predicted positions reveals a systematic difference, the reason for which is not known.