Ultraviolet absorption bands of ionic compound clusters: Onset of crystalline structures in lsqb;Csn+1Inrsqb;+,n=1ndash;13

摘要

Starting from the large photofragmentation cross sections recently reported for masshyphen;selected alkalindash;halide cluster and nanocrystal beams lsqb;X. Li, R. D. Beck, and R. L. Whetten, Phys. Rev. Lett.68, 3420 (1992)rsqb;, we have obtained optical absorption spectra for ionic compound clusters in the ultraviolet chargehyphen;transfer bands. The smaller members of the CsI series, 3n14, show distinct absorption edges in the 4.6ndash;5.8 eV regionmdash;just below the bulk exciton band near 5.9 eVmdash;and the smallest cluster with an onset near the bulk is Cs14I13+, which is also a (3,3,3) nanocrystal. To help account for these features, we have computed minimumhyphen;energy structures for the entire series using a classical electrostatic potential, and have derived a simple model for the absorption energy, following deBoer. The locations of the spectral features are compared to the predicted energies, and good qualitative agreement is found.