Ultraviolet absorption bands of ionic compound clusters: Onset of crystalline structures in [Cs_n+1I_n]⁺, n=1-13

Starting from the large photofragmentation cross sections recently reported for mass-selected alkali-halide cluster and nanocrystal beams [X. Li, R. D. Beck, and R. L. Whetten, Phys. Rev. Lett. 68, 3420 (1992)], we have obtained optical absorption spectra for ionic compound clusters in the ultraviolet charge-transfer bands. The smaller members of the CsI series, 3 < n < 14, show distinct absorption edges in the 4.6-5.8 eV region-just below the bulk exciton band near 5.9 eV - 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 minimum-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.