Island size distributions and number densities have been derived from scanning tunneling microscopy images of Ni deposited on cleaved GaAs͑110͒ at room temperature and above. For submonolayer coverages, this system forms three-dimensional ͑3D͒ reacted islands with the degree of reaction dependent upon the growth temperature. The form of the measured island size distributions is consistent with nucleation of the islands via an adatom-substrate exchange process-i.e., the critical island size is zero. For large island sizes and room-temperature growth, plots of the area-normalized island size distribution versus the scaled variable s/s av ͑with s av the average island size͒ all fall on a common curve. In contrast to expectations for exchange nucleation, the total number density of islands is found to decrease with temperature. A rate equation model has been extended to account for the growth of 3D islands and to incorporate the possibility of first-order rate processes ͑rate proportional to monomer density͒. Model results show that the observed island size distribution can be produced by any first-order process, and suggest characteristics of the actual growth kinetics relevant to this ternary system. ͓S0163-1829͑96͒03235-3͔

We report emission spectra and core-hole lifetimes for rare-gas and alkali-metal impurities in alkalimetal host lattices. The results lead to new proposed explanations for the anomalous core absorption edges reported in earlier work.