Core excitons in amorphous magnesium alloys

Abstract

High-resolution photoabsorption spectra were obtained in the range 40-200 eV for Mg-group-V alloys. Sharp line structure was discovered for amorphous alloys of Mg-Bi and Mg-Sb near the photoabsorption threshold of the Mg $2p$ core level. These lines were strongest for compositions corresponding to the intermetallic compounds in the crystalline solid, and are thought to be due to the formation of core excitons. The positions and shapes of the exciton lines contain information about short-range screening, ionicity, and excited-state lifetime in the amorphous solids. Variation in the line shape of the threshold is described by a critical exponent, and appears to be continuous between compositions having insulating conductivity and compositions approaching metallic conductivity. This supports the idea that the metallic $L_{\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}\mathrm{I}}$ threshold is a remnant exciton singularity. Exchange interaction alters the relative strengths of the threshold doublet which originates in the spin-orbit splitting of the $2p$ initial state. This electron-hole exchange suggests a compact excition wave function. Furthermore, for sufficiently long screening lengths the excitons sharpened enough to uncover additional structure, possibly due to higher-order members of an exciton series. The fact that excitons were not observed in crystalline alloys of Mg-Bi is attributed to a change in conductivity and to the nature of the critical exponent in the ordered phase.