Low-Energy Optical Absorption Peak in Aluminum and Al-Mg Alloys

Measurements of the absorptivity of Al and dilute Al-Mg alloys were made at 4.2 K in the 0.2- to 5-eV energy region by a calorimetric technique. In addition to the well-known conductivity peak around 1.6 eV, another peak has been found near 0.5 eV. These peaks are present in the alloys, broadened, but unshifted, as well as in Al. The peaks are at about $2\left|V_{200}\right|$ and $2\left|V_{111}\right|$, respectively, where the $V$'s are Fourier coefficients of the pseudopotential.