Optical Studies of Metal Alloys

Abstract

The optical properties of metals and alloys have been studied experimentally and theoretically, with particular emphasis on the infrared behaviour. The optical absorption of dilute alIoys of AuTi, AuV, AuMn, AuFe and AuCo, prepared by vacuum evaporation, have been measured over the range 0.1 to 6.0 eV. The measurements were obtained using a differential reflectance technique in which the reflectivity of the alloy is compared with a sample of the host metal, thereby isolating the influence of the impurity element. The extra absorption observed in the alloys below the Au interband threshold has been interpreted in terms of the virtual bound state model for the impurities. To facilitate this interpretation a new analytical model for the virtual bound state dielectric constant has been derived, based on the generalized Drude modeI. The virtual bound state for AuCo has a Lorentzian density of states centred 0.14 +- 0.03 eV below the Fermi level with a half-width at half height of 0.13 +- 0.04 eV. For AuTi, the virtual bound state is positioned 0.5 +- 0.2 eV above the Fermi 1evel with a half-width of 0.6 +- 0.2 eV, and for AuV it is situated right at the Fermi level with a half-width of 0.4 +- 0.1 ev. AuMn and AuFe have magnetically split virtual bound states, with the width of the higher energy peak being consistent with the overall trend – a broadening of the virtual bound state as the impurity is changed across the 3d series from Co to Ti. The analysis of these alloys suggests that not all of the impurity electrons occupy the virtual bound state, indicating the existence of other types of state associated with the impurity. In the interband region, the principal impurity effect is a broadening and positive shift of the Au edge at 2.5 eV. Another cause of extra absorption in metals is the anomalous skin effect. In a theoretical study this has been treated classically using a convective derivative to describe the non-local behaviour. This approach yields a simple and accurate means by which to evaluate the optical properties, In the infrared and visible, where computational difficulties are encountered with the exact theories, it provides a more complete description than the approximations presently in use. Other alloy systems have been studied optically including the intermetallic compound Al10V and ptNi. In Al10V, a large Drude effect coupled with strong interband absorption (including a transition at 0.3 eV) leads to a low infrared reflectivity. The addition of small amounts of Ni to Pt leads to extra Drude absorption and a broadening of the interband edge at 0.4 eV. The techniques of sample preparation by evaporation and differential spectroscopy have been applied to metal-hydrogen alloys. Extra structure in the infrared reflectivity brought about by the addition of hydrogen to vanadium has been identified in this way.