Colours and other optical properties of binary noble metal-noble metal alloys

The method of ellipsometry is applied to the study of the optical properties of Ag-Au, Ag-Cu and Au-Cu alloys and the results are interpreted in terms of alloy colour and electronic properties. Variation of colour with alloy composition is placed on a quantitative basis for the first time and discussed in terms of the most fundamental properties of the materials. Use of ellipsometry in the investigation also makes possible the assessment of the effects of tarnish and other contaminant films. The influence of sample preparation on the optical properties is discussed, and the importance of using ‘real’ surfaces, in order to make possible the industrial and commercial application of the work, is emphasised. A survey is given of previous studies both theoretical and experimental that are related to the noble metals or the experimental technique used. Sample requirements are discussed and the preparation methods adopted are described. Relevant electronic, optical and colorimetric theory is presented and unified in a computational system that takes full advantage of the great power of modern computers. Ellipsometric results are given and compared with the optical results of workers using other techniques. The effect of cuprite tarnish on the properties and on the accuracy of their determination is presented in detail. It is found that by far the most significant determinant of noble alloy colours is the width of the d-FS gap near to L. Tarnishing of copper rich alloys has a visually observable and measurable effect only when prolonged over many hours, but a rapidly formed film is probably responsible for an ellipsometrically observed increase in the saturation of these red metals that borders on the perceptible