Optical strength of mirrors on fluorite substrates subjected to cw radiation from a chemical CO laser

Abstract

The effects of annealing high-purity polycrystalline gold foils (+99.999",) with thickness d = 25 pn at 870 i 20 'C under reduced chlorine pressure pc,, (in the range 11 5 pc,, 5 330 Pa) are reported. Detailed size-effect measurements at temperatures over the range 4.2 < T < 20 K on samples with 2 5 d 5 25 pn are presented and interpreted according to the theory of Soffer. The results show that the chlorine annealing increases the extrapolated bulk resistivity ratio R R R , of the foil from +600 to +70000 for the optimum annealing conditions, where RRR = R2,, R4.2 K. Also a temperatureindependent product of the bulk resistivity p , and the bulk electron mean free path i,. p,;., = (9.6 rt: 0.6) x 10-"f2m2, is found. With this value of p,;.,, the data compare very well with Soffer's theory for a surface roughness parameter r = /i:j.e z 1. where h is the root mean square surface roughness and j.e is the Fermi wavelength of the conduction electrons. for freshly annealed samples while r >> I for surfaces etched in aqua regia. In both cases r is independent of temperature. Kondo-effect measurements reveal the presence of 3.0 i 0.1 at PPM Fe in the as-received Au foils. The decreasing p , (4.2 K ) is attributed firstly to the physical removal of the Fe impurities, as previously found by Walker. and secondly to the clustering, passivation and removal of other impurities, probably accompanied by reduced grain-boundary scattering. The results of the CI2 annealing are compared with the well-known oxygen annealing method for noble metals, applied on some of our A u foils. I t is inferred from this that the former method is much more effective ( R R R , being roughly ten times better). Important ageing effects in R R R , at 300 K. established in both methods, are reported for the first time and are tentatively correlated to changes in grain-boundary scattering with time. A StesmunsA second example originates from the field of magnetic resonance in metals, i.e. the conduction electron spin resonance (CESR). Here, it is well known that trying to extend observations of CESR signals to metals other than those already well documented, requires very high-purity materials. From the observations by Monod and Janossy (1977) of transmission electron spin resonance (TESR) in gold via the surface-coating technique, it is clear that gold can be observed directly, without the troublesome surface coatings, provided the gold foils used are suitably purified.The usual methods of purification, e.g. distillation, zone refining, etc, are all limited by the residual impurity level and sample shape. For metals like Cu, Pd, Ag, Ta and Au, another well-known method of 'purification' exists, as manifested by the increase of the residual conductivity at low temperatures (for a review, see Fickett 1974). The method, which consists of the anneal at high temperatures (close to the melting point of the metal concerned) in a certain pressure of oxygen or air, can be applied to all sample shapes and is especially useful for foils and films, which are requi...