The specular reflectance of aluminum films evaporated in ultra-high vacuum has been measured with precision in the 0.3- to 32-µ wavelength region. The measured reflectance of these films, which is good to ±0.1%, is significantly higher than that of films prepared under the usual high-vacuum conditions. The reflectance values are now in very good agreement in the 6- to 32-µ wavelength region with those calculated from theory using the bulk value of the dc conductivity and an effective electron density Neff≈2.6 free electrons/atom. Hence the discrepancy between theory and experiment in the infrared which has previously been reported for aluminum now appears to be resolved. However, at shorter wavelengths the reflectance is lower than would be predicted from theory. One possible explanation is that the relaxation time is wavelength dependent near the region where interband transitions become important.
H. E. BENNETT, M. SILVER, and E. J. ASHLEY, "Infrared Reflectance of Aluminum Evaporated in Ultra-High Vacuum," J. Opt. Soc. Am. 53, 1089-1095 (1963)