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Infrared measurements over the range of 2 to 38 µ have been carried out for a number of p-type silicon specimens, including boron doped single crystal specimens. At liquid helium temperature these specimens exhibit optical absorption by neutral impurities involving (a) photoionization of bound charge carriers, which is accompanied by photoconductivity, and (b) optical excitation of bound charge carriers from the ground state to higher energy states, which appears as relatively narrow bands at wave lengths beyond the photoionization limit. Three excitation bands are observed in the boron doped specimens at 28.7, 31.2 and 36.0 µ, which are attributed to ls-4p, ls-3p and ls-2p transitions, respectively. The energy levels of the boron centers calculated from these bands correspond closely to those of a hydrogen model having an ionization energy of 0.046 ev. and indicate an effective mass for holes = 0.45 m and a radius for the ground state orbit of 13.5 A• The boron ionization energy is independent of concentrations up to densities of 6 X 1017 impurity centers/cm.8 A second type of impurity center having an ionization energy of 0.07 e.v. is also observed in a polycrystalline specimen. For this center the excitation bands do not conform to those of a simple hydrogen model.

The far-infrared optical properties of KC1 and KBr have been measured at r=300°K by the method of asymmetric Fourier-transform spectroscopy. This method allows the simultaneous measurement of the amplitude and phase shifts of the radiation reflected or transmitted by the crystals. From reflectance data, the real and imaginary parts of the complex index of refraction were obtained in the spectral region from approximately 40 to 360 cm" 1 . Detailed numerical calculations for KC1 were performed by employing a model in which the radiation was assumed to interact with the k=0 TO phonon mode via the first-order dipole moment and the subsequent decay of this mode by two-phonon processes. The frequency-dependent Hermitian and anti-Hermitian parts of the k = 0 self-energy, arising from the cubic-lattice anharmonicity, were calculated by using the phonon frequencies and eigenvectors obtained from the Karo-Hardy deformationdipole model. The complex index of refraction of KC1 was calculated from the self-energy and the results are in reasonable agreement with experiment. Finally, a comparison is then made for KBr between the calculations performed by Cowley and our experimental data. Phys. Rev. 163, 836 (1967). 6 and he included the contributions to the dielectric constant from the second-order dipole moment in addition to the first-order moment. Cowley did not have detailed experimental data with which to compare his 4

R6sum6.-Nous exposons dans ses lignes gknkrales la mkthode de la fonction diklectrique et ses limites dans la description des propriktks optiques des solides, en particulier des mktaux. Nous dkcrivons aussi des methodes pour deduire cette fonction a partir des expkriences. Nous soulignons I'importance d'erreurs dkrivant de la prkparation des kchantillons plutbt que de la technique de mesure. On discute les donnkes expkrimentales pour diffkrents Clkments mktalliques. Abstract.-A description of the optical properties of solids, especially metals, in terms of a dielectric function is outlined, along with some of its limitations. Methods of deriving this function from measurements are outlined as well. Errors due to sample preparation, rather than measurement errors, are emphasized. Experimental data for several elemental metals is discussed. Article published online by EDP Sciences and available at http://dx.

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