The reflectivity and the transmission of InSb crystals ($n$-type) were measured between 20 and 200μ. The reflectivity curve has a sharp maximum at 54.6μ and the transmission curve shows a low transmission at the same wavelength due to lattice vibration of this crystal. The transmission curve shows a weak absorption at 28.3μ due to the overtone of that lattice vibration. The reflectivity curve shows another flat peak at longer wavelengths. The reflectivity minimum between these two peaks shifts from 160 to 72μ with increase of temperature from -35 to 130°C, and the maximum value of the flat peak increases with increase of temperature and is 80% at temperatures of 100°C or more.

The index of refraction $n$ and the absorption constant $K$ were calculated in the region in which both the reflectivity and the transmission were measured. Using the values of $n$ and $K$ thus obtained, the concentration of free electrons $N$ and their mobility $b$ were obtained, using the theory of free electrons. Then $n$, $K$, and the reflectivity, $R$, were calculated for the region in which the transmission could not be measured. The values of $R$ thus obtained agree well with the observed values. This shows that the theory of free electrons holds in InSb except in the region in which the effects of lattice vibration in this crystal are dominant.

• Received 2 September 1955

DOI:https://doi.org/10.1103/PhysRev.101.526