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The refractive index is measured for PbEuTe alloys in the 3.3-12-microm spectral region. Each alloy is grown with a different Eu concentration. The range of Eu concentrations studied is between 0.76 and 6.5 at %. All measurements are performed on samples maintained at room temperature. We use a classical dispersion relation to establish a simple empirical relationship that describes the dependence of the refractive index as a function of both the percentage of the Eu concentration and the infrared wave number.

A wavelength-stabilized tunable diode laser and a reflective optical system can produce fringe-free spectra with a noise level of [10.sup.-5] absorbance units and a precision of [10.sup.-5 cm.sup.-1] in the wave-number scale.