Infrared emission spectra from metal samples with oxide surface layers are shown to be very advantageously studied using the spectrum-ratioing facility of a recording infrared interferometer. The emission from a given sample is ratioed against that from a black-body emitter at the same temperature so as to give emittance as a function of wavenumber directly. This method has very useful application to irregularly shaped metal emitters. In the absence of selective reflection there is a direct correspondence between emission and absorption spectra for thin layers of an emitting substance. However, the presence of selective reflection leads to reduced emission and to considerable differences in the appearance of "absorption" and emission spectra in regions of strong absorption. Emission spectra obtained from copper plates heated, above 150°C, for different periods in air are shown clearly to indicate the presence of cuprous, Cu(I), and cupric, Cu(II), oxides in the surface layer.
D. Kember and N. Sheppard, "The Use of Ratio-recording Interferometry for the Measurement of Infrared Emission Spectra: Applications to Oxide Films on Copper Surfaces," Appl. Spectrosc. 29, 496-500 (1975)