# DETERMINATION OF THE INDEXES OF REFRACTION IN THE INFRARED SPECTRUM

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 Title: DETERMINATION OF THE INDEXES OF REFRACTION IN THE INFRARED SPECTRUM Creators: Lecomte, J. Issue Date: 1958 Publisher: Ohio State University Abstract: “Till now, the literature has afforded very many references concerning the absorption spectra in the infrared, but, on the other hand, the determinations of the indexes of refraction are not very many. This lack appears to be deplored as the way the liquids and solids absorb could not be made clear without knowing the dispersion. This is the reason why we started, at the Sorbonne, carrying out systematically various methods on the determination, in the infrared spectrum, of indexes of refraction. As you know, one of the main difficulties is due to the very strong absorption shown by the liquids and the solids in the sometimes very broad regions of our spectral range. We remind that about 15 years ago, in our services at the Sorbonne, we had made use of a method by total reflection, which already had led us to interesting results up to wavelengths of about 2.5 $\mu$. With the contribution of Mine. Vincent-Geisse, we have improved the well-known interferential method, by using as an interferometer a variable spacing absorption cell with a thoroughly optical flat surface of NaCl or KBr plates and covered with a semi-transparent deposit of arsenic pentaselemide. This substance appears to be particularly suitable, according to M. F. Gans and Mine. G. Wagner, on account of its high and constant reflecting power, between the visible and about 17 $\mu$. The use of a double beam spectrograph makes easier the absorption bands, weak as well as medium. In about one day, we can obtain the dispersion curve of a substance between approximatively 3 and 15 $\mu$. The determination of the interference orders is made according to a new method avoiding the difficulties and the uncertainties with which all those who used this interferential method had been faced. It leads us to the indexes of refraction of various organic substances (benzene, saturated aliphatic alcohols, carbon tetrachloride and carbon disulphide, chloroform, 1, 2-dichloro- and 1, 2-dibromo-ethanes . . . ) in the spectral range indicated hereafter. Instead of disposing the liquid directly between the interferometer plates, the method can be extended to substances soluble in convenient solvents. As concerns solids, Mlle. Jeromee successfully reached interesting results by means of the method of Christiansen filters, using as liquids containing the powder in suspension those whose index of refraction had been determined by the interferential method said before. As you know, the Christiansen filters lead with a feeble accuracy-to the determination of indexes of refraction but while working in suitable conditions size of grain of powder, thickness of the filter, and above all, suitable choice of the liquid so that its dispersive curve cuts under a convenient angle the dispersive curve of the solid those techniques being of a great help in numerous cases, in the present problem. Finally, with highly absorbing solids, Mme. Cameo-Bosco started working out the well-known method through reflection, by means of a new dispositive to be adapted to infrared commercial spectrographs and using an incidence angle of only a few degrees: the measures can accordingly be taken without difficulty in series, with a scope of research works almost unexplored. Let us mention, at last, the working out of a refractometer in infrared by Mme. Vincent-Geisse, M. Gans and Mme. G. Wagner, to be used with infrared commercial spectrographs, and means to start measuring as systematically and automatically as possible, the indexes of refraction of glasses and optical instruments.” Description: Author Institution: Laboratoire de Recherches Physiques \'{a} la Sorbonne URI: http://hdl.handle.net/1811/7780 Other Identifiers: 1958-L-07