Investigations in the infra-red region of the spectrum. Part IV. -The monochromator method in the infra-red

Abstract

Absorption spectra in the infra-red are usually obtained by passing radiation from a Nernst filament through an absorption tube, and converging it to form an image of the filament at the collimator slit of the spectrometer. The radiation traverses the resolving train, and an image of the collimetor slit is focused on the thermoplie slit; a direct record of the energy curve of the Nernst filament is then made by means of a thermopile connected to a high sensitivity galvanometer, or an equivalent device. A shutter is inserted between the filament and collimator slit, and the percentage absorption at any wave-length is most conveniently determined by noting the galvanomater defection on opening the shutter when ( a ) an empty, and ( b ) a full absorption tube lies in the path of the radiation. This necessitates the use of a device such as a rocker to exchange the full for the empty tube at each spectrometer setting. The most important factor in the design of this rocker is that the empty and full tubes occupy accurately the same position when either is in the path of the beam. If this condition is not satisfied, the image of the filament on the collimator slit may be displaced, and different longitudinal sections of the image will be presented at each interchanges; the intensity or radiation traversing any longitudinal section of this image varies rapidly from the centre to the edge, and the displacement will give a difference in galvanometer deflections for the empty and full tubes even when no absorption occurs. In certain work which we propose to carry out later we shall require long absorption tubes of stainless steel, and a rocker of inconveniently heavy construction would be necessary. There is a further disadvantage of the rocker method in its use with gases; leads from a pumping system must either be flexible or be disconnected after each change in pressure. High purity of absorbing gas is desirable and rubber joins are to be avoided, hence glass to glass position seals must be after each adjustment of pressure, or a large glass spiral of sufficient flexibility must be permanently incorporated in the system. The first method is almost impracticable; the second gives erroneous results in the determination of the very small gas pressures necessary when the fundamental bands of certain substances are under examination; it is of considerable importance to know this gas pressure accurately, and in some cases these bands are so intense that pressures of less than 1 mm. are required to give a maximal absorption of 50 per cent. under the conditions of the experiments.