low-Temperature Microcell for Infrared Study of Condensed Gases James
E. Stewart,'
National Bureau of Standards, Washington
have been devised for T obtaining infrared spectra of small amounts of material with liquid or solid ECHNIQUES
samples, which employ microcells, lowpowered microscopes with refracting optics, and higher-powered microscopes with reflecting optics. The standard 10-cm. cell for gases has a volume of about 200 ml., or about 20 nil. per cm. of path length. The development of folded-path cells (4) has decreased this ratio to about 9.5 and 6.5 for 1- and 10meter cells, respectively. About 0.25 ml. of a typical gas sample at atmospheric pressure will yield a satisfactory spectrum when expanded into a 10meter cell. This is about the same amount obtained in a separation made by vapor phase chromatography. The folded-path gas cells derive their efficiency from repeated absorption by the same material, but the use of diverging radiation requires large crosssectional areas in these cells. Another approach is to reduce the area of the beam of radiation with appropriate optics, collimate it, and pass it through a long cell just large enough to accommodate the radiation. The length of such a cell is inconvenient. This disadvantage, as well as the necessity for collimation, can be avoided by condensing the gas into a thin film a t a reduced temperature. The cell described here is the result of a first attempt to exploit this possibility. The cell, shown in Figure 1, is based on a conventional single Dewar vessel, with a brass cold finger soldered to a Kovar cup a t the bottom of the liquid nitrogen container. A thin (
