June 1954 L E T T E R S T O T H E E D I T O R 505

Infrared Absorption of Liquid Water from 2 to 42 Microns

EARLE K . PLYLER AND NICOLO ACQUISTA National Bureau of Standards, Washington, D. C.

(Received March 8, 1954)

transfer of heat from cell to cell in animal tissue at body tempera-

FIG. 1. The infrared absorption spectrum of liquid water for three cell thicknesses from 2 to 10 microns.

ture involves the radiation at long wavelengths. The spectrum of water has been determined by many observers but there is a scarcity of data on the quantitative determinations for liquid water, expecially beyond 10 microns. Measurements from 2 to 10 microns were also repeated, for it was found that the older data1

for the absorption of water was in error in the percentage of transmission for the 3-micron region.

Figure 1 shows the percentage of transmittance of three cell thicknesses of water as measured with a Perkin-Elmer Model 21 double beam spectrometer using a NaCl prism. Calcium fluoride windows with spacers of different thicknesses were used for absorption cells in this region. The 0.03-millimeter shim was measured and the thickness of the other shims was determined by applying Beer's law at different wavelengths of the spectrum. The thicknesses determined in this manner checked within 15 percent of each other for the different wavelengths. The transmis­sion curves in Fig. 1 and Fig. 2 have been corrected for the decrease in transmission produced by the cell windows.

The results shown in Fig. 2 were obtained with a single beam spectrometer, using prisms of potassium bromide, cesium bromide, and cesium iodide. The absorption cells were made with windows of silver chloride and thallium bromide-iodide. The thallium

FIG. 2. The infrared absorption spectrum of liquid water from 10 to 42 microns.

bromide-iodide windows had optical flat surfaces which permitted the making of thin cells. The small irregularities in the transmis­sion curve for the 0.005-millimeter cell are attributed to errors in the reduction of data in the regions of atmospheric absorption of water vapor. The broad maximum of absorption occurring at about 15.5 microns appears to be the center of a band extending from 10 to 40 microns. Previous measurements by Rubens and Ladenburg2 from 2 to 18 microns showed a band at 14.8, while Sohm3 showed several small bands between 15 and 25 microns. These bands observed by Sohm were not found in this work. Cartwright4,5 made measurements on the absorption of water from 50 to 150 microns with cells of 0.0125-millimeter thickness. His results show a gradual increase in transmission in this region without any separate maxima.

The absorption band at 15.5 microns does not appear in the spectrum of the vapor and is attributed to intermolecular vibra­tions arising in groups of H2O molecules existing in liquid water. This vibration combines with the fundamentals at 3 and 6 microns and produces the two bands at 2.55 and 4.74 microns which are not present in the spectrum of the vapor. The combination band occurring at 2.55 microns has much less intensity than the band at 4.74 microns.

1 J. Lecomte, Le Spectre Infrarouge (Librairie Scientifique, Albert Blanchard, Paris, 1928). 2 H. Rubens and E. Ladenburg, Le Radium 6, 33 (1909). 3 Monica Sohm, Z. Physik 116, 34 (1940). 4 C. H. Cartwright, Nature 135, 872 (1935). 5 C. H. Cartwright, Nature 136, 181 (1935).

I N a number of different researches on transfer of energy the transmittance of water in the liquid state is needed. The