June 1969 L E T T E R S TO T H E E D I T O R 779

Strengths of Twenty Lines in the v3 Band of Water Vapor*

HAROLD J. BABROV† AND ANTHONY R. HEALY The Warner & Swasey Company, Control Instrument Division

32-16 Downing Street, Flushing, New York 11354 (Received 12 December 1968)

INDEX HEADINGS : Spectra; Water; Infrared ; Absorption.

We have measured the strengths of 20 strong lines in the v3 band of H2O. Previously, in this laboratory, Babrov and Casden1 deter­mined the strengths of 42 moderately strong lines in the v1 and v3 bands using a modified curve-of-growth method to determine the strength of a line.

It was stated in Ref. 1 that one of the criteria for successful use of this modified curve-of-growth method is that the line be opti­cally thin. For the sample path used in the previous work (3.98 cm), many of the strong lines in the v3 band were not optically thin. In the present work, a shorter path length was used, to make the stronger lines in the band optically thin. The experimental technique and the apparatus used were the same as those de­scribed in Ref. 1 with the exception of the sample cell that was used and the technique for measuring the water-vapor pressure. Also, a simplified correction procedure for background absorption was used.

For this work, a 0.565-cm Monel-metal cell was used, with CaF2 windows. The sample cell was held at 420K±1K. The water-

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vapor pressure was measured using a water-handling system de­scribed previously.2 We had two problems which were minimal in the previous work with lines of moderate strength. First we had the problem of maintaining a constant pressure within the sample cell during the measurements. At the beginning of our experi­ments, due to adsorption on the walls of the cell, the water-vapor pressure would steadily decrease, reaching a total loss of about 40% over a period of twenty four hours. This effect was apparently absent in the previous work partly because the ratio of volume to surface area of the quartz sample cells was five times as great as with our present cell. Another contributing factor to the change of this effect is the change of the materials used in the construction of the sample cell. Because similar tests with non-adsorbing gases, such as CH4 and CO2, produced no such effect and because the system was vacuum tested, all possibilities of a leak were ruled out. This reduction of pressure was made negligible by maintaining a direct connection between the sample cell and the water-han­dling system; we then added sufficient water vapor to saturate the adsorption process. The pressure in the cell was read from the manometer in the water-handling system and was monitored con­stantly during the course of the measurements. In addition, the line at 3802.97 cm-1, measured in Ref. 1, was scanned each time the twenty lines of the present experiment were scanned; the pressure was calculated from the measured equivalent width and known strength of this line. The pressure obtained from the spectroscopic measurement gave consistently good agreement with the manometric pressure.

The second problem in our present work was that these very strong lines had an intrinsically higher background absorptance (measured absorption with the cell evacuated). This problem was surmounted by pumping the monochromator to a lower pressure than previously (7×10 - 5 torr) and purging the source unit for a longer time before starting the measurements. We adopted a pro­cedure used only occasionally in the previous work: The equivalent widths of all of the background lines were first measured relative to the line at 3837.85 cm-1. Then, the equivalent width of this line was measured before and after each run, and the equivalent widths of the background lines were calculated from the known ratios and this measured equivalent width.

The calculation of the experimental strengths from the spectra and pressure measurements is described in Ref. 1. The steps in­volved in calculating the line strengths from the primary data, the integrated absorptance, pressure, and temperature are given in Tables I through VII.3 Table I lists the pressure of water vapor and the equivalent width of the standard background line (at 3837.85 cm-1) for each run. Table II lists the ratios of equivalent widths of the lines studied to the equivalent width of the standard line, for a complete background spectrum. Tables III through VI are similar to tables with similar titles in Ref. 1, so they require no further explanation. The line strengths are shown in Table VII. The first column of Table VII lists the frequencies of the lines studied, in descending order, the following three columns give the upper-state rotational quantum numbers J', Ka', Kc', of each line4

and the following three columns give the lower-state rotational quantum numbers J", Ka", Kc". The next two columns are the experimental line strengths and the strengths calculated from the rigid-rotator approximation, respectively, at 420 K. The last column in Table VII is the ratio of the measured line strength to the rigid-rotator strength, the F-factor.

Overlapping by small neighboring lines was important only for the line at 3950.00 cm-1, which was overlapped by lines at 3949.50 and 3950.13 cm-1. We calculated the combined effect of the over­lapping lines to be 9%. This calculation is crude and could be in error by 30% to 40%; therefore the additional error due to over­lapping may be as high as 4% for the line at 3950.00 cm-1. For all other lines, the errors from the uncertainty in the base line, inaccuracy of the half-width, and the overlap by neighboring lines should amount to 12% at worst, as explained in Ref. 1.

The strong lines studied in this work show only a gradual in­crease of the F-factors as the frequency decreases. There are no

TABLE VII. Strengths of lines in the V3 band of H2O.

highly perturbed lines, in contrast to the sample of moderately strong lines described in Ref. 1 which included many highly per­turbed lines. Even the line at 3950.00 cm-1, listed as perturbed in Ref. 4, follows the same trend as the other strong lines studied. The over-all variation of the F-factors is not as large as that of the lines studied previously, because these strong lines are closer to the band center; the rotational quantum numbers for these lines were lower than for those of Ref. 1.

We are most grateful to Mr. Luke Bow for his painstaking reduction of the experimental data.

* Supported by U. S. Air Force, Arnold Engineering Development Center, Tullahoma, Tennessee.

† Present address: Physics Department, Indiana University-South Bend, South Bend, Indiana 46615. 1 H. J. Babrov and F. Casden, J. Opt. Soc. Am. 58, 179 (1968). 2 G. J. Maclay.J.Chem. Phys.43, 185 (1964).

3 For Tables I through VI, order Document NAPS-00293 from ASIS National Auxiliary Publications Service, c/o CCM Information Sciences, Inc., 22 West 34 Street, New York, New York 10001, remitting $1.00 for microfiche or §3.00 for photocopies.

4 D. M. Gates, R. F. Calfee, D. W. Hansen, and W. S. Benedict, Line Parameters and Computed Spectra for Water Vapor Bands at 2.7μ, Natl. Bur. Std. (U. S.) Monograph 71 (U. S. Gov't. Printing Office, Washington, D. C , 1964).