INFRARED DISPERSION OF GASES
Loading...
Date
1951
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Ohio State University
Abstract
These measurements were originally undertaken to investigate the discrepancies between the temperature-invariant part of the dielectric constant and the index of refraction extrapolated to zero frequency. Van Vleck had shown that unless the measurements of absorption in the infrared bands were seriously in error, these bands could not contribute enough to the index to reduce this disagreement appreciably. He suggested that, since absolute measurements are extremely difficult, a measurement of dispersion in the neighborhood of the bands would be desirable. The first measurements were made on HCl with hollow prism equipment similar to what had been used by others for measurements on $CO_{2}$. The value obtained for du/dr or the ``effective charge'' for the vibration was $1.00 -10^{-10} esu$, in good agreement with that obtained by Bourgin from intensity measurements, and showing that the uncertainty in dielectric constant measurements must be somewhat greater than generally assumed. The success of the method in determining the equivalent of infrared intensities encouraged the improvement of the equipment and a survey of the dispersion of simple gases. The most significant improvement was a divided thermopile receiver which enabled determination of image position to about 0.001 mm, and, together with other improvements, resulted in a tenfold increase in both speed and accuracy. The next gas studied was methane. From the infrared-active bending mode a value of $0.31-10^{18}esu$ was obtained for the moment of the CH bond. The stretching mode gives $du/dr=0.562-10^{-10} esu$, for the same bond. For CO and NO we obtained $du/dr=3.18\cdot10^{-10}$ and $2.24\cdot10^{-10}$ esu, respectively. Thus, these molecules which have negligible dipole moments have rather strong infrared vibration bands. Very careful investigations, with further improvements in equipment were made on HCl and DCl to determine the sign of du/dr. The values obtained were 1.12 for HCl and 1.145 for DCl, which indicates that the sign of du/dr is negative. The values for du/dr were 0.954 for HCl and 0.001 for DCl. Measurements on methane, ethylene, and acetylene show that du/dr for CH bond is approximately the same for all of them while the bond moment varies in the ratio 1:2:3.
Description
Author Institution: Department of Physics, University of Wisconsin