Knowledge Bank

A spectrometer for measurement of microwave absorption by gases at high temperature has been $constructed,1,2$ and with it spectra of gaseous NaCl, KCl, CsCl, and TlCl have been obtained. Microwaves pass through a 5-foot absorption cell which can be held at temperatures at least as high as $875\circ C$. Absorption lines are modulated by Stark effect to give sensitive detection. At approximately $775\circ C$ the pure rotational transition $J=1\to 2$ of NaCl was observed. Frequencies for the two Cl isotopes and various vibrational states are listed in Table I. These give $Bc(Cl35)=6536.9\pm 0.3Mc,\alpha e(Cl35)=48.1\pm 0.1Mc$, and the internuclear distance $re=2.3606\pm 0.0003\backslash AA$. Frequency measurements of the absorption lines were made with a frequency standard. However, experimental conditions gave lines several megacycles broad, which limited the precision of measurements which could be easily obtained to that indicated in Table I. A measurement of the ratio of intensities of the $NaCl37v=0$ and $NaCl35v=3$ lines gives a value for the vibrational frequency $\omega e(35)=378cm-1$ if the dipole moment is assumed independent of vibrational state. Including a reasonable variation of the dipole moment, this measurement may be in error $\pm 15\%$. It agrees well, however, with a value of $380cm-1$ obtained by $Levi.3$ At approximately $715\circ C$, the $J=5\to 6$ transition of CsCl was observed and lines listed in Table II were measured. These give values $Be(Cl35)=2163.8\pm 0.2Mc,\alpha e(Cl35)=10.06\pm 0.06$ Mc, and the internuclear distance $re=2.9041\pm 0.0003\backslash AA$. The observed spectrum of Kcl gave molecular constants in agreement with those found by measurement of rotational transitions in molecular $beams.4$ A rich spectrum of lines between 25,000 Mc and 23,500 Mc was $observed2$ in TlCl vapor at approximately $305\circ C$. This spectrum showed no obvious regularities, and cannot be produced by a [FIGURE] diatomic molecule, so that the vapor of TlCl must contain a considerable amount of dimers or some other combination of Tl and Cl. It may be noted that the $re$ value of CsCl is 4% less than the value of $3.02\pm .03\backslash AA$ obtained from electron diffraction measurements of the average overall vibrational states at $1200\circ C$, but falls within the experimental error of a molecular beam $measurements5$ which is $2.88\pm 0.03\backslash AA$. Likewise the $re$ value for NaCl is 5% less than the value of $2.48\pm 0.03\backslash AA$ given by electron diffraction $measurements.6$ This discrepancy is unexplained. The value of $\alpha e$ for $CsCl35$ obtained from molecular beam measurements of the product of the dipole moment and the moment of $inertia5$ is $15.6\pm 1.5$ Mc. The large discrepancy between this result and the directly measured value given here may be due to an incorrect assumption about the variation of the dipole moment with vibration state. We are very grateful to Mr. C. O. Dechert, foreman of the Columbia Radiation Laboratory machine shop, for considerable aid in the design and construction of the apparatus, as well as to Mr. A. P. Marshall and others of the machine shop staff. We also appreciate the help of Messrs. A. Javan and W. A. Hardy with several of the experimental measurements.