EVIDENCE OF COLLISIONAL NARROWING IN THE INFRARED LASER STARK SPECTRUM OF $CH_{3}CN^{*}$
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Date
1981
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Ohio State University
Abstract
The lineshape of the $JKM=211+111$ transition in the infrared-laser Stark spectrum of the $\nu_{4}$ a band of $CH_{3}CN$ has been recorded by means of a computer-controlled spectrometer with the P(20) transition of an $N_{2}O$ laser as the source. The lineshapes were first analyzed and the results reported at the Symposium on Molecular Spectroscopy in 1979. The Lorentz widths obtained by assuming the theoretical value of the Doppler width were plotted against the pressure in the range 0.01-1.0 torr. A linear variation in the high pressure region led to a zero-pressure intercept below the origin and the low-pressure data showed a marked deviation from a straight line. The original data, taken with a sample cell with a 1 mm spacing between the Stark electrodes, and some more recent data, taken with a sample cell with a 3 mm spacing, have now been analyzed by taking into account the phenomenon of collisional narrowing (Dicke $narrowing^{1}$). The standard deviation of the difference between observed and calculated intensities is reduced significantly by inclusion of collisional narrowing. In addition, a narrowing parameter can be selected that eliminates the curvature in the low pressure region of the plot of Lorentz width $vs$ pressure and that leads to an intercept at zero pressure equal to the calculated value of the combined effect of wall collision broadening and finite team transit time. The collision broadening parameter, 70 MHz/torr, is close to the previously-reported value, which was derived form the slope of the higher pressure points obtained by the Voigt profile analysis. The narrowing parameter is approximately one-third the collision-broadening parameter and somewhat greater than twice the collision rate predicted from gas law data.
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$^{*}$ This work was supported by the National Science Foundation. $^{1}$ R. H. Dicke, Phys. Rev. 89, 472 (1953); J. P. Wittke and R. H. Dicke, Phys. Rev. 103, 620 (1956).
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