$CH_{3}I$: ISOTOPE SHIFT FOR $^{129} I$ IN THE $\nu_{6}$ BAND
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Date
1980
Authors
Wahlen, M.
Tucker, G.
Marczak, E.
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Publisher
Ohio State University
Abstract
The isotope shift in the $\nu_{6}$ band of $CH_{3} I$ has been measured for the radioactive isotope $^{129} I$ (half the $=2.0 \times 10^{7}$ yrs) with respect to the stable isotope $^{127}$I, using tunable diode laser spectroscopy. The radioactive gas (mixture of $CH_{3}^{129}I$ and $CH_{3}^{127}I$) was probed at low pressure (0.73 Torr) in a 3.30 m cell. Twenty-nine transitions free from interference in the $^{R}Q (3,J)$ (at $909.2 cm^{-1}$) and $^{R}Q(4,J)$ (at $917.1 cm^{-1}$) branches up to J = 33 have been investigated. The isotope shift for $CH_{3}^{129} I$ with respect to $CH_{3}^{127} I$ measured on a line to line basis was found to be $\Delta\nu = (-0.0138 \pm 0.0006)$ $cm^{-1}$. No variation with J could be observed for the shift above the experimental uncertainty. The lines were fully resolved and the measured FWHM at p=0.73 Torr of 39 MHz was compatible with our previously measured values of the Doppler width of 28 MHz and of the self-broadening coefficient of $(36\pm3)$ MHz $Torr^{-1}$ for $CH_{3}^{127}I$. Proof of unambiguous line identification was obtained from the precise linear correlation between the measured frequency differences $\Delta \nu_{J} =\nu [^{R}Q(K,J + 1)] - \nu [^{R}Q(K,J)]$ of corresponding line pairs in the isotopic species. For the lines investigated $\Delta \nu_{J}$ increased with J from about 0.02 to $0.05 cm^{-1}$. The abundance ratio $^{129}I/^{127}I$ in the radioactive mixture was determined spectroscopically form the intensities of $^{R}Q(3,16)$ and amounted to $(0.034 \pm 0.003)$. An independent mass-spectrometrical determination yielded a ratio of $(0.030 \pm 0.001)$. Absolute line strength were measured for this line to be $S= (1.89 \pm 0.19) \times 10^{-21}$ $cm^{-1}$ $Molecule^{-1}$ $cm^{2}$ for $CH3$129I and $S=(1.84 \pm 0.18)^{-1} \times 10^{-21}$ $cm^{-1}$ for $Molecule^{-1}$ $cm^{2}$ for $CH_{3}^{127}I$.
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