THE INVERSION MOTION IN THE Ne - NH$_{3}$ VAN DER WAALS DIMER STUDIED VIA MICROWAVE SPECTROSCOPY
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Date
2007
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Publisher
Ohio State University
Abstract
The umbrella inversion motion of the ammonia molecule leads to a tunneling splitting of the ammonia energy levels. Transitions between rotational energy levels in different tunneling states lie in the microwave region and can be studied using a Fourier transform microwave spectrometer. Our study of Ne - NH$_{3}$ investigates the effect of the neon atom on the NH$_{3}$ inversion motion. Twelve transitions in total have been measured for the inversion motion in $^{20}$Ne - $^{14}$NH$_{3}$ (and several minor isotopomers: $^{22}$Ne - $^{14}$NH$_{3}$, $^{20}$Ne - $^{15}$NH$_{3}$, $^{22}$Ne - $^{15}$NH$_{3}$). The gas sample contained 0.3\% NH$_{3}$ in 4-5 atm of Ne. Due to isotope enrichment of $^{22}$Ne (9.25\% naturally abundant) in the molecular expansion, the $^{22}$Ne - NH$_{3}$ signals had similar intensities to the $^{20}$Ne - NH$_{3}$ signals. To distinguish the isotopomers, an enriched sample of 3\% $^{20}$Ne (99.95\%, Cambridge Isotopes) with 0.3\% NH$_{3}$ in a helium backing gas was used. The results from this enriched sample study show an anomalous isotope effect where the inversion transitions of the heavier $^{22}$Ne isotopomers are observed at higher frequencies. This isotope effect is not observed with the Ne - $^{15}$NH$_{3}$ isotopomers, whose inversion transitions were measured at lower frequencies than the analogous Ne - $^{14}$NH$_{3}$ dimers.
Description
Author Institution: Department of Chemistry, University of Alberta, Edmonton, AB, CANADA