LAURA E. DOWNIE, JULIE M. MICHAUD AND WOLFGANG JÄGER, Department of Chemistry, University of Alberta, Edmonton, AB, CANADA.
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 - NH3 investigates the effect of the neon atom on the NH3 inversion motion. Twelve transitions in total have been measured for the inversion motion in 20Ne - 14NH3 (and several minor isotopomers: 22Ne - 14NH3, 20Ne - 15NH3, 22Ne - 15NH3). The gas sample contained 0.3% NH3 in 4-5 atm of Ne. Due to isotope enrichment of 22Ne (9.25% naturally abundant) in the molecular expansion, the 22Ne - NH3 signals had similar intensities to the 20Ne - NH3 signals. To distinguish the isotopomers, an enriched sample of 3% 20Ne (99.95%, Cambridge Isotopes) with 0.3% NH3 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 22Ne isotopomers are observed at higher frequencies. This isotope effect is not observed with the Ne - 15NH3 isotopomers, whose inversion transitions were measured at lower frequencies than the analogous Ne - 14NH3 dimers.