15min:

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.