The Pure Inversion-Tunneling Transition of Ammonia in Helium Droplets

The pure inversion tunneling transition of ammonia embedded in superfluid helium droplets has been measured for the two isotopomers $^{14}$NH$_3$ and $^{15}$NH$_3$. The transitions were found at 21.72\,GHz for $^{14}$NH$_3$ and at 20.68\,GHz for $^{15}$NH$_3$ and are therefore shifted by 1.97\,GHz and 1.94\,GHz to the red compared to the gas phase values of the two isotopomers, respectively [Pickett {\it et al.}, J. Quant. Spectrosc. Radiat. Transfer {\bf 60}, 883 (1998).]. The measured peaks consist of a broad background with a width at half maximum of 1.5\,GHz and an unexpectedly sharp peak with a width at half maximum of only 15\,MHz sitting on top of the broad background. The difference between gas phase and droplet spectra will be discussed and a interpretation of the complex line shape will be given.