1. The inversion motion in the Ne – NH 3 van der Waals dimer studied via microwave spectroscopy Laura E. Downie, Julie M. Michaud and Wolfgang Jäger Department of Chemistry, University of Alberta Edmonton, AB, Canada June 21 st, 2007

2. 2 Fascinating ammonia 1934: inversion in ammonia first observed 1 Beginning of a new field: microwave spectroscopy Since then, ammonia and its umbrella motion have been the focus of hundreds of papers 1. C. E. Cleeton and N. H. Williams, Phys. Rev. 45, 234 (1934).

3. 3 Weakly bound complexes with ammonia Rare gas atom - NH 3 complexes: –Ne - NH 3, 1 Ar - NH 3, 2 Kr - NH 3, 3 and Xe - NH 3. 4 Complexes where the inversion motion was investigated: –Ne - NH 3 (inversion splitting) 1 –Ar - NH 3 (pure inversion transitions) 2 –Xe - NH 3 (inversion splitting) 4 1.J. van Wijngaarden, W. Jäger, J. Chem. Phys. 115, 6504 (2001). 2.E. Zwart, et al. J Chem. Phys. 95, 793 (1991). 3.J. van Wijngaarden, W. Jäger, Mol. Phys. 99, 1215 (2001). 4.Q. Wen, W. Jäger, Manuscript in preparation.

4. 4 Motivation for studying inversion motion in Ne - NH 3 To understand the inversion motion in ammonia - rare gas atom dimers A step to larger rare gas clusters with ammonia Helium nanodroplet experiments in the IR studied the inversion motion 1 and predicted ground state transition frequency to be ~24 GHz (MW study presented TJ01) 1.M. N. Slipchenko, A. F. Vilesov, Chem. Phys. Lett. 412, 176 (2005).

5. 5 NH 3 monomer 36 cm -1 0.79 cm -1 2072 cm -1 Pairs of energy levels in the double well due to tunneling motion of ammonia

6. 6 NH 3 inversion potential 0.79 cm -1 36 cm -1

7. 7 Quantum number nomenclature Rotational quantum numbers Free ammonia: j, k Rg-ammonia: J, K Internal Rotor States Σ states: angular momentum perpendicular to radial coordinate (K=0) Π states: angular momentum parallel to radial coordinate (|K|=1) J K j k

8. 8 para-NH 3 energy levels (j = 1, |k| = 1) a s Here, unlike ortho-NH 3, both tunneling components are present and are either symmetric or asymmetric with respect to inversion NH 3 ΣsΣs ΣaΣa Π lower Π upper Rg - NH 3

9. 9 Ar - NH 3 transitions j = 1, |k| = 1 MW region Σ a Π lower a s Inversion Transitions Σ s J = 0 2 3 4 5 6 1

10. 10 Experimental set-up Gas mixture: 0.05-0.3% NH 3 in Ne or 2% 20 Ne in He.

11. 11 Example transition for Ne - NH 3 inversion 22931.97 22932.44 22932.98 Frequency / MHz

12. 12 Pure inversion transitions observed Centre frequency of  a -  s (MHz) J = 1 - 1J = 2 - 2J = 3 - 3 20 Ne - NH 3 23218.822932.422699.5 22 Ne - NH 3 23238.722971.222742.0 20 Ne - 15 NH 3 22178.221915.921694.9 22 Ne - 15 NH 3 22196.321952.221736.0 Next step: try to find transitions within the  s and  a states

13. 13 Search region of J = 2 - 1 in ∑ states

14. 14 Transition at ~13297 MHz Frequency (MHz)

15. 15 Transitions found within  s and  a states Centre frequency (MHz) Assignment (to be confirmed) 13010.9  a, J = 2 - 1, 20 Ne 13019.1 ? 13085.9 ? (  a, J = 2 - 1, 22 Ne) 13297.9  s, J = 2 - 1, 20 Ne 13348.1 ? (  s, J = 2 - 1, 22 Ne) 13533.5 ? 13842.09 ?

16. 16 Ne - NH 3 inversion energy levels (j=1, |k| = 1) ΣsΣs ΣaΣa J =0 2 3 4 Measured Want to measure 1

17. 17 Conclusions Pure inversion transitions of Ne - NH 3 have been assigned Rotational transitions within the inversion states have been observed and tentatively assigned Additional studies will hopefully confirm the assignments

18. 18 Acknowledgements Jäger and Xu groups Funding from: Thank you for your attention