The inversion motion in the Ne – NH3 van der Waals dimer studied via microwave spectroscopy

Laura E. Downie, Julie M. Michaud and Wolfgang Jäger

Department of Chemistry, University of AlbertaEdmonton, AB, Canada

June 21st, 2007

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Fascinating ammonia

• 1934: inversion in ammonia first

observed1

• 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).

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Weakly bound complexes with ammonia

• Rare gas atom - NH3 complexes:– Ne - NH3,1 Ar - NH3,2 Kr - NH3,3 and Xe - NH3.4

• Complexes where the inversion motion was investigated:– Ne - NH3 (inversion splitting)1

– Ar - NH3 (pure inversion transitions)2

– Xe - NH3 (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.

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Motivation for studying inversion motion in Ne - NH3

• 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 motion1 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).

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NH3 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

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NH3 inversion potential

0.79 cm-1

36 cm-1

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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 jk

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para-NH3 energy levels (j = 1, |k| = 1)

a

s

Here, unlike ortho-NH3, both tunneling components are present and are either

symmetric or asymmetric with respect to inversion

NH3 Σs

Σa

Πlower

Πupper

Rg - NH3

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Ar - NH3 transitions j = 1, |k| = 1

MW region

Σa

Πlower

a

s

Inversion Transitions

ΣsJ = 0

23

4

5

6

1

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Experimental set-up

Backing Pressure: between 5 and 80 atm

Precursors: 2% Molecular H2 or pH2, 0.02% OCS in He

Diffusion pump backed by a rotary fore vacuum pump

Stationary MirrorMovable Mirror (tuned by Motormike )

Nozzle

Gas mixture:

0.05-0.3%

NH3

in Ne or

2% 20Ne

in He.

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Example transition for Ne - NH3 inversion

22931.97 22932.44 22932.98 Frequency / MHz

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Pure inversion transitions observed

Centre frequency of a - s (MHz)

J = 1 - 1 J = 2 - 2 J = 3 - 320Ne - NH3 23218.8 22932.4 22699.5

22Ne - NH3 23238.7 22971.2 22742.0

20Ne - 15NH3 22178.2 21915.9 21694.9

22Ne - 15NH3 22196.3 21952.2 21736.0

Next step: try to find transitions within the s and a states

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Search region of J = 2 - 1 in ∑ states

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Transition at ~13297 MHz

Frequency (MHz)

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Transitions found within s and a states

Centre frequency (MHz)

Assignment (to be confirmed)

13010.9 a, J = 2 - 1, 20Ne

13019.1 ?

13085.9 ? (a, J = 2 - 1, 22Ne)

13297.9 s, J = 2 - 1, 20Ne

13348.1 ? (s, J = 2 - 1, 22Ne)

13533.5 ?

13842.09 ?

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Ne - NH3 inversion energy levels (j=1, |k| = 1)

Σs

Σa

J =0

2

3

4

Measured

Want to measure

1

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Conclusions

• Pure inversion transitions of Ne - NH3 have been assigned

• Rotational transitions within the inversion states have been observed and tentatively assigned

• Additional studies will hopefully confirm the assignments

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Acknowledgements

• Jäger and Xu groups• Funding from:

Thank you for your attention