The pure Inversion-Tunneling Transition The pure Inversion-Tunneling Transition of Ammonia in Helium Dropletsof Ammonia in Helium Droplets

Rudi Lehnig and Wolfgang Jäger

Department of Chemistry, University of Alberta, Edmonton, AB Canada

• Gas-phase like (that means rotationally resolved) spectra of dopant molecules.

• Increased moment of inertia (dopant drags normal fluid helium density with it).

• Line widths range from 75 MHz to several GHz.

• Microwave spectroscopy may help to understand the line-broadening mechanisms(low-pass filter for droplet excitations).

Helium Nanodroplet SpectroscopyHelium Nanodroplet Spectroscopy

pure MW spectroscopy:• J = 4 – 3, J = 5 – 4 of HCCCN (10.5 – 14.5 GHz)

Reinhard et al., PRL 82, 5036 (1999). • Callegari et al., JCP 113, 4636 (2000).• J = 1 – 0 of HCN and DCN (58 – 74 GHz)

Conjusteau et al., JCP 113, 4840 (2000).

MW-IR double resonance:• 1 of HCCCN

Callegari et al., JCP 113, 4636 (2000).• 3 of OCS

Grebenev et al., JCP 113, 9060 (2000). Kunze et al., JCP 116, 7473 (2002).

Previous Microwave StudiesPrevious Microwave Studies

diffusion pump8000 L / min

turbo pump700 L / min

turbo pump700 L / min

turbo pump700 L / min

turbo pump340 L / min

cryostatca. 14 K

skimmer500 μm

doping cell

microwave resonator

quadrupolemass-spec

nozzle5 μm

Helium Nanodroplet SpectrometerHelium Nanodroplet Spectrometer

ℓHe-cooledbolometer, 1.5 K

radiation

2060 cm-1: 1 photon for 400 helium atoms8 GHz: 18 photons for 1 helium atom

Depletion SpectroscopyDepletion Spectroscopy

Implementation of Microwave ResonatorImplementation of Microwave ResonatorCavity mirrorsHelium droplet beam

Use of TWT microwave amplifier; output power: 10 Wreflectivity R ~ 0.98; total stored power: ~ 50 * Pin

Inversion Motion of AmmoniaInversion Motion of Ammonia

Microwave Inversion Transition of NHMicrowave Inversion Transition of NH33@He@HeNN10

-5 d

eple

tion

diffusion pump8000 L / min

turbo pump700 L / min

turbo pump700 L / min

turbo pump700 L / min

turbo pump340 L / min

cryostatca. 14 K

skimmer500 μm

doping cell

microwave resonator

quadrupolemass-spec

nozzle5 μm

Helium Nanodroplet SpectrometerHelium Nanodroplet Spectrometer

ℓHe-cooledbolometer, 1.5 K

flag

Microwave Inversion Transition of NHMicrowave Inversion Transition of NH33@He@HeNN10

-5 d

eple

tion

single ammonia molecule

sharp feature is saturated

Pick-up Pressure; Saturation BehaviourPick-up Pressure; Saturation Behaviour

Microwave Inversion Transition of Microwave Inversion Transition of 1515NHNH33@He@HeNN

Possible Causes for the Sharp PeakPossible Causes for the Sharp Peak

• Effect of Bose-Einstein Condensate fraction?

• Two different relaxation channels?

• Saturation effect, Lamb Dip?

• Maser action of ammonia molecule?

• The overall feature looks like the P-, Q-, R-branches of a vibrational band.

Energy level splitting through coupling of ammonia rotation with center-of-mass motion of ammonia in the droplet (particle in a box model).

Rotational wavefunction is identical and inversion wavefunction nearly identical Splitting is similar for both states.

Line-Broadening MechanismLine-Broadening Mechanism

K. K. Lehmann, Mol. Phys. 97, 645 (1999).I. Reinhard, C. Callegari, A. Conjusteau, K. K. Lehmann, and G. Scoles, PRL 82, 5036 (1999).K. K. Lehmann, JCP 126, 024108 (2007).

20.0 20.5 21.0 21.5 22.0 22.5 23.0 23.5Frequency / GHz

m’13121110

98

641

13121110

98

641

m”

Frequency / MHz

Inte

nsity

Simulated Line-ShapeSimulated Line-Shape

E = f1 m2 E = h (m+1/2) E = B m(m+1)

Results of SimulationsResults of Simulations

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AcknowledgementsAcknowledgements

• Chemistry Design and Manufacturing Facility

• Canada Foundation for Innovation• ASRA, ISRIP• NSERC• University of Alberta