A FABRY-PERÓT CAVITY PULSED FOURIER TRANSFORM W-BAND SPECTROMETER WITH A

PULSED NOZZLE SOURCE.

GARRY S. GRUBBS II, CHRISTOPHER T. DEWBERRY AND

STEPHEN A. COOKE,

Department of Chemistry, The University of North Texas, P. O. Box 305070, Denton, Texas, 76203,

U.S.A.

J. Phys. Chem. 71, (1979), 2723

Rev. Sci. Instrum. 52, (1981), 33

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Cavity based FTMW spectroscopy:

•Most spectrometers 6 – 26 GHz.

•Several operate from ~2 GHz to ~40 GHz

Below 1 GHz:

This is our measurement of the 36 cm galactic methanol line, JKaKc = 110 – 111, detected in Sagitarius II. 500 averaging cycles.

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W-band (75 – 110 GHz)

• Ultimately we hope to look at the pure rotational spectra of simple heavy element-containing species.

• For diatomic molecules containing one very heavy element and one light element rotational constants can be large (~ 50 GHz).

• Numerous lanthanide- and actinide-containing molecules have ground states with high orbital angular momentum. Often low J rotational levels are missing.

232Th17O

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Prior work.

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J. Chem. Phys , 115, (2001), 6007

“With this spectrometer, we were able to observe the

J: 6 – 5 line of O13C34S in natural abundance by

integration of 600 shots with a repetition rate of 5 Hz”.FT-3mm

Int. J. Infrared and Millimeter Waves 4, (1983), 733

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Kolbe and Leskovar spectrometer 1

•Klystron source from 67 to 73 GHz, passed through a high efficiency doubler

•Fabry-Perót cavity. Spherical mirrors, 50 mm in diameter, 148 mm radius of curvature, 74 mm apart. Loaded Q ~ 109000.

•Static gas at low pressure.

•Absorption spectroscopy.

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Rev. Sci. Instrum. 56, (1985), 97

•Time domain experiments.

•Static gas technique.

•30 MHz Modulated frequency doubler.

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Int. J. Infrared and Millimeter Waves 7, (1986), 1329

•Numerous circuit improvements.

•Static gas.

•Dismantled more than 15 years ago.FT-3mm

Our Fourier transform W-band spectrometer:

75 – 110 GHz

Frequency/MHz

50 100 150 200 250 300 350 400

High Frequency Test Circuit1. MW synthesizer2. Power divider3. SPST switch4. SSB modulator5. Active multiplier chain6. LNA (W-band)7. LNA (RF)8. O-scope

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Small T vacuum chamber

Pumped using Varian V-250 turbo pump backed by Varian SD40 rotary vane pump.

Pulsed nozzle located perpendicular to the axis of microwave propagation.

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Active Multiplier Chain (Millitech)

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W-band pin diode switches (Millitech)

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Model No: Bandwidth/GHz NF/dB (Max) Gain/dB (Min)

SLW-15-5 75-110 4.5 20

Low Noise Amplifier (Spacek Labs)

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Coupling of microwaves into the cavity:

Will likely be an iterative process:

1. Simply try butting the end of the waveguide up against the mirror. Waveguide will terminate on to waveguide dimensioned circular holes passing through the mirror??

2. Try wire antenna. (Tough because of waveguide size)

3. Use horn antenna. Very efficient coupling but significantly reduce the Q.

On going…

Need most of the circuit components to be inside the vacuum chamber.

Waiting for machining of cooling blocks and vacuum chamber.

Potential problems concerning the coupling of electromagnetic radiation into the cavity.

Temporary/short term fall back is a “Q=1 experiment” using opposing horn antenna (on order).

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Graduate Students:

Kerry EtchisonSmitty Grubbs IIChris Dewberry

Physics Machine Shop.

Chris Steve Smitty Kerry

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We gratefully acknowledge financial support from:

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