1. 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.

2. J. Phys. Chem. 71, (1979), 2723 Rev. Sci. Instrum. 52, (1981), 33 FT-3mm

3. Cavity based FTMW spectroscopy: Most spectrometers 6 – 26 GHz. Several operate from ~2 GHz to ~40 GHz

4. Below 1 GHz: This is our measurement of the 36 cm galactic methanol line, J KaKc = 1 10 – 1 11, detected in Sagitarius II. 500 averaging cycles. FT-3mm

5. 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. 232 Th 17 O FT-3mm

6. Prior work. FT-3mm

7. J. Chem. Phys, 115, (2001), 6007 “With this spectrometer, we were able to observe the J: 6 – 5 line of O 13 C 34 S in natural abundance by integration of 600 shots with a repetition rate of 5 Hz”. FT-3mm

8. Int. J. Infrared and Millimeter Waves 4, (1983), 733 FT-3mm

9. 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. FT-3mm

10. Rev. Sci. Instrum. 56, (1985), 97 Time domain experiments. Static gas technique. 30 MHz Modulated frequency doubler. FT-3mm

11. Int. J. Infrared and Millimeter Waves 7, (1986), 1329 Numerous circuit improvements. Static gas. Dismantled more than 15 years ago. FT-3mm

12. Our Fourier transform W-band spectrometer: 75 – 110 GHz 1.MW synthesizer 2.Power divider 3.SPST switch 4.SSB modulator 5.Active multiplier chain 6.LNA (W-band) 7.LNA (RF) 8.O-scope FT-3mm

14. 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. FT-3mm

15. Active Multiplier Chain (Millitech) FT-3mm

16. W-band pin diode switches (Millitech) FT-3mm

17. Model No: Bandwidth/GHz NF/dB (Max) Gain/dB (Min) SLW-15-5 75-110 4.5 20 Low Noise Amplifier (Spacek Labs) FT-3mm

18. 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.

19. 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). FT-3mm

20. Graduate Students: Kerry Etchison Smitty Grubbs II Chris Dewberry Physics Machine Shop. Chris Steve Smitty Kerry FT-3mm

21. We gratefully acknowledge financial support from: FT-3mm