The Search is Over: Design and Applications of a Chirped Pulse Fourier Transform Microwave (CP- FTMW) Spectrometer for Ground State Rotational Spectroscopy Kevin O. Douglass, Gordon G. Brown, Brian C. Dian, Scott Geyer, Brooks H. Pate University of Virginia Department of Chemistry

Summary of Chirped Pulse Advantages Power reduction over transform limited pulses Decouples bandwidth from pulse duration 10 4 reduction in power requirements Signal scales as Allow frequency multiplication (Increase Initial Bandwidth)

Chirped Pulse Generation: x8 Scheme x4 x2 7.5 – 18.5 GHz To Experiment Chirped Pulse Frequency Sweep – MHz Phase locked oscillator 9.9 GHz 10 MHz Rb Oscillator 4 GHz Arb. Waveform Generator Dual Channel Independent Trigger Laser/TWT Trigger Scope Trigger 0-80 dB Programmable Atten. Single Sideband Filter Sweep – GHz Sweep – GHz 19.8 GHz High Power TWT Amplifier x2 Low Power Loop

1 GHz100 MHz10 MHz Chirped Pulse vs Transform Limited Pulse Phase Control: Chirped pulse excitation* Chirped pulse Signal α 1/SQRT(bandwidth) Transform limited pulse Signal α 1/bandwidth * J.C. McGurk, T.G. Schmalz, and W.H. Flygare, J. Chem. Phys. 60, 4181 (1974). CP= TLP=

Phase Control: Chirped pulse excitation* * J.C. McGurk, T.G. Schmalz, and W.H. Flygare, J. Chem. Phys. 60, 4181 (1974). Chirped Pulse vs Transform Limited Pulse Chirped pulse Signal α 1/SQRT(bandwidth) CP= TLP= Signal-to-Noise is independent of Bandwidth!! 1 GHz 100:1 S/N  10 shots to cover 10 GHz 10 GHz :1 S/N  10 avgs. to get ≈ S/N Same # of valve pulses to obtain equivalent S/N and spectral range

11 GHz CP-FTMW Spectrometer Pulse Monitor 12 GHz Oscilloscope (40 Gs/s) Free Induction Decay 0.5– 11.5 GHz Arbitrary Waveform Generator Chirped Pulse Frequency Sweep 4 GS/s x GHz TWT Amplifier FID acquisition and Fourier transform 9.9 GHz PDRO 2 GHz Bandwidth Multi-Nozzle Sample Feed GHz PDRO IR Multipass

11 GHz Chirped Pulse

Broadband FTMW Chamber Design Double ridge waveguide horns replace the narrow band cavity Allows for broadband transmission and detection 7.5 – 18 GHz (with no moving parts) Q≈1, averages are needed to compensate for the factor of (Q) 1/2 loss in S/N

100 Shots: 20 s acquisition ~ 2  mol sample consumption Pure Rotational Spectrum of Suprane 20  s of FID Acquisition (80 kHz linewidth, FWHM) shots 20 μ s gate: 45 min. acquisition B-F Equivalent 0.1% Suprane in He/Ne Choose Your Sensitivity

100 Shots: 20 s acquisition ~ 2  mol sample consumption Pure Rotational Spectrum of Suprane 20  s of FID Acquisition (80 kHz linewidth, FWHM) 0.1% Suprane in He/Ne Choose Your Sensitivity ~500:1 S/N in 20 seconds Cavity has moved 5 MHz

Epifluorohydrin Pure Rotational Spectrum A (MHz) B (MHz) C (MHz) μ a 1.42* μ b 3.12 μ c 0.34 *B3LYP G**

Epifluorohydrin “Minor” Conformer and Carbon-13 Rotational Spectra Minor conformer Main conformer 13 C-1 13 C-2 13 C A (MHz) B (MHz) C (MHz) μ a 0.41* μ b 0.48 μ c 0.06 *B3LYP G**

Chirped Pulse Generation: x8 Scheme x4 x2 7.5 – 18.5 GHz To Experiment Chirped Pulse Frequency Sweep – MHz Phase locked oscillator 9.9 GHz 10 MHz Rb Oscillator 4 GHz Arb. Waveform Generator Dual Channel Independent Trigger Laser/TWT Trigger Scope Trigger 0-80 dB Programmable Atten. Single Sideband Filter Sweep – GHz Sweep – GHz 19.8 GHz High Power TWT Amplifier x2 Low Power Loop

Epifluorohydrin: Double Resonance M. Nakajima, Y. Sumiyoshi, Y. Endo, Rev. Sci. Instrum. 73 (2002) 165. Chirped pulse sets up coherence over the full bandwidth of the spectrum 7.5 – 18 GHz Single Frequency pulse (~1 MHz FWHM) destroys coherence on resonance ARB MW Horn x pump

Epifluorohydrin: Double Resonance

Linear sweep 7.5 – 14 GHz Linear sweep GHz pump MHz * Epifluorohydrin: Double Resonance *

Advantages of CP-FTMW Source optimization (for new species) –Temperature ramp Double Resonance: –Frequency agility of the arbitrary waveform generator (change freq. in 250 ps) –Selective excitation for double resonance ~ 100 Double resonance scans in 1 hr

Conclusions We have built a FTMW spectrometer with chirped pulse excitation GHz in a single valve pulse! 500:1 S/N in 20 seconds Equivalent S/N as cavity in averages –45 minutes (possible 17 minutes) Double (multi) resonance experiments from a single source over full bandwidth of spectrometer

Future Experiments Sensitivity enhancements Power upgrade to 3 kW (factor of 3 in S/N) Coaxial design with off axis parabolic mirror –Possible factor of 10 in S/N Fully 2-D Microwave Spectrum Chirped pulse – Chirped pulse

Acknowledgements Richard Suenram Pate Lab Group Members Funding: NSF Chemistry SELIM Program NSF MRI Program (with Tom Gallagher, UVa Physics) University of Virginia John D. and Catherine T. Macarthur Foundation The Jeffress Trust

Propyne-NH 3 Dimer