Chirped-Pulse Fourier Transform mm-Wave Spectroscopy from GHz Brent J. Harris, Amanda L. Steber, Justin L. Neill *, Brooks H. Pate University of Virginia, Department of Chemistry, University of Virginia, McCormick Rd, PO Box , Charlottesville, VA *University of Michigan Department of Astronomy, University of Michigan 500 Church St., Ann Arbor, MI 48109

mm-Chirped Pulse Spectrometer GHz GHz 258 – 295 GHz 220 – 325 GHz WR 3.4 Low IF (MHz – GHz) DC – 33 GHz Static Gas Experiments

Using the Power Molecular transitions saturate upon absorption on the order of μWatts. (mTorr) Fast frequency sweeps “spread” the power across a broad frequency range. (no saturation) For CPFT, weak pulse limit: more power = more signal For single transform limited pulses, the rest of the power is unused. Translating power into speed

Chirped-Pulse at 1mm Doppler dephasing dominates below 1mTorr (~1.5 us) Time domain signal after 45dB amplification Sensitivity is achieved by time domain averaging Scan rate: 36 GHz / 2 μs 18,000,000 GHz/s Short recovery from excitation compared to measurement time

Maximum Bandwidth Spectroscopy Tektronix: DPO/DSA/MSO70000 Series 100GS, 33GHz Bandwidth At 100GS, each trace 200k points Essentially 100% duty cycle for up to 250 Million points (1250 FIDs). Compatible for coupling measurement to transient events like 10Hz LASER Acetaldehyde,

Applications Compatible for coupling to transient events Pulse-jet synthesis Discharge, short-lived species Double-resonance spectroscopy LASER, dynamical studies (Can probe the time domain through 1250 FIDs) Suppose you want to interrogate one line

Narrow Band Sweeps Signal scales as (BW) 1/2 in weak pulse limit Acetaldehyde: 130:1 Methanol: 90:1 Methyl Formate 60:1 So, how to get the advantage out smaller bandwidth chirps??? Acetaldehyde,

Segmenting vs Fullband Segmenting: better signal strength, but longer experiment (50 FIDs) The result is : Same sensitivity Fullband: can signal average in equivalent time

Segmented CP & Real Time Averaging Agilent: U1084 Acquiris 8-bit High Speed PCIe Digitizer with on-board Signal Processing (4GS/s) Essentially 100% duty cycle up to 16 Million back-to back acquisitions Approach 100% duty cycle: Trace detections of analytes Number of data points per FID: 8,000 (seg) vs 200,000 (full) Signal averaging very stable!

Segmented CPFT vs Absorption * S.M. Fortman, I.R. Medvedev, C. F. Neese, F.C. De Lucia, Ap J, 2010, 725, 1682 FASST Absorption Spectroscopy * 6 m path length ~70GHz in 40 s CPFT Spectroscopy 4 m path length ~30GHz in 10 ms (1000X faster for equivalent sensitivity)

Segmented CPFT vs Absorption Tradeoff: Resolution 3X line width compared to FASSST Hallmarks of Segmented CPFT: - Measured against zero background - Simple frequency calibration - Minimal data manipulation/processing FFT (parallelizable for segmented) gain correction - Sensitivity Improvement by phase unwrapping of the magnitude spectrum to recover the absorption and dispersion line shapes. Could see 2X better line resolution for CPFT. Tradeoff: Spectral Purity AWG LO purity creates spurs and images Improvement by advances in AWGs. Also, fast switching MW synthesizers.

Conclusions and Future Directions Chirped-Pulse Fourier Transform spectroscopy translates the high power available in THz devices into speed. Sensitivity is achieved 1000X faster than the fastest absorption techniques. Full band swept experiment rep rate makes the technique compatible for coupling with transient laser events. Segmented sweeping of the spectrum results in equal sensitivity in the weak pulse limit and is accompanied with cost reduction in signal processing (both time and $). Essentially 100% duty cycle in time domain averaging can be achieved with real time digitizers. The speed of broadband detection of weak emitting analytes makes mm-Wave spectrum a good space for analytical chemistry.

Acknowledgements This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE Pate Lab NSF CCI (Center for Chemistry of the Universe) CHE