Chirped-Pulse Fourier Transform mm-Wave Spectroscopy from 260-

295GHz

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 400319, Charlottesville, VA 22904

*University of MichiganDepartment of Astronomy, University of Michigan

500 Church St., Ann Arbor, MI 48109

mm-Chirped Pulse Spectrometer

2-3.5 GHz

10.8-12.3 GHz 258 – 295 GHz

220 – 325 GHzWR 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 Series100GS, 33GHz Bandwidth

At 100GS, each trace 200k pointsEssentially 100% duty cycle for up to 250 Million points (1250 FIDs).

Compatible for coupling measurement to transient events like 10Hz LASER

Acetaldehyde,

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: 130:1Methanol: 90:1Methyl Formate 60:1

So, how to get the advantage out smaller bandwidth chirps???

Acetaldehyde,

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 Digitizerwith 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 Spectroscopy4 m path length~30GHz in 10 ms

(1000X faster for equivalentsensitivity)

Segmented CPFT vs Absorption

Tradeoff: Resolution3X 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 PurityAWG 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-0809128

Pate LabNSF CCI (Center for Chemistry of the Universe)

CHE-0847919