1. Frequency Band Performance Comparisons for Room-Temperature Chirped Pulse Millimeter Wave Spectroscopy
Justin L. Neill, Brent J. Harris, Robin L. Pulliam, Matt T. Muckle
BrightSpec, Inc., 770 Harris St. Suite 104b, Charlottesville, VA, USA.
Brooks H. Pate
Department of Chemistry, University of Virginia, McCormick Road, Charlottesville, VA, USA.
June 24, 2016
FC10

2. Overview
Goal of the study: To offer a quantitative performance comparison between room temperature chirped-pulse millimeter wave spectrometers operating in different frequency bands for rapid analytical characterization of complex mixtures.
Criteria:
Correct identifications (needs: high resolution, good frequency coverage)
Particular challenge is finding a weak component in a strong ‘forest’
Range of molecular coverage (needs: bandwidth, matched to molecules of interest)
Sensitivity (needs: high power, well matched to strong transitions)
Ability to do advanced pulse sequences (need: high power to reach p/2, p pulse conditions)
Acknowledgement: This material is based upon work supported by the U.S. Army under Contract No. W31P4Q-15-C-0019.
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3. The Theory
The basic population/frequency factors favor higher frequency….
Simulations at 313 K
Propylene
Acrolein
Anisole
Note: Very small molecules (e.g. NH3, H2O, CO, NO….) have wider frequency spacings and do not emit in every band.
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4. The Instruments ….but other band-dependent effects also play a role.
Specification
GHz
(W-Band)
GHz
GHz
Source power (typ.)
30 mW*
30 mW
2 mW
SSB Conversion Loss
10 dB
8.5 dB
11.5 dB
Beam Size (FWHM)
4.0 cm
1.8 cm
1.2 cm
Typical Transmission Efficiency (power)
40%
35%
15%
Typical FID dephasing time
(1/e2, Doppler limited, OCS)
3 ms
900 ns
450 ns
*More power available (Gallium Nitride pulsed amplifiers)
Beam size is set by parabolic mirrors – and limited by desire to have Rayleigh length of ~80 cm
The bigger beam size at W-band allows us to interact with more molecules and achieve better
sensitivity – signal proportional to A1/2L – if we can achieve the same Rabi flip
angle excitation (and slower dephasing at W-band allows this!)
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5. The Measurements
Broadband Survey: Segmented Chirped Pulse Millimeter Wave Spectroscopy
Chirped pulses generated by 12 GS/s arbitrary waveform generator
(Keysight M8190, typ. -60 dBc spur-free dynamic range)
Detection made on 4 GS/s digitizer w/real time signal accumulation
(Keysight U1084A)
Targeted Monitoring:
Achieve “p/2” resonance condition for optimum sensitivity
Can perform double resonance measurements to recover
selectivity in complex mixtures
Time (ms)
Intensity (V)
J.L. Neill et al., Opt. Express 2013, 21,
A.L. Steber et al., 68th ISMS, 2013, Talk WH12.
B.J. Harris et al., 68th ISMS, June 2013, Talk WH13.
B.J. Harris et al., Proc. SPIE 9362, Terahertz, RF, MMW and Sub-MMW Technology and Applications VIII, , Feb 2015.
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6. The Measurements Optical:
Stainless steel sampling chamber, 65 cm length
CF63 end ports (I.D. 6 cm), Teflon windows
Dursan coated, heating capability
Off-axis parabolic mirrors, 75 mm eff. focal length
20-25 dBi standard gain feed horns
GHz FT-MRR Spectrometer
in Discovery form factor
(Reconfigurable)
Calibrated mixture – six components at 1% each, balance N2
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7. Partial Pressure 3s Sensitivity (mTorr)
The Results
Single Component Reference Samples
Example: Ethanol
GHz, 2.4 mTorr
GHz, 1.3 mTorr
H2O
GHz, 2 mTorr
Molecule
Partial Pressure 3s Sensitivity (mTorr)
10 min broadband survey
GHz
GHz
GHz
CH3CN
0.11
0.006
OCS
0.16
0.019
1.1
MeOH
2.0
1.2
4.1
EtOH
2.5
0.91
1.00
THF 11
1.9
6.2
Acetic Acid
3.3
0.52
1.65
Acrolein
0.72
0.12
2.6
10 minute high-dynamic-range surveys
(segment bandwidths ~30 MHz)
Signal to noise ratio ~2,000:1 (s ~ 2x10-4 mV)
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8. Quantifying Spectral Confusion
Most molecules (especially as they get bigger) can occupy a lot of channels….this can impose a practical detection limit in trace scenarios – finding a weak component in a dense forest.
Complexity metric: fraction of channels occupied as a function of dynamic range
Ethanol
Apodization window:
Kaiser-Bessel (b = 8)
Linewidths (FWHM):
GHz: 0.55 MHz
GHz: 1.2 MHz
GHz: 2.8 MHz
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9. Quantifying Spectral Confusion
Most molecules (especially as they get bigger) can occupy a lot of channels….this can impose a practical detection limit in trace scenarios – finding a weak component in a dense forest.
Complexity metric: fraction of channels occupied as a function of dynamic range
N,N-Dimethylacetamide (Common diluent for analysis of solid samples by headspace)
Apodization window:
Kaiser-Bessel (b = 8)
Linewidths (FWHM):
GHz: 0.55 MHz
GHz: 1.2 MHz
GHz: 2.8 MHz
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10. Cell Pressure Dependence
Optimal Pressure for Nitrogen/Air-Based Mixtures
GHz, 4 us gate (‘high-res’)
GHz, 2 us gate (‘low-res’)
GHz, 1.8 us gate
An advantage of FT spectroscopy with apodization windows – the lineshape is nearly pressure invariant at the peak sensitivity point
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11. Broadband Mixture Spectra
ACN (off screen)
ACN (off screen)
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12. Sensitivity – Broadband FTmmW
Concentration detection limits in N2 (10 minute scan)
Molecule
GHz
GHz
Sensitivity
Ratio
Simulation
Ratio (calc.)
High-Res, 10 mTorr
30 mTorr
Acetonitrile
14 ppm
0.7 ppm 19 31
Methanol
140 ppm
34 ppm
4.2 11
Ethanol
270 ppm
70 ppm
4.0 12
Dichloromethane
315 ppm
22 ppm 14 21
2-Propanol
900 ppm
160 ppm
5.4
Tetrahydrofuran
1800 ppm
175 ppm 10
9.8
*Higher power amplifiers available at W-band, potential for factor of ~2 improvement
*Resolved hyperfine structure at W-band that collapses at GHz
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13. Sensitivity – Targeted FTmmW
For single lines, we can tailor pulse length to p/2 condition for each transition –
measure nutation curves directly in the matrix
Acetonitrile
Methanol
Sensitivity Results – Targeted Spectroscopy (1 minute per component)
Molecule
GHz
GHz
Ratio
Low-Res, 20 mTorr
30 mTorr
Acetonitrile
0.63 ppm
0.10 ppm
6.2
Methanol
4.0 ppm
2.1 ppm
1.9
Ethanol
19.3 ppm
4.4 ppm
4.4
Dichloromethane
14.4 ppm
1.6 ppm
9.1
2-Propanol
24 ppm
12 ppm
2.1
Tetrahydrofuran
43 ppm
18 ppm
2.4
Note: These molecules are polar enough that p/2 condition can be reached in a time < T2; so a more powerful source would not improve sensitivity.
For molecules with dipole moments < 0.5 D it would improve sensitivity.
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14. Approximate LDL in N2, Targeted, 1 minute
Higher Frequencies
GHz gives the best coverage for small molecules:
Molecule
Approximate LDL in N2, Targeted, 1 minute
GHz
GHz
GHz
Other transitions
H2O --
40 ppb
22 GHz + other low-m transitions
NH3
60 ppb
24 GHz
H2S
520 ppb
168, 216 GHz NO
2 ppm
150, 250 GHz CO
1 ppm
115, 230 GHz
PH3
200 ppb
340 ppb
H2CO
>1 ppm
14 ppb
HCN
~200 ppb
6 ppb
10 ppb
But we are power starved for almost all molecules:
HCN (m = 2.98 D)
PH3 (m = 0.57 D)
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15. Conclusions
Optimal sensitivity for most molecules suitable for room temperature FTmmW spectroscopy is found between GHz, but…
Room-temperature FT-millimeter wave spectroscopy at lower frequencies can give competitive sensitivity performance for many molecules, and:
-Better complex mixture performance (greater dynamic range before confusion)
-Lower cost
Working at higher frequencies can give access to smaller molecules while maintaining reasonable sensitivity to larger molecules – however, less suited to complex mixtures (broader linewidths, higher line density, harder to achieve p/2 or p excitation for advanced measurements)
The best band depends on the application!
6/24/2016

16. Thank you! Millimeter-Wave FT-MRR Analyzer Microwave FT-MRR Analyzer
770 Harris St. #104b | Charlottesville, VA 22903
Ph: (434)
Millimeter-Wave FT-MRR Analyzer
(Fourier Transform-Molecular Rotational Resonance)
Microwave FT-MRR Analyzer
Discovery Series
First Installation, U. Valladolid
Pulsed Jet CP-FTMW Spectroscopy
Enantiomeric Excess (by 3-Wave Mixing)
BrightSpec ONE
D. Patterson et al., Nature 2013, 497,
S. Lobsinger et al. J. Phys. Chem. Lett. 2015, 6,
Room Temperature CP-mmW
6/24/2016