1. Nathan Seifert, Wolfgang Jäger University of Alberta
A New GHz Chirped-Pulse Fourier Transform Microwave (CP-FTMW) Spectrometer:
Instrumental Analysis & Initial Molecular Results
Nathan Seifert, Wolfgang Jäger
University of Alberta

2. (Nearly) 10 Years of Broadband Rotational Spectroscopy
2008: First instrument paper published in Rev Sci Instrum
Many groups around the world with chirped pulse instrumentation!
Now:
Molecular structure
UAlberta, Umanitoba (RF08), U Virginia (WG07)
Max Planck CFEL (next talk!), E Illinois Univ. (RH04),
Amherst College (RA02), Wesleyan University (TC02),
Universidad de Valladolid (WC05), Universidad de Pais Vasco (TH04),
and more!!!
Reaction Kinetics
MIT, Missouri (MI02), Argonne Nat. Lab. (WH02)
Astrochemically-Relevant Molecular Species
Harvard (RA03), UC Davis (WE11), Koln (TH08)
Analytical Chemistry (e.g. Head space analysis)
Brightspec, LLC (WE09, WG10)
G. B. Park, R. W. Field, J. Chem. Phys. 2016, 144,

3. CP-FTMW Spectroscopy at UofA
Current CP-FTMW Spectrometer: Example Measurement
12.0 GHz CW
11.0 – 11.9 GHz
12.1 – 13.0 GHz
Arbitrary waveform generator
100 MHz – 1 GHz
1-4 µs chirped pulse
25 W solid state amplifier
Low noise
high gain amplifier
500 MHz – 2.5 GHz
Molecular signal, GHz
13.5 GHz CW
Doppler limited; T2 ca. 5 µs
Free induction decay
High speed digitizer
Fourier transform

4. CP-FTMW Spectroscopy at UofA
Current CP-FTMW Spectrometer: Primary Bottlenecks
Low power
25 W amplifier limits
sensitivity;
generally scales as P1/2 up to
> 500 W
Arbitrary waveform generator
Limited bandwidth:
Requires frequency
synthesizer as local
oscillator to cover entire band
Spectral impurities
Mixing artifacts derived from
synthesizer’s spurious signals
intense enough to require
background subtraction
Low noise
high gain amplifier
Molecular signal, GHz
High speed digitizer
Slow data processing:
Collecting FIDs a slow process
-- practically limits repetition
rate to ca. 1 Hz
Room for improvement everywhere!

5. phase-locked oscillator
GHz CP-FTMW design
Improved power:
Approximately 2x power
relative to previous
generation CP-FTMW
instrument
Low-compression switch:
High power-handling SPDT
switch (RF Lambda) used to
reduce CW noise power and
limit compression of pulse
Improved bandwidth:
Only doubler required to
reach required band;
can output up to 4.8 GHz
for direct measurements
Arbitrary waveform generator
1-3 GHz
1-8 µs chirped pulse
+15 dBm
+46 dBm
2-6 GHz x2
-15 dBm
Improved gain:
Higher directionality/ gain
antennae and improved low
noise amplifier improve
molecular signal detection
Low noise
amplifier
GHz
High speed digitizer
Cleaner heterodyning:
PLO has order of magnitude
improvements in spectral
purity relative to synthesizers
Higher speed digitization:
Significant improvements over
Previous gen digitizer at UofA.
At 20 µs detection length, 25Gs/s
6 frames per valve pulse --- effective
maximum rate is 10Hz.
6.1 GHz
phase-locked oscillator

6. Initial Results: ortho-dichlorobenzene
FWHM: 75 kHz
35Cl / 37Cl
35Cl / 37Cl
Amplitude (mV)
Expt., 7k average (10 min)
Value / MHz
Onda & Yamaguchi[1]
New A
(13)
(34) B
(7)
(24) C
(4)
821.15(79)
1.5χaa
-56.9(20)
-61.90(88)
0.25(χbb -χcc )
-6.0(13)
-6.11(20)
χab --
-55.3(51) / 48.3(47)
RMS
0.134
0.009
Revised prediction (new constants)
Frequency (MHz)
[1] M. Onda, I. Yamaguchi, J. Mol. Struct. 1976, 34, 1-7.

7. Methyl Lactate Comparison
2-6 GHz CP-FTMW
Amplitude (mV)
Frequency (MHz)
8-18 GHz CP-FTMW, after fixes
S/N: 730k avg
Power (arb units)
S/N ( 𝑃 1/2 ): 2000 avg
Frequency (MHz)
average  1 acqusition
730k average  1 acqusition
Or, at 0.5 Hz (3 acq/s)  1 second
Or, at 2 Hz (12 acq/s)  1 second
At face value, the new chirp is winning by a bit, BUT….
Intrinsic intensity (log10 Trot = 2 K:
: -3.55
: -2.27
Therefore, the 303 is intrinsically 19 times more intense
At equivalent performance to the new chirp, the 303 – 202 should be 1 second.

8. Deep Averaging: (2-fluoroethanol)4
0.1% 2-fluoroethanol, 3 atm He
Experiment
B3LYP-D3/ g(d,p)
A / MHz
(96)
558.7 B
(17)
303.0 C
(20)
262.8
DJ / kHz
0.070(3)
DJK
-0.035(2)
N / σ / kHz
18 / 7.3
GHz, 1.34 million averages
New tetramer detection!
Intensity (µV)
Frequency (MHz)

9. Future Goals & Plans
First significant study completed thanks to new instrument: (trifluoroethanol)3 (see RG06!)
Traveling wave tube amplification coming soon
2-8 GHz, ~400 W
Heated sample reservoir & gas nozzle
Promising initial results with a number of test molecules
Laser Ablation
(D)- (left) / (L)- (right) tartaric acid,
addition and insertion based
monohydrates
Only possible with laser ablation!
(mp. 172 °C)
One step ahead of us!
Alonso group from Valladolid reports on
monomer tomorrow (WC05)
LABVIEW-free spectrometer control software
VISA-compliant suite written in full Python
MW-MW Double Resonance & other multiresonant pulse sequences
Easy to program with current-generation AWGs

10. Thank you! Acknowledgements Yunjie Xu, Javix Thomas Mohamad al-Jabiri
Elijah Schnitzler, David Loewen
Chemistry Dept. Machine Shop
Thank you!