Nathan Seifert, Wolfgang Jäger University of Alberta A New 2.0-6.0 GHz Chirped-Pulse Fourier Transform Microwave (CP-FTMW) Spectrometer: Instrumental Analysis & Initial Molecular Results Nathan Seifert, Wolfgang Jäger University of Alberta

(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, 200901.

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, 11-13 GHz 13.5 GHz CW Doppler limited; T2 ca. 5 µs Free induction decay High speed digitizer Fourier transform

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, 11-13 GHz High speed digitizer Slow data processing: Collecting FIDs a slow process -- practically limits repetition rate to ca. 1 Hz Room for improvement everywhere!

phase-locked oscillator 2.0-6.0 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 0.1-4.1 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

Initial Results: ortho-dichlorobenzene FWHM: 75 kHz 202 - 101 212 - 111 35Cl / 37Cl 35Cl / 37Cl Amplitude (mV) Expt., 7k average (10 min) Value / MHz Onda & Yamaguchi[1] New A 1930.02(13) 1930.26(34) B 1431.16(7) 1432.73(24) C 821.670(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.

Methyl Lactate Comparison 2-6 GHz CP-FTMW Amplitude (mV) Frequency (MHz) 8-18 GHz CP-FTMW, after fixes 101-000 S/N: ~8000:1 @ 730k avg 303-202 Power (arb units) S/N ( 𝑃 1/2 ): 170:1 @ 2000 avg Frequency (MHz) 170:1 @ 2000 average  4:1 @ 1 acqusition 8000:1 @ 730k average  11.3:1 @ 1 acqusition Or, at 0.5 Hz (3 acq/s)  7:1 @ 1 second Or, at 2 Hz (12 acq/s)  39:1 @ 1 second At face value, the new chirp is winning by a bit, BUT…. Intrinsic intensity (log10 scale) @ Trot = 2 K: 101-000 : -3.55 303-202 : -2.27 Therefore, the 303 is intrinsically 19 times more intense At equivalent performance to the new chirp, the 303 – 202 should be 740:1 @ 1 second.

Deep Averaging: (2-fluoroethanol)4 0.1% 2-fluoroethanol, 3 atm He Experiment B3LYP-D3/6-311++g(d,p) A / MHz 561.19622(96) 558.7 B 289.3763(17) 303.0 C 250.0267(20) 262.8 DJ / kHz 0.070(3) DJK -0.035(2) N / σ / kHz 18 / 7.3 3.0-4.5 GHz, 1.34 million averages 918-826 615-505 624-514 New tetramer detection! Intensity (µV) Frequency (MHz)

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

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