CALIFORNIA INSTITUTE OF TECHNOLOGY Direct Digital Synthesis Chirped Pulse Fourier Transform Spectrometers for the Classroom & Research 70 th ISMS – June 25 th, 2015 G.A. Blake, B. Carroll, I. Finneran Division of Chemistry & Chemical Engineering, Division of Geological & Planetary Sciences Outline: How might we use the digital electronics revolution to build cost effective tools for teaching & research? Shaping a hands-on laboratory experience. Coupling data driven discovery/theory in the classroom and in the laboratory. Closing Remarks

Orders of Magnitude in Light-Matter Interactions Before diving into a specific technique/lab, remind students of the big picture! Our particular focus is on the THz region of the spectrum, hence this plot: 1 THz 1 ps 33 cm mm 48 K 4.1 meV

Classical Microwave Spectroscopy Believe it or not, the Ch 6 (junior Physical Chemistry) lab in Pasadena ran off of this instrument until 2014: Hewlett-Packard Journal June1971 J = 1 J = 0 F = 3/2 F = 5/2 F = 3/2 F = 1/2 F = 5/2 F = 3/2 eqQ = 0eqQ > 0eqQ < eqQ eqQ eqQ eqQ eqQ eqQ Stark experiment, molecular structure (TC04, J. Muenter)

Microwave Spectroscopy & Digital Electronics II. “Wireless communications advances will soon drive the cost of microwave components for spectrometers/sensors such that they are nearly free.” – F.C. De Lucia “Wireless communications advances will soon drive the cost of microwave components used in mass market applications such that they are nearly free.” – F.C. De Lucia /GAB modification. B. C. Dian et al., Science 320, 924 (2008). G. G. Brown et al., Review of Scientific Instruments 79, (2008). The ongoing revolution. I. Awesomely clever ideas:

Microwave Spectroscopy & Digital Electronics To build strong classroom/academic lab and research environments, try when possible to build instruments in a modular fashion. AmplifierDigital Sampling ~$10k-$40k per GHz bandwidth ~$1k/W – low end $100-$500/W – high end ~$10k-$20k per GHz bandwidth 1 W – 200 W per GHz AWG/AFG What’s needed for a CP-FTMW instrument ? (bleeding edge numbers, now 1-2 years old)

Microwave Spectroscopy & Digital Electronics To build strong classroom/academic lab and research environments, try when possible to build instruments in a modular fashion. What might we do for the chirp? Frequency Time Direct Digital Synthesis! AD9914 Finneran, Carroll, Holland RSI, 84, (2013)

Microwave Spectroscopy & Digital Electronics X Band Waveguide Stark cell SPST Switch To build strong classroom/academic lab and research environments, try when possible to build instruments in a modular fashion. Goal: Rebuild the Ch 6 lab, reusing the existing waveguide and pumps, for ~$7.5k. Carroll

Microwave Spectroscopy & Digital Electronics To build strong classroom/academic lab and research environments, try when possible to build instruments in a modular fashion. Goal: Rebuild the Ch 6 lab, reusing the existing waveguide and pumps, for ~$7.5k. Carroll

Teaching Lab CP-FTMW - Parts To build strong classroom/academic lab and research environments, try when possible to build instruments in a modular fashion. 200 MHz Bandwidth 1 GS/s > 1kHz acquisition Onboard FFT, including windowing (interactive) 60,000 Waveform memory Carroll ISMS (2014)

DDS CP detects 16 O 12 C 32 S, 16 O 12 C 34 S, 16 O 13 C 32 S, 16 O 12 C 33 S Time domain spectroscopy Coherence Simple analysis Strong links to other experiments (NMR, EPR) Teaching Lab CP-FTMW - Performance To build strong classroom/academic lab and research environments, try when possible to build instruments in a modular fashion. O 13 CS 50,000 shots/50 sec Carroll

Example ties to class discussions. I. Ch 1b/21b & FT-NMR. With B 0 applied along z-axis, the sample acquires a net magnetization (more spins along the field than anti-parallel). Apply RF B 1 field in xy-plane, all spins now precess with the same phase. Spins dephase in ~1 sec (liquids). FT to get spectrum. Detect xy-plane emission, this is called the Free Induction Decay (FID). time

Some Upgrades are Inexpensive, Others… To build strong classroom/academic lab and research environments, try when possible to build instruments in a modular fashion. Valon 5009 (23 MHz-6 GHz, $550), two oscillators, provides DDS clock and LO. U1084A Acqiris 8-bit 4 GS/s digitizer (Agilent/Keysight, ~$22k in 2013). Many parts readily upgradable to ‘research quality’ specs. Amplifiers will be a long term issue. Carroll, Finneran

WRD750, L~10m. 3W amplifier. LO=13 GHz, DSB. 100 sec/1M shots, can run 5x faster. 16(5, 12)- 17(4, 13) / 2(0, 2)- 3( -1, 3) To build strong classroom/academic lab and research environments, try when possible to build instruments in a modular fashion. But, only modestly more powerful amps enable waveguide experiments. And the DDS can easily drive segmented chirp studies. High Capability DDS CP-FTMW - Waveguides Carroll, Finneran

For simplicity, we use a double sideband implementation, to generate a total of 4 GHz of data/LO setting… Requires LO shifting to deconvolute the spectrum, but this works well unless the line density is very high. We find good performance with modest pumping (only a 6” DP)… 13 C Finneran, Carroll High Capability DDS CP-FTMW – Supersonic Jets

SOFIA Water Clusters Ring puckering modes I. Finneran, J. Good High Capability DDS CP-FTMW – Jets II. Need to try 3-D printed slit transitions! (TE02)

Python Fitting GUI Autofit Seifert, NA, Finneran, IA et al. "Autofit, an automated fitting tool for broadband rotational spectra, and applications to 1- Hexanal." Journal of Molecular Spectroscopy 312 (2015): SPCAT/SPFIT (~20 ms per run per thread) Pickett, Herbert M. "The fitting and prediction of vibration-rotation spectra with spin interactions." Journal of Molecular Spectroscopy (1991): Bringing in additional tools: Autofit (& TE10, RC04)

Bringing in additional tools: Autofit (& RC04) and Double-Resonance MHz MHz Pump Check LO = 12 GHz Single Tone On Single Tone Off Pump Check On,Off,Off,On… RH08, RH09 I. Finneran, B. Carroll, M. Allodi

Closing Thoughts & Acknowledgements So, what are we doing in Pasadena? *CP-FTMW as one of the many Ch 6 experiments (do the lab in a day, write up results). *Working to develop an extended ‘Ch 6x’ lab in which the students construct 1-2 experiments (diode laser), characterize the instrument and take/analyze data. *And, building on the NCF, Coker, and SUNY Purchase experience, develop modules on modestly complex species where both ab initio predictions and lab studies are undertaken. Tie to Ch 1, 21. These developments and research were, of course, enabled by significant support, listed below, but driven by the talented students and postdocs in the group. Thank you for your attention!