Broadband Rotational Spectroscopy Raymond C. Ferguson interview for the Beckman Center for the History of Chemistry Brooks H. Pate Department of Chemistry University of Virginia E. Bright Wilson, Jr (1986) “You said earlier that microwave hasn’t played the role that NMR has. Of course it’s nowhere near playing the role that NMR does. It’s a little hard to say what should have been done, but we could have done better. Still, it’s a marvelous tool, and I still love it, quite frankly. I wish I could go on and do more with it.”

Acknowledgements National Science Foundation (Chemistry, CCI, MRI, I-Corps) National Radio Astronomy Observatory VA NC Alliance LSAMP University of Virginia David Pratt, Steve Shipman, Bob Field, David Perry, Tom Gallagher Mike McCarthy, Tony Remijan, Phil Jewel, Susanna Widicus-Weaver Rick Suenram, Frank Lovas, David Plusquellic Zbyszek Kisiel, George Shields, Berhane Temelso, Jeremy Richardson, Stuart Althorpe, David Wales, Alberto Lesarri, Sean Peebles, Rebecca Peebles, Gamil Guirgis, Jim Durig, Isabelle Kleiner, Bob McKellar, Kevin Lehmann Pate Broadband Rotational Spectroscopy Group Gordon Brown, Kevin Douglass, Brian Dian, Steve Shipman Matt Muckle, Justin Neill, Dan Zaleski, Brent Harris, Amanda Steber, Nathan Seifert, Cristobal Perez, Simon Lobsiger, Luca Evangelisti

Broadband rotational spectroscopy has been used to explore interesting questions in the structure of water clusters including isomerism, quantum effects, tunneling, and hydrogen bond cooperativity. Broadband Rotational Spectroscopy The chirped-pulse Fourier transform (CP-FT) technique offers significant advantages for broadband rotational spectroscopy.. Chirped-pulse Fourier transform spectroscopy techniques have been extended to mm-wave spectroscopy for high- speed spectrum acquisition.

Basics of Molecular Rotational Spectroscopy The Effect of Temperature Pulsed Jet CP-FTMW Spectroscopy (2-50 GHz) mm-Wave – to – THz CP-FTMW Spectrometers GHz (x24, 30 mW) GHz (x48, 3 mW) GHz (x72, 0.5 mW) 300 K The Effect of Molecular Size GHz CP-FT Spectrometer 3-8 heavy atoms Polar Molecules

Fourier Transform Rotational Spectroscopy Kyle Crabtree RE03 G.S. Grubbs II WJ08 Wei Lin WJ09

What is the Advantage of Chirped-Pulse Excitation? Transform Limited Pulse: Bandwidth is determined by the pulse duration:  ~ (1/t pulse ) Chirped-Pulse: Linear frequency sweep from f 1 to f 2 where the pulse duration and pulse bandwidth (f 2 -f 1 ) are chosen independently. Excitation Bandwidth is Decoupled from the Pulse Duration Transition Line Width ~ 1MHz (T 2 ~ 1  s) Spectrometer Bandwidth ~ 30 GHz (t pulse ~ 33 ps) “High Resolution Spectroscopy”

Moore’s Law Applied to Scope Bandwidth When is Chirped Pulse Fourier Transform Spectroscopy Advantageous ? 1)The spectrum is high- resolution (1/T 2 << Freq Range) 2)The available power is much higher than the power required for saturation (P > > P sat ) 3)High-speed digital electronics are available Bandwidth (GHz)

C.Perez et al., CPL 571, 1-15 (2013) Low Frequency (2-8 GHz) Chirped-Pulse Fourier Transform Microwave Spectrometer Key Features: 1) Improved signal averaging throughput (Tektronix) makes it possible to signal average to 10M FIDs 2) Low-noise TWTA (500 W) 3) High directionality microwave horn antennas 4) Multinozzle (5) capability Time reduction: (5) 2 = 25 Sample reduction: 5 Macroscopic Dipole

Rotationally Resolved Studies of Water Clusters Trimer Tetramer Pentamer Hexamer “Cage” Octamer D 2d Octamer S 4 Dimer Dyke, T. R., Muenter, J. S., J. Chem. Phys Liu, K.; Brown, et al. Nature 1996, 381, 501. N. Pugliano and R. J. Saykally, Science Liu, K.; Brown, M.G.; Cruzan, J.D.; Saykally, R.J. Science 1996, 271, 62. K. Liu, J. D. Cruzan, R. J. Saykally, Science Cruzan, J.D..et al Science 1996, Richardson, J. O. et al., J. Phys. Chem. A, Article ASAP (2013). Microwave Spectroscopy THz Spectroscopy

Low Frequency CP-FTMW Spectroscopy: 2-8 GHz Normal Water Spectrum: 3 Hexamers 2 Heptamers 5 Nonamers 4 Decamers 7 Undecamers 2 Tridecamer 1 Pentadecamer 700 transitions (140 MHz of bandwidth) 1700 transitions at 3:1 signal-to-noise ratio or higher unassigned T rot < 10K Isotope Spiking: ~15% H 2 18 O

Water Clusters Identified in a Single Measurement (H 2 O) 6 (H 2 O) 7 (H 2 O) 9 (H 2 O) 10 (H 2 O) 13 (H 2 O) 15 (H 2 O) 11

Isomers of the Water Hexamer: (H 2 O) 6 rms O … O bond length differences: ~0.01 Angstrom All three isomers observed neon carrier Only the CAGE is observed with argon Prism Cage Book The (D 2 O) 6 prism has a relatively lower energy by ~0.1 kcal/mol

Isomer Stability and ZeroPoint Vibrational Energy ∆E* = ∆E = δ ZPE Luca Evangelisti FD 12 Simple Isomer System for ZPVE: 12 isomers of (HOD)(H 2 O) 5 Hexamer Cage Energy E el (H 2 O) 6

Absolute ZPVE (kcal/mol)Relative ZPVE (kcal/mol) HarmonicAnharmonicHarmonicAnharmonic cage-H1->D cage-H2->D cage-H3->D cage-H4->D cage-H5->D cage-H6->D cage-H7->D cage-H8->D cage-H9->D cage-H10->D cage-H11->D cage-H12->D Neon Expansion

Argon Expansion Absolute ZPVE (kcal/mol)Relative ZPVE (kcal/mol) HarmonicAnharmonicHarmonicAnharmonic cage-H1->D cage-H2->D cage-H3->D cage-H4->D cage-H5->D cage-H6->D cage-H7->D cage-H8->D cage-H9->D cage-H10->D cage-H11->D cage-H12->D

Water Nonamers and Decamers Structures: RI-MP2/aug-cc-pVDZ Energies: RI-MP2/CBS Rel. Energies: kcal/mol

Structures of Water Nonamers and Decamers RI-MP2/aug-cc-pVDZ Structure Parameter is O---O Bond Length: Correlates with H-bond strength and O-H stretch frequency 10-PPD110-PPS1

Simple Ideas for Hydrogen Bond Cooperativity Saykally Hydrogen Bond Cooperativity Result Up to 20% of the hydrogen bond network energy comes from three body effects

Simple Ideas for Hydrogen Bond Cooperativity Saykally Hydrogen Bond Cooperativity Result

Hydrogen Bond Cooperativity and Cluster Geometries

The Original Challenge: Develop a Rotational Spectroscopy Technique Compatible with Pulsed Laser Excitation PCCP Perspectives

Segmented Chirped-Pulse Fourier Transform Spectroscopy Separate AWG Channels Generate Chirp Segments (Blue) and Local Oscillator (LO) Frequency (Blue) with Phase Reproducibility AWG Output of GHz Linearly Addresses the Frequency Range 260 – 295 GHz LO Shifting Reduces Required Detection Bandwidth Output Power: mW brightspec.com

Real-Time Measurement Performance Noise level: mV Ethyl Cyanide Single Sweep Spectrum

Coherent Pulse Experiments Large Rabi Flip Angles are Achieved in the GHz Frequency Range

Coherent Measurements in Fourier Transform mm-wave Spectroscopy Hahn Echo Sequence to Measure Collisional Relaxation Rate and Make Mass Estimate Gavin W. Morley

Pulse Echoes and Collision Rates “Voigt Profile Model”

Pulse Echoes and Collision Rates “Voigt Profile Model”

Opportunities to Find New Problems to Solve V. Alvin Shubert FD02 J.U. Grabow, Angew. Chem. 52, (20130). V.A Shubert, D. Schmitz, D. Patterson, J.M Doyle, and M. Schnell, Angew. Chem. 52, (2013). K.K. Lehmann (submitted) Chirality Reaction Kinetics Rydberg Molecules Analytical Chemistry Yan Zhou RE05 David Grimes RE07

Is our data valuable? Radio Astronomy (CDMS, JPL, Splatalogue) Molecular Discovery Analytical Chemistry What types of data should we archive? Do we also need to provide analysis tools? Nathan Seifert FD 04 James McMillan RA06 Christian Enders RA 01 Joanna Corby WF 01

The Second Wave of Technology Brandon Carroll RE01 The Electronics for Rotational Spectroscopy Will Be Free! Microwave Synthesizer 100 MHz – 6 GHz$6 Direct Digital Synthesis (Chirps) $50 GaN Microwave Amplifiers (2-18 GHz, 25 W)$ 1-10 /W Valon 5008 Dual Synthesizer $395 Mini Circuits SMA Adapter $8.95

Broadband rotational spectroscopy has been used to explore interesting questions in the structure of water clusters including isomerism, quantum effects, tunneling, and hydrogen bond cooperativity. Broadband Rotational Spectroscopy The chirped-pulse Fourier transform (CP-FT) technique offers significant advantages for broadband rotational spectroscopy.. Chirped-pulse Fourier transform spectroscopy techniques have been extended to mm-wave spectroscopy for high- speed spectrum acquisition.