Broadband Rotational Spectroscopy

Raymond C. Ferguson interview for the Beckman Center for the History of Chemistry

Brooks H. PateDepartment of Chemistry

University of Virginia

http://faculty.virginia.edu/bpate-lab/

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.”

AcknowledgementsNational Science Foundation (Chemistry, CCI, MRI, I-Corps)National Radio Astronomy ObservatoryVA NC Alliance LSAMPUniversity of Virginia

David Pratt, Steve Shipman, Bob Field, David Perry, Tom GallagherMike McCarthy, Tony Remijan, Phil Jewel, Susanna Widicus-WeaverRick Suenram, Frank Lovas, David PlusquellicZbyszek 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 GroupGordon Brown, Kevin Douglass, Brian Dian, Steve ShipmanMatt 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

260-290 GHz (x24, 30 mW)520-580 GHz (x48, 3 mW)780-870 GHz (x72, 0.5 mW)

300 K

The Effect of Molecular Size

260-290 GHz CP-FT Spectrometer3-8 heavy atoms

Polar Molecules

Fourier Transform Rotational Spectroscopy

Kyle Crabtree RE03 G.S. Grubbs II WJ08Wei Lin WJ09

What is the Advantage of Chirped-Pulse Excitation?

Transform Limited Pulse: Bandwidth is determined by the pulse duration: Dn ~ (1/tpulse)

Chirped-Pulse: Linear frequency sweep from f1 to f2 where the pulse duration and pulse bandwidth (f2-f1) are chosen independently.

Excitation Bandwidth is Decoupled from the Pulse Duration

Transition Line Width ~ 1MHz (T2 ~ 1 ms)Spectrometer Bandwidth ~ 30 GHz (tpulse ~ 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/T2 << Freq Range)

2) The available power is much higher than the power required for saturation

(P > > Psat)

3) High-speed digital electronics are available

Band

wid

th (G

Hz)

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 D2d Octamer S4

Dimer

Dyke, T. R., Muenter, J. S., J. Chem. Phys. 1974 2929.

Liu, K.; Brown, et al. Nature 1996, 381, 501.N. Pugliano and R. J. Saykally, Science 1992 257 1937.Liu, K.; Brown, M.G.; Cruzan, J.D.; Saykally, R.J. Science 1996, 271, 62.K. Liu, J. D. Cruzan, R. J. Saykally, Science 1996 271 929.Cruzan, J.D..et al Science 1996, 271 59.Richardson, J. O. et al., J. Phys. Chem. A, Article ASAP (2013).

Microwave Spectroscopy

THz Spectroscopy

Low Frequency CP-FTMW Spectroscopy: 2-8 GHzNormal Water Spectrum:3 Hexamers2 Heptamers5 Nonamers4 Decamers7 Undecamers2 Tridecamer1 Pentadecamer

• 700 transitions (140 MHz of bandwidth)• 1700 transitions at 3:1 signal-to-noise ratio or higher unassigned

Trot < 10K

Isotope Spiking: ~15% H218O

Water Clusters Identified in a Single Measurement (H2O)6 (H2O)7

(H2O)9

(H2O)10

(H2O)13 (H2O)15

(H2O)11

Isomers of the Water Hexamer: (H2O)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 (D2O)6 prism has a relatively lower energy by ~0.1 kcal/mol

Isomer Stability and ZeroPoint Vibrational Energy

∆E* = +0.22

∆E = +0.07

δZPE δZPE

Luca EvangelistiFD 12

Simple Isomer System for ZPVE:

12 isomers of (HOD)(H2O)5 Hexamer Cage

Ener

gy

Eel

(H2O)6

  Absolute ZPVE (kcal/mol) Relative ZPVE (kcal/mol)

  Harmonic Anharmonic Harmonic Anharmonic

cage-H1->D1 91.819 89.676 0.000 0.000

cage-H2->D2 91.831 89.689 0.012 0.013

cage-H3->D3 91.836 89.691 0.017 0.015

cage-H4->D4 91.829 89.691 0.010 0.015

cage-H5->D5 91.827 89.695 0.008 0.019

cage-H6->D6 91.855 89.707 0.036 0.031

cage-H7->D7 91.845 89.708 0.026 0.032

cage-H8->D8 91.881 89.725 0.062 0.049

cage-H9->D9 91.972 89.815 0.153 0.139

cage-H10->D10 91.985 89.829 0.166 0.153

cage-H11->D11 91.994 89.837 0.175 0.161

cage-H12->D12 92.020 89.863 0.201 0.187

Neon Expansion404 -303

Argon Expansion

  Absolute ZPVE (kcal/mol) Relative ZPVE (kcal/mol)

  Harmonic Anharmonic Harmonic Anharmonic

cage-H1->D1 91.819 89.676 0.000 0.000

cage-H2->D2 91.831 89.689 0.012 0.013

cage-H3->D3 91.836 89.691 0.017 0.015

cage-H4->D4 91.829 89.691 0.010 0.015

cage-H5->D5 91.827 89.695 0.008 0.019

cage-H6->D6 91.855 89.707 0.036 0.031

cage-H7->D7 91.845 89.708 0.026 0.032

cage-H8->D8 91.881 89.725 0.062 0.049

cage-H9->D9 91.972 89.815 0.153 0.139

cage-H10->D10 91.985 89.829 0.166 0.153

cage-H11->D11 91.994 89.837 0.175 0.161

cage-H12->D12 92.020 89.863 0.201 0.187

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-PPD1 10-PPS1

Simple Ideas for Hydrogen Bond CooperativitySaykally Hydrogen Bond Cooperativity Result

Up to 20% of the hydrogen bond network energy comes from three body effects

Simple Ideas for Hydrogen Bond CooperativitySaykally 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 ChirpSegments (Blue) and Local Oscillator (LO) Frequency (Blue) with Phase Reproducibility

AWG Output of 2.0-3.5 GHzLinearly Addresses the Frequency Range 260 – 295 GHz

LO Shifting Reduces Required Detection Bandwidth

Output Power: 30-40 mW

brightspec.com

Real-Time Measurement Performance

Noise level: 0.002 mV

Ethyl CyanideSingle Sweep Spectrum

Coherent Pulse Experiments

Large Rabi Flip Angles are Achieved in the 260 -295 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 http://en.wikipedia.org/wiki/Spin_echo

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, 11698 (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 ChemistryYan 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 $6Direct Digital Synthesis (Chirps) $50GaN Microwave Amplifiers (2-18 GHz, 25 W) $ 1-10 /W

Valon 5008 Dual Synthesizer$395www.valontechnology.com

Mini CircuitsSMA 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.