Pulsed EPR: and at High Magnet Fields

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Pulsed EPR: Instrumentation and Practice at High Magnet Fields Alisa Leavesley 17.06.2019 1

Transcript of Pulsed EPR: and at High Magnet Fields

Page 1: Pulsed EPR: and at High Magnet Fields

Pulsed EPR: Instrumentation and Practice at 

High Magnet FieldsAlisa Leavesley

17.06.2019

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What is electron paramagnetic resonance (EPR)?

• Types of unpaired electrons:• Transition metals• Free radicals • Defects

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μw

12 μ

12 μ

Magnetic fieldElectric field

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What information does EPR provide?

• Identify presence, quantity, and type of paramagnetic species

• Inform on molecular structure, environment, and dynamics

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Continuous Wave (CW)

Pulsed

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Local spin environment and dynamics are determined by multi‐spin interactions

Dipolar interactions Hyperfine interactions

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n

e

Nuclear spin (I)

Electron spin (S) 

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Pulsed EPR: an outline of the talk

• Why use it?• How do you get data? (Instrumentation)• Examples of practical applications (Practice)

• DEER• ELDOR• EDNMR• ESEEM• ENDOR• HYSCORE

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e‐ ‐ e‐ interactions

e‐ ‐ n+ interactions

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B0

Why use pulsed EPR?  Target specific parts of the EPR spectrum to study 

More detailed information about spin interactions (e‐ ‐ e‐ & e‐ ‐ n) and dynamics

I = 1S = ½ 

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Advantages of high field/frequency EPR• Improve sensitivity & polarization

• Improve g‐factor resolution

• Reduce zero‐field splitting effects

7Clarkson, R. B. et al Molec. Phys. 1998, 95, 1325-1332.Bagryanskaya, E.G. et al Phys. Chem. Chem. Phys. 2009, 11, 6700-6707.

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Instrumentation for pulsed high field EPR

CommercialHome‐built

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95 GHz

263 GHz St. Andrews: 95 GHz

Goethe: 180 GHz

UCSB: 194 GHz

Berlin: 360 GHz

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UCSB 194 GHz home‐built instrument overview

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Features:Modified NMR  magnetCryogenic temperaturesQuasi‐optical designBroad‐band solid‐state μw source

Versatile μw manipulation 

Siaw, T.A., Leavesley, A., Lund, A., Kaminker, I., Han S. J. Magn. Reson. 2016, 264, 131-153.Leavesley, A., Kaminker, I., Han, S. eMagn. Reson. 2018, 7.

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High frequency pulses: generation and requirements

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Control of pulse length, amplitude, νμw, and φμw

Methods to cut pulses from cw sources• Pin diode switches• Mixers• Arbitrary waveform generator (AWG)

C. Armstrong, “The Truth about Terahertz”, IEEE Spectrum, August 2012 

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Basic quasi‐optical design for EPR detection

11Siaw, T.A., Leavesley, A., Lund, A., Kaminker, I., Han S. J. Magn. Reson. 2016, 264, 131-153.

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Solid state source‐based high frequency EPR detection schemes

Heterodyne

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HomodyneBolometer

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EPR signal: Free induction decay & echoesFree Induction Decay Echo

13Prasad P.V., Storey P. Magnetic Resonance Imaging. In: Molecular Biomethods Handbook. Humana Press, 949‐973.Wikipedia, Hanh echo, https://commons.wikimedia.org/wiki/File:HahnEcho_GWM.gif.

Spin relaxation FID

Fourier Transform Frequency

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1.0

0.8

0.6

0.4

0.2 Nor

m. I

nt. E

cho

int.

0.60.50.40.30.20.10.0

Delay (s)

T1e at 193.598 GHz Fit

T1 = 0.15481 ± 0.00314 T2 = 0.023732 ± 0.000547 A = 0.50673 ± 0.0095 B = 0.47446 ± 0.00933

Examples of classic EPR relaxation acquisitionsSpin lattice relaxation (T1e): Phase memory time (Tm):

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1.0

0.8

0.6

0.4

0.2

0.0N

orm

. Int

. Ech

o in

t.

7x10-6654321

Delay (s)

10 mM, 10 K, 193.598 GHz data fit

Tm = 2.3774e-06 ± 1.05e-08 A = 1.4483 ± 0.00471 B = 1 ± 0

10 mM trityl, 1 M Urea in 6:3:1 d8‐glycerol:D2O:H2O at 10 K

Unpublished work

Vary td Vary tau

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Hovav, Y. et al Phys. Chem. Chem. Phys. 2015, 17, 226-244.Leavesley, A., et al Phys. Chem. Chem. Phys. 2017, 19, 3596-3605.

Correlation EPR: Electron‐electron double resonance (ELDOR) for electron depolarization profiles

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Electron spectral diffusion (eSD) affects the e‐ depolarization

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eSD transfers electron polarization across the EPR spectrum

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weak dipolar interactions = poor eSD

strong dipolar interactions = good eSD

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Baseline defects result from AMC hysteresis effects

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1‐source ELDOR

Leavesley, A., Kaminker, I., Han, S. eMagn. Reson. 2018, 7.40 mM 4‐amino TEMPO

AL1

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Folie 17

AL1 Do I unclude the 2-source modification to the instrumentation? Or move straight into more practical applications?Alisa Leavesley; 12.06.2019

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Induction and reflection mode operation

Reflection mode limited to 2 µs pulses with 0.5% duty cycle

Leavesley, A., Kaminker, I., Han, S. eMagn. Reson. 2018, 7.

Modifications for 2‐source functionalityTd is on the order of TM

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Background free ELDOR measurements with 2‐source configuration

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1‐source ELDOR 2‐source ELDOR

Leavesley, A., Kaminker, I., Han, S. eMagn. Reson. 2018, 7.40 mM 4‐amino TEMPO

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20Leavesley, A., Kaminker, I., Han, S. eMagn. Reson. 2018, 7.

AWG operation improves pulse control 

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AWG‐chirp pulses have broader excitation profiles and improve refocused echo intensities

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0 200 400 600 800 1000time / ns

real imaginary

MHzchirp 2.3

Leavesley, A. Kaminker, I. Han, S. eMagn. Reson. 2018, 7.

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Transition between hole burning ELDOR and ELDOR detected NMR: elucidating hyperfine interactions

22Leavesley, A., Kaminker, I., Han, S. eMagn. Reson. 2018, 7.

ν ν ν

I = S = ½ 

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Hyperfine interaction identification via electron spin echo envelop modulation (ESEEM)

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Tπ/2 π/2 π/2τ

τ

Deligiannakis, Y. Rutherford, A.W. J. Am. Chem. Soc.1997, 119, 4471‐4480

ττ

Moro, F., Turyanska, Y., et al. Sci. Reports 2015, 5, 10855. 

Nuclear spins modulate the echo decay

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Hyperfine interaction identification via electron‐nuclear double resonance (ENDOR)

242D ENDOR: Hyperfine correlation spectroscopy (HYSCORE)

π π

τ

τπ/2

π

νμw

νrf

Möbius, K., Lubitz, W., Cox, N., Savitsky, A. Magnetochem. 2018, 4(4), 50.Blok, H., Disselhorst, J., et al. J. Magn. Res. 2005, 173, 49–53.

31P

55Mn |mS = ‐3/2, mI = ‐3/2>

55Mn |mS = 5/2, mI = 3/2> ZnGeP2:Mn2+

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Comparison of 1D pulsed EPR‐based hyperfine interaction detection methods

500 scans

100 scans

Napela, A. et al. J. Magn. Reson. 2014, 242, 203-213.

Technique Transition NMR freq. limitation Other limitations

ESEEM Allowed e‐e ~ < 60 MHz Moderate T1n and 

NMR linewidths

ENDOR Allowed e‐e & n‐n

Freq. dependent artifacts weak signals

EDNMR Forbidden e‐n > 5 MHz

Long saturation pulse 

broadening

Cox, N. et al Methods in Enzymology, 2015, 563, 211-249. 25

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Double electron spin resonance: spin distance distributions (DEER)

2 ν 2 3 1

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π/2

πτ2

τ1 π

π

νdetect

νexcite

τ1 τ2

t

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DEER acquisition: raw signal to electron spin distance distributions

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Background/noise correction

FT

Convert to distance distribution

ν 2 3 1

Dockter, C. Volkov, A. et al. PNAS 2009, 106, 18485‐18490.

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Measuring electron spin distances: polymer brushes

28Leavesley, A. Jain, S., et. al. Phys. Chem. Chem. Phys. 2018, 20, 27646‐27654.

Supplied by Prof. Andrzej Rajca ‐University of Nebraska‐Lincoln

Dendrimer (9 spins)Tammes problem predicts:2.31 nm 1st neighbor2.98 nm 2nd neighbor3.46 nm 3rd neighbor3.65 nm 4th neighbor3.77 nm  5th neighbor

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Measuring electron spin distances: proteinsSpin labeling proteins

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Conclusions

• The basic what, why, & how of pulsed high field EPR

• Common pulse sequences for applications

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Model overview

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Quasi optical platform for PE THz EPR spectroscopy and microscopy

Cryogenic Ltd magnet• 12 T• Sweepable• 100 mm Ø bore

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Probe model: piezo and optical interface

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Approach 1: blueApproach 2: mauve

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Probe model: optical performance simulations

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Approach 2:Mirror XP loss 

(dBi)HM Loss (dBi)

Approach 1: M1 ‐22.19 ‐25.20

Approach1: M2 ‐22.26 ‐25.27

Approach 2: M1 ‐24.16 ‐27.17

Approach 2: M2 ‐23.57 ‐26.58

Approach 1:

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Quasi optical bridge for co‐ & cross‐ polar detection

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WG‐45M250

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AcknowledgementsUCSB• Dr. Ilia Kaminker• Dr. Alicia Lund• Prof. Songi Han• Dr. Jessica Clayton• Prof. Mark Sherwin

Weizmann Institute of Science• Prof. Shimon Vega

Thomas Keating Ltd• Dr. Richard Wylde• Dr. Kevin Pike• Georg Sebek

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Funding:

• Dr. Ting Ann Siaw• Dr. Asif Equbal• Dr. Sheetal Jain• Blake Wilson• Dr. Nick Agladze

• Prof. Daniella Goldfarb