pulsed epr: and at high magnet fields
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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
What information does EPR provide?
• Identify presence, quantity, and type of paramagnetic species
• Inform on molecular structure, environment, and dynamics
3
Continuous Wave (CW)
Pulsed
Local spin environment and dynamics are determined by multi‐spin interactions
Dipolar interactions Hyperfine interactions
4
n
e
Nuclear spin (I)
Electron spin (S)
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
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.
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
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.
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
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.
Solid state source‐based high frequency EPR detection schemes
Heterodyne
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HomodyneBolometer
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
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
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
eSD transfers electron polarization across the EPR spectrum
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weak dipolar interactions = poor eSD
strong dipolar interactions = good eSD
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
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
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
20Leavesley, A., Kaminker, I., Han, S. eMagn. Reson. 2018, 7.
AWG operation improves pulse control
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.
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
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
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+
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
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
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.
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
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
Probe model: piezo and optical interface
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Approach 1: blueApproach 2: mauve
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:
Quasi optical bridge for co‐ & cross‐ polar detection
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WG‐45M250
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
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