28/02/2014
Overview
Electron Paramagnetic Resonance
and Dynamic Nuclear Polarisation
Electron paramagnetic resonance
• What it is
• Why it’s useful
AS:MIT CH916
Gavin W Morley,
Department of Physics,
University of Warwick
Gavin W Morley, EPR and DNP
Dynamic nuclear polarization
• Why it’s useful
• What it is
Gavin W Morley, EPR and DNP
Electron paramagnetic resonance
(EPR)
…NMR for electrons:
The crucial difference is that the
electron magnetic moment is 660
times larger than that of a proton
Magnetism
Paramagnetism:
Diamagnetism:
Ferromagnetism:
Electron spins tend to:
follow
oppose
ignore
…an applied magnetic field
Also known as
… electron spin resonance (ESR)
… or electron magnetic resonance (EMR)
… or ferromagnetic resonance (FMR)
Gavin W Morley, EPR and DNP
Gavin W Morley, EPR and DNP
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28/02/2014
Magnetic resonance
Magnetic resonance
Photons
reflected
Photons
reflected
Magnetic field
Magnetic field, B
Isidor Isaac Rabi
(1898 – 1988)
Energy
of a
spin
S=½
Photon
energy
Energy
of a
spin
H = g µB B S
Pieter Zeeman
(1865 - 1943)
Magnetic field
Magnetic field, B
Gavin W Morley, EPR and DNP
Gavin W Morley, EPR and DNP
Electron paramagnetic resonance
Photons
reflected
Electron paramagnetic resonance
Photons
reflected
Magnetic field, B
S=½
I=½
Magnetic field, B
Energy
of a
spin
system
H = g µB B S - g N µN B I
Bruker
J van Tol, L-C Brunel & RJ Wylde,
Rev Sci Instrum 76, 076101 (2005)
Gavin W Morley, EPR and DNP
Magnetic field, B
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Electron paramagnetic resonance
Iz = ½
Photons
reflected
Iz = -½
Electron paramagnetic resonance
Iz = ½
Photons
reflected
A
Iz = -½
A
Magnetic field, B
Magnetic field, B
H = g µB B S- gN µN B I
+ASI
S=½
I=½
Energy
of a
spin
system
TEMPO
Gavin W Morley, EPR and DNP
Electron paramagnetic resonance
Iz = ½
Photons
absorbed
Iz = -½
Electron paramagnetic resonance
Iz = 1
Photons
absorbed
A
Iz = 0
Magnetic field, B
TEMPO
J van Tol, L-C Brunel & RJ Wylde,
Rev Sci Instrum 76, 076101 (2005)
Iz = -1
A
Magnetic field, B
Gavin W Morley, EPR and DNP
£20/g from Aldrich
J van Tol, L-C Brunel & RJ Wylde,
Rev Sci Instrum 76, 076101 (2005)
Magnetic field, B
Gavin W Morley, EPR and DNP
Stable free radicals:
BDPA
Nitroxides
DPPH
£20/g from Aldrich
TEMPO
J van Tol, L-C Brunel & RJ Wylde,
Rev Sci Instrum 76, 076101 (2005)
£20/g from Aldrich
Gavin W Morley, EPR and DNP
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Electron paramagnetic resonance
Iz = 1
Photons
absorbed
Iz = 0
Iz = -1
Magnetic field
modulation
(~10 to 100 kHz)
Electron paramagnetic resonance
Iz = 1
Differential
absorption
Iz = 0
A
Advantage:
noise only at
modulation
frequency
Iz = -1
Magnetic field, B
TEMPO
Differential
absorption
£20/g from Aldrich
J van Tol, L-C Brunel & RJ Wylde,
Rev Sci Instrum 76, 076101 (2005)
Magnetic field, B
Gavin W Morley, EPR and DNP
Gavin W Morley, EPR and DNP
Electron paramagnetic resonance
Electron paramagnetic resonance
spectrometer
Motional narrowing occurs due to
molecular rotation at higher temperature
Magic angle spinning is not used in EPR
because you can’t spin fast enough
TEMPO
Microwave
bridge
Console
Computer
S
J van Tol, L-C Brunel & RJ Wylde,
Rev Sci Instrum 76, 076101 (2005)
Gavin W Morley, EPR and DNP
£20/g from Aldrich
Electromagnet (up to 1.5 T) or
superconducting magnet for higher field
Gavin W Morley, EPR and DNP
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Electron paramagnetic resonance
spectrometer
Electron paramagnetic resonance
spectrometer
Bridge
source
Continuous-wave (CW) EPR so far…
detector
Pulsed EPR next
Modulation coils
S
Microwave resonator
Gavin W Morley, EPR and DNP
Gavin W Morley, EPR and DNP
Pulsed electron paramagnetic
resonance spectrometer
Bridge
Pulsed electron paramagnetic
resonance
High power pulsed
source
Protected
detector
Felix Bloch (1905-1983)
Modulation coils off
Photo courtesy Stanford
News Service
Bruker probe
S
Bloch sphere
Microwave resonator
Gavin W Morley, EPR and DNP
Gavin W Morley, EPR and DNP
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Pulsed electron paramagnetic
resonance
Pulsed EPR
J van Tol, L-C
Brunel & RJ
Wylde, Rev Sci
Instrum 76,
076101 (2005)
resonance condition
g µB Bres = h f
Bres
~100 MHz width
Pulse width < 500 MHz
Resonator ringing deadtime
Short T2 and T2* compared to NMR
Spins polarize (thermalize) on
timescale T1
f
π/
f
2
Free induction decay (FID) on
timescale T2*
Gavin W Morley, EPR and DNP
π/
2
Homogeneous (T2) can be much longer
than inhomogeneous (T2*) so most pulsed
EPR uses spin echo
Gavin W Morley, EPR and DNP
Rotating Frame
Spin echo
In rotating frame
Gavin W Morley, EPR and DNP
Gavin W Morley, EPR and DNP
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Pulsed EPR
Spin echo decay
Double electron-electron resonance (DEER) allows distances
between two electron spins in the range 2 to 6 nm to be measured
(cf < 1 nm by NMR for two nuclear spins)
Dipolar coupling α 1/r3
Site directed spin labelling with
one or more TEMPO
Erwin L Hahn (born 1921)
Photo: AIP Emilio Segre Visual Archives, Stephen Jacobs Collection
Gavin W Morley, EPR and DNP
Gavin W Morley, EPR and DNP
Electron nuclear double resonance
(ENDOR)
Photons
reflected
Iz = ½
Iz = -½
Why do ENDOR instead of NMR?
A
Magnetic field, B
Energy
of a
spin
system
Electron nuclear double resonance
(ENDOR)
H = g µB B S- gN µN B I
+ASI
S=½
I=½
• only need ~1010 spins instead of ~1015
• NMR may be difficult near the electron spin
Energy
of a
spin
system
Magnetic field, B
Gavin W Morley, EPR and DNP
H = g µ B B0 S - g N µ N B0 I
+ASI
S=½
I=½
Magnetic field, B0
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Electron nuclear double resonance
(ENDOR)
EPR language:
B0 is static magnetic field
B1 is EPR magnetic field
B2 is NMR magnetic field
ENDOR for quantum computing
Classical
computer
Quantum
computer
Bits
Qubits
0 or 1
α |0⟩ + β |1⟩
(NMR language: B1 is NMR magnetic field)
H = g µ B B0 S - g N µ N B0 I
+ASI
S=½
I=½
Energy
of a
spin
system
Magnetic field, B0
Gavin W Morley, EPR and DNP
Spin qubits in silicon
Gavin W Morley, EPR and DNP
Other EPR applications
• Quality control in beer, wine etc
• Radiation dose received by
teeth
Image by
Manuel Vögtli (UCL)
Gavin W Morley, EPR and DNP
Gavin W Morley, EPR and DNP
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Overview
Dynamic nuclear polarization (DNP)
Electron paramagnetic resonance
• What it is
• Why it’s useful
Dynamic nuclear polarization
• More signal
Dynamic nuclear polarization
• Why it’s useful
• What it is
Gavin W Morley, EPR and DNP
Gavin W Morley, EPR and DNP
Dynamic nuclear polarization (DNP)
Dynamic nuclear polarization (DNP)
Boltzmann polarization:
ܲ=
n2
n1
Gavin W Morley, EPR and DNP
݊ଵ − ݊ଶ
݊ଵ + ݊ଶ
Polarization
∆ா
݊ଶ
ି
= ݁ ௞ಳ ்
݊ଵ
Electrons
0.01
Protons
ܲ=
݊ଵ − ݊ଶ
݊ଵ + ݊ଶ
1E-3
1E-4
∆E
Boltzmann polarization:
1
0.1
Magnetic field = 10 T
1E-5
0.01
0.1
1
10
Temperature (K)
100
So transfer electronic
polarization to nuclei
Gavin W Morley, EPR and DNP
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Dynamic nuclear polarization (DNP)
Initial polarizations:
Dynamic nuclear polarization (DNP)
Nuclear magnetic resonance (NMR)
Electrons > 95%
Nuclei < 0.1%
For 8.6 T and 3 K
Gavin W Morley, EPR and DNP
Dynamic nuclear polarization (DNP)
Electron paramagnetic resonance (EPR)
Gavin W Morley, EPR and DNP
Dynamic nuclear polarization (DNP)
Electron paramagnetic resonance (EPR)
Forbidden transitions:
zero quantum and
double quantum
Gavin W Morley, EPR and DNP
Gavin W Morley, EPR and DNP
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Magnetic resonance
Photons
reflected
Magnetic resonance
S=½
I=½
Iz = ½
Photons
reflected
A
Magnetic field, B
Energy
of a
spin
system
H = g µB B S - g N µN B I
+ASI
S=½
I=½
Energy
of a
spin
system
Magnetic field, B
Magnetic field, B
Gavin W Morley, EPR and DNP
Gavin W Morley, EPR and DNP
Magnetic resonance
Energy of a
spin system
↑↑
(↑↓ + ↓↑)/√2
↓↓
Singlet, F = 0:
(↑↓ - ↓↑)/√2
Breit-Rabi
diagram
Magnetic resonance
Energy of a
spin system
A S I = A/2 (S+ I- + S- I+)
+ A Sz Iz
S± = Sx ± i Sy
Magnetic field, B
H = g µB B S- gN µN B I + A S I
Gavin W Morley, EPR and DNP
Gregory Breit
(1899-1981)
Magnetic field, B
H = g µB B S- gN µN B I
Triplet, F = 1:
Iz = -½
Magnetic field, B
H = g µB B S - g N µN B I + A S I
Gavin W Morley, EPR and DNP
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Dynamic nuclear polarization (DNP)
Electron paramagnetic resonance (EPR)
Dynamic nuclear polarization (DNP)
Solid effect DNP
more
Forbidden
also gets weaker at
high magnetic fields
at high magnetic fields
Gavin W Morley, EPR and DNP
Gavin W Morley, EPR and DNP
Dynamic nuclear polarization (DNP)
Dynamic nuclear polarization (DNP)
Cross effect
Solid effect DNP
Thermal Mixing
ee-
e-
n
n
n
e-
also gets weaker at
high magnetic fields
Gavin W Morley, EPR and DNP
e-
en
also get weaker at
high magnetic fields
Gavin W Morley, EPR and DNP
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Dynamic nuclear polarization (DNP)
Dynamic nuclear polarization (DNP)
Gyrotron DNP pioneered
By Bob Griffin’s group
at MIT
KJ Pike et al., J Mag Res 215, 1 (2012)
See also Bruker’s 263 GHz AVANCE™
Ray Dupree, Steven Brown, Mark Newton, Kevin Pike, Andrew Howes,
Tom Kemp, Mark Smith et al. at Warwick,
see KJ Pike et al., J Mag Res 215, 1 (2012)
Gavin W Morley, EPR and DNP
Gavin W Morley, EPR and DNP
Dynamic nuclear polarization (DNP)
EPR + NMR = Electron nuclear double
resonance (ENDOR)
Gavin W Morley, EPR and DNP
Dynamic nuclear polarization (DNP)
EPR + NMR = Electron nuclear double
resonance (ENDOR)
ENDOR-DNP:
ENDOR-DNP:
A S Brill, PRA 66 043405 (2003)
A S Brill, PRA 66 043405 (2003)
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Background
Dynamic nuclear polarization (DNP)
Our research
Dynamic nuclear polarization (DNP)
EPR + NMR = Electron nuclear double
resonance (ENDOR)
GWM, J van Tol, A Ardavan, K Porfyrakis, J Zhang & GAD Briggs,
PRL 98, 20501 (2007)
GWM, K Porfyrakis, A Ardavan & J van Tol, AMR 34, 347 (2008)
ENDOR-DNP:
ENDOR-DNP:
A S Brill, PRA 66 043405 (2003)
A S Brill, PRA 66 043405 (2003)
See also: B Epel, A Poppl, P
Manikandan, S Vega & D Goldfarb,
JMR 148 388 (2001)
Gavin W Morley, EPR and DNP
Background
Gavin W Morley, EPR and DNP
Our research
Dynamic nuclear polarization
Dynamic nuclear polarization
240 GHz excitation, A/ħ = 16 MHz
Temperature jump
gain x50 from DNP
and x200 from temp
x10,000 total
1
0.1
Polarization
CW EPR signal
ENDOR-DNP at 4 K
Thermalized at 4 K
After ENDOR-DNP
TDNP
0.01
TNMR
1E-3
1E-4
Magnetic field = 10 T
See ArdenkjaerLarsen et al, PNAS
100, 10158 (2003)
1E-5
8.569
8.570
8.571
Magnetic field (T)
Gavin W Morley, EPR and DNP
8.572
0.01
0.1
1
10
100
Temperature (K)
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Conclusions
Dynamic nuclear polarization
Temperature jump with dissolution
plus
Gavin W Morley, EPR and DNP
EPR history: first observed in 1944 by Zavoisky in
Kazan State University and developed
independently at the same time by Bleaney at
Oxford University.
Basic textbook: Weil, Bolton & Wertz, Electron
paramagnetic resonance, Wiley (1994).
Pulsed EPR textbook: Schweiger & Jeschke,
Principles of pulse EPR, OUP 2001.
Arnaud
Comment and
Rolf Gruetter,
Lausanne
Dynamic nuclear polarization
For a review see: T Maly et al., DNP at high
magnetic fields, Journal of Chemical Physics,
128, 052211 (2008)
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