Electron Paramagnetic Resonance
and Dynamic Nuclear Polarisation
AS:MIT CH916
Gavin W Morley,
Department of Physics,
University of Warwick
Gavin W Morley, EPR and DNP
Overview
Electron paramagnetic resonance
• What it is
• Why it’s useful
Dynamic nuclear polarization
• Why it’s useful
• What it is
Gavin W Morley, EPR and DNP
Overview
Electron paramagnetic resonance
• What it is
• Why it’s useful
Dynamic nuclear polarization
• Why it’s useful
• What it is
Gavin W Morley, EPR and DNP
Overview
Electron paramagnetic resonance
• What it is
…NMR for electrons
Gavin W Morley, EPR and DNP
Electron paramagnetic resonance
(EPR)
… or electron spin resonance (ESR)
Gavin W Morley, EPR and DNP
Magnetism
Paramagnetism:
Electron spins tend to:
follow
Diamagnetism:
oppose
Ferromagnetism:
ignore
…an applied magnetic field
Gavin W Morley, EPR and DNP
Magnetic resonance
An isolated spin:
Isidor Isaac Rabi
(1898 – 1988)
Energy
of a
spin
Photon
energy
Magnetic field
Gavin W Morley, EPR and DNP
Magnetic resonance
Isidor Isaac Rabi
(1898 – 1988)
Energy
of a
spin
Photon
energy
Magnetic field
Gavin W Morley, EPR and DNP
Magnetic resonance
Photons
reflected
Magnetic field
Isidor Isaac Rabi
(1898 – 1988)
Energy
of a
spin
Photon
energy
Magnetic field
Gavin W Morley, EPR and DNP
Magnetic resonance
Photons
reflected
Magnetic field
Isidor Isaac Rabi
(1898 – 1988)
Energy
of a
spin
Photon
energy
Magnetic field
Gavin W Morley, EPR and DNP
Magnetic resonance
Photons
reflected
Magnetic field
Isidor Isaac Rabi
(1898 – 1988)
Energy
of a
spin
Photon
energy
Magnetic field
Gavin W Morley, EPR and DNP
Magnetic resonance
Photons
reflected
S=½
Magnetic field, B
Energy
of a
spin
H = g μB B S
Pieter Zeeman
(1865 - 1943)
Magnetic field, B
Gavin W Morley, EPR and DNP
Electron paramagnetic resonance
Photons
reflected
Magnetic field, B
Bruker
J van Tol, L-C Brunel & RJ Wylde,
Rev Sci Instrum 76, 076101 (2005)
Gavin W Morley, EPR and DNP
Electron paramagnetic resonance
Photons
reflected
S=½
I=½
Magnetic field, B
Energy
of a
spin
system
H = g μB B S - gN μN B I
Magnetic field, B
Gavin W Morley, EPR and DNP
Electron paramagnetic resonance
Photons
reflected
S=½
I=½
Magnetic field, B
Energy
of a
spin
system
H = g μB B S - gN μN B I
Magnetic field, B
Gavin W Morley, EPR and DNP
Electron paramagnetic resonance
Photons
reflected
Iz = ½
Iz = -½
A
Magnetic field, B
Energy
of a
spin
system
H = g μB B S - gN μN B I
+ASI
S=½
I=½
Magnetic field, B
Gavin W Morley, EPR and DNP
Electron paramagnetic resonance
Iz = ½
Photons
reflected
Iz = -½
A
Magnetic field, B
Some stable
free radicals:
BDPA
Nitroxides
DPPH
TEMPO
J van Tol, L-C Brunel & RJ Wylde,
Rev Sci Instrum 76, 076101 (2005)
Gavin W Morley, EPR and DNP
£20/g from Aldrich
Electron paramagnetic resonance
Photons
absorbed
Iz = ½
Iz = -½
A
Magnetic field, B
TEMPO
J van Tol, L-C Brunel & RJ Wylde,
Rev Sci Instrum 76, 076101 (2005)
Gavin W Morley, EPR and DNP
£20/g from Aldrich
Electron paramagnetic resonance
Photons
absorbed
Iz = 1
Iz = 0
Iz = -1
A
Magnetic field, B
TEMPO
J van Tol, L-C Brunel & RJ Wylde,
Rev Sci Instrum 76, 076101 (2005)
Gavin W Morley, EPR and DNP
£20/g from Aldrich
Electron paramagnetic resonance
Photons
absorbed
Iz = 1
Iz = 0
Iz = -1
Magnetic field
modulation
(~10 to 100 kHz)
Differential
absorption
Magnetic field, B
Gavin W Morley, EPR and DNP
Advantage:
noise only at
modulation
frequency
Electron paramagnetic resonance
Differential
absorption
Iz = 1
Iz = 0
A
Iz = -1
Magnetic field, B
TEMPO
J van Tol, L-C Brunel & RJ Wylde,
Rev Sci Instrum 76, 076101 (2005)
Gavin W Morley, EPR and DNP
£20/g from Aldrich
Electron paramagnetic resonance
Differential
absorption
Iz = 1
Iz = 0
Iz = -1
Magnetic field, B
TEMPO
J van Tol, L-C Brunel & RJ Wylde,
Rev Sci Instrum 76, 076101 (2005)
Gavin W Morley, EPR and DNP
£20/g from Aldrich
Electron paramagnetic resonance
spectrometer
Microwave
bridge
Console
S
Electromagnet (up to 1.5 T) or
superconducting magnet for higher field
Gavin W Morley, EPR and DNP
Computer
Electron paramagnetic resonance
spectrometer
Bridge
source
Circulator
detector
Modulation coils
S
Microwave resonator
Gavin W Morley, EPR and DNP
Electron paramagnetic resonance
spectrometer
Bridge
source
Circulator
detector
Modulation coils
S
Microwave resonator
Gavin W Morley, EPR and DNP
Electron paramagnetic resonance
spectrometer
Bridge
source
Circulator
detector
Modulation coils
S
Microwave resonator
Gavin W Morley, EPR and DNP
Electron paramagnetic resonance
spectrometer
CW EPR so far…
Pulsed EPR next
Gavin W Morley, EPR and DNP
Pulsed electron paramagnetic
resonance spectrometer
Bridge
High power pulsed
source
Protected
detector
Modulation coils off
S
Microwave resonator
Gavin W Morley, EPR and DNP
Pulsed electron paramagnetic
resonance
Gavin W Morley, EPR and DNP
Pulsed electron paramagnetic
resonance
resonance condition
g mB Bres = h f
Bres
Polarize (thermalise) on
timescale T1
f
p/
2
Gavin W Morley, EPR and DNP
Free induction decay (FID) on
timescale T2*
Pulsed EPR
J van Tol, L-C
Brunel & RJ
Wylde, Rev Sci
Instrum 76,
076101 (2005)
f
p/
2
Gavin W Morley, EPR and DNP
~100 MHz width
Pulse width < 500 MHz
Resonator ringing  deadtime
Short T2 and T2* compared to NMR
Homogeneous (T2) can be much longer
than inhomogeneous (T2*) so most pulsed
EPR uses spin echo
Rotating frame
Gavin W Morley, EPR and DNP
Rotating frame
Gavin W Morley, EPR and DNP
Spin echo
In rotating frame
Gavin W Morley, EPR and DNP
Spin echo
In rotating frame
Gavin W Morley, EPR and DNP
Spin echo decay
Erwin L Hahn (born 1921)
Photo: AIP Emilio Segre Visual Archives, Stephen Jacobs Collection
Gavin W Morley, EPR and DNP
Spin echo decay
Erwin L Hahn (born 1921)
Photo: AIP Emilio Segre Visual Archives, Stephen Jacobs Collection
Gavin W Morley, EPR and DNP
Pulsed EPR
Double electron-electron resonance (DEER) allows distances
between two electron spins in the range 2 to 6 nm to be measured
(For two nuclear spins NMR only measures up to 1 nm)
Dipolar coupling  1/r3
Site directed spin labelling with
one or more TEMPO molecule
Gavin W Morley, EPR and DNP
Electron nuclear double resonance
(ENDOR)
Photons
reflected
Iz = ½
Iz = -½
A
Magnetic field, B
Energy
of a
spin
system
H = g μB B S - gN μN B I
+ASI
S=½
I=½
Magnetic field, B
Gavin W Morley, EPR and DNP
Electron nuclear double resonance
(ENDOR)
Why do ENDOR instead of NMR?
• only need ~1010 spins instead of ~1015
• NMR may be difficult near the electron spin
Energy
of a
spin
system
H = g μB B0 S - gN μN B0 I
+ASI
S=½
I=½
Magnetic field, B0
Gavin W Morley, EPR and DNP
Electron nuclear double resonance
(ENDOR)
B0 is static magnetic field
B1 is EPR magnetic field
B2 is NMR magnetic field
Energy
of a
spin
system
H = g μB B0 S - gN μN B0 I
+ASI
S=½
I=½
Magnetic field, B0
Gavin W Morley, EPR and DNP
ENDOR for quantum computing
Classical
computer
Quantum
computer
Bits
Qubits
0 or 1
α 0 + β 1
Gavin W Morley, EPR and DNP
Spin qubits in silicon
Image by
Manuel Vögtli (UCL)
Gavin W Morley, EPR and DNP
Other EPR applications
• Quality control in beer, wine etc
• Radiation dose received by
teeth
Gavin W Morley, EPR and DNP
Overview
Electron paramagnetic resonance
• What it is
• Why it’s useful
Dynamic nuclear polarization
• Why it’s useful
• What it is
Gavin W Morley, EPR and DNP
Dynamic nuclear polarization (DNP)
Dynamic nuclear polarization
• More signal
Gavin W Morley, EPR and DNP
Dynamic nuclear polarization (DNP)
Thermal (Boltzmann)
equilibrium:
Define polarization
as:
n2
n1
Gavin W Morley, EPR and DNP
ΔE
Dynamic nuclear polarization (DNP)
Boltzmann polarization:
1
0.1
Polarization
Electrons
0.01
Protons
1E-3
1E-4
Magnetic field = 10 T
1E-5
0.01
0.1
1
10
Temperature (K)
Gavin W Morley, EPR and DNP
100
So transfer electronic
polarization to nuclei
Coupled electron-nuclear spin system
Energy
of a
spin
system
H = g μB B0 S - gN μN B0 I
+ASI
S=½
I=½
Magnetic field, B0
Gavin W Morley, EPR and DNP
Dynamic nuclear polarization (DNP)
Initial polarizations:
Electrons > 95%
Nuclei < 0.1%
For 8.6 T and 3 K
Gavin W Morley, EPR and DNP
Dynamic nuclear polarization (DNP)
Nuclear magnetic resonance (NMR)
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 transition
Gavin W Morley, EPR and DNP
Dynamic nuclear polarization (DNP)
Electron paramagnetic resonance (EPR)
more
Forbidden
at high magnetic fields
Gavin W Morley, EPR and DNP
Dynamic nuclear polarization (DNP)
Solid effect DNP
also gets weaker at
high magnetic fields
Gavin W Morley, EPR and DNP
Dynamic nuclear polarization (DNP)
Solid effect DNP
also gets weaker at
high magnetic fields
Gavin W Morley, EPR and DNP
Dynamic nuclear polarization (DNP)
Cross effect
Thermal Mixing
ee-
e-
n
n
n
e-
e-
en
also get weaker at
high magnetic fields
Gavin W Morley, EPR and DNP
Dynamic nuclear polarization (DNP)
Gyrotron DNP pioneered
by Bob Griffin’s group
at MIT
Also see KJ Pike et al, J Mag Res 215, 1 (2012)
Gavin W Morley, EPR and DNP
Dynamic nuclear polarization (DNP)
A Zagdoun et al, JACS 135 12790 (2013)
Gavin W Morley, EPR and DNP
Dynamic nuclear polarization
Temperature jump
1
Polarization
0.1
TDNP
plus
gain x50 from DNP
and x200 from temp
 x10,000 total
0.01
TNMR
1E-3
1E-4
Magnetic field = 10 T
1E-5
0.01
0.1
1
10
Temperature (K)
Gavin W Morley, EPR and DNP
100
See ArdenkjaerLarsen et al, PNAS
100, 10158 (2003)
Dynamic nuclear polarization
Temperature jump with dissolution
plus
Gavin W Morley, EPR and DNP
Arnaud
Comment and
Rolf Gruetter,
Lausanne
Conclusion
Electron paramagnetic resonance
• What it is
• Why it’s useful
Dynamic nuclear polarization
• Why it’s useful
• What it is
Gavin W Morley, EPR and DNP