Transcript endor - University of Warwick

Electron nuclear double
resonance (ENDOR)
Gavin W Morley,
Department of Physics,
University of Warwick
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Electron nuclear double
resonance (ENDOR)
Overview
- Why do ENDOR?
- Continuous-wave ENDOR
- Pulsed ENDOR with:
-
Selective pulses
Non-selective pulses
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Electron nuclear double
resonance (ENDOR)
- Why do ENDOR?
- More sensitive than NMR
- “EPR-detected NMR” (electron has a
larger magnetic moment, flips faster and
can be detected more sensitively)
- NMR may be impossible due to nearby
electron spin
- Higher resolution than EPR
- Extra selection rules
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Birth of ENDOR
George Feher (born
1924) Photo from AIP
Emilio Segre Visual
Archives
Image by Manuel Vögtli (UCL)
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Electron nuclear double
resonance (ENDOR)
EPR-detected NMR: how?
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Electron paramagnetic resonance
Photons
reflected
Iz = ½
Iz = -½
Magnetic field, B
Energy
of a
spin
system
Magnetic field, B
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
S=½
I=½
Electron paramagnetic resonance
Photons
reflected
Iz = ½
Iz = -½
S=½
I=½
Magnetic field, B
Energy
of a
spin
system
You need to record an
EPR spectrum before
trying ENDOR
Magnetic field, B
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
ENDOR
Photons
reflected
Iz = ½
Iz = -½
S=½
I=½
Magnetic field, B
Energy
of a
spin
system
RF in
Magnetic field, B
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Two ENDOR transition frequencies
For isotropic A
Microwave
photons
reflected
Microwave
photons
reflected
“Weak coupling”
RF frequency
“Strong coupling”
RF frequency
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Electron paramagnetic resonance
Photons
reflected
Iz = ½
Iz = -½
Magnetic field, B
Energy
of a
spin
system
Magnetic field, B
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Electron paramagnetic resonance
Energy
of a
spin
system
Magnetic field, B
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Electron nuclear double
resonance (ENDOR)
• EPR-detected NMR: how?
B0 is static magnetic field
B1 is EPR magnetic field
B2 is NMR magnetic field
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Electron nuclear double
resonance (ENDOR)
• EPR-detected NMR: how?
B0 is static magnetic field
B1 is EPR magnetic field
B2 is NMR magnetic field
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Electron nuclear double
resonance (ENDOR)
• EPR-detected NMR: how?
B0 is static magnetic field
B1 is EPR magnetic field
B2 is NMR magnetic field
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Electron nuclear double
resonance (ENDOR)
• EPR-detected NMR: how?
B0 is static magnetic field
B1 is EPR magnetic field
B2 is NMR magnetic field
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Continuous Wave ENDOR
EPR-detected NMR
George Feher
Photo from AIP Emilio
Segre Visual Archives
Image by Manuel
Vögtli (UCL)
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Continuous Wave ENDOR
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Continuous Wave ENDOR
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Continuous Wave ENDOR
CW ENDOR is the desaturation of a
saturated EPR transition by providing an
extra T1e relaxation route via NMR
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Continuous Wave ENDOR
Hale & Mieher, Phys Rev 184 739 (1969)
(following Feher, Phys Rev 114, 1219 (1959))
(use FM and lock-in)
νNMR (MHz)
CW ENDOR is the desaturation of a
saturated EPR transition by providing an
extra T1e relaxation route via NMR
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Continuous Wave ENDOR
Hale & Mieher, Phys Rev 184 739 (1969)
(following Feher, Phys Rev 114, 1219 (1959))
νNMR (MHz)
Image by Manuel
Vögtli (UCL)
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
B Koiller, R B
Capaz, X Hu
and S Das
Sarma, PRB
70, 115207
(2004)
Continuous Wave ENDOR
Hale & Mieher, Phys Rev 184 739 (1969)
(following Feher, Phys Rev 114, 1219 (1959))
νNMR (MHz)
CW ENDOR effect is typically a few % of the EPR signal
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Continuous Wave ENDOR
Advantage of CW ENDOR:
Observe sharpest ENDOR resonances
Disadvantage of CW ENDOR:
CW ENDOR line intensity depends on a
delicate balance between relaxation
rate and excitation power. Jack Freed
did the relaxation theory for this for
molecules in solution.
George Feher
Photo from UCSD
This is compared with experiments in solution in:
Plato, Lubitz & Mobius, J Phys Chem 85, 1202 (1981)
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Pulsed ENDOR
Use a π pulse for nuclei, but there are two
main pulse sequences for electrons:
1. Davies ENDOR: π pulse then echo
readout with all selective (long) pulses.
2. Mims ENDOR uses a stimulated echo
with non-selective (short) pulses
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Davies ENDOR
MW:
RF:
π/
2
π
π
π
Roy Davies,
Royal Holloway,
University of
London
As with CW ENDOR, sweep RF frequency to get a spectrum.
Use long, selective MW pulses to burn a hole  smaller signal.
However, there are no “blind spots” which is an advantage over Mims ENDOR.
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Rotating frame
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Rotating frame
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Spin echo
In rotating frame
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Spin echo
In rotating frame
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Davies ENDOR
Product operator notation:
Electron-nuclear two-spin order, 2SzIz
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Davies ENDOR
RF pulse duration
is an important
parameter to set
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Davies ENDOR
Davies ENDOR efficiency,
FDavies= 50%
Echo
height
RF frequency
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Davies ENDOR
Product operator notation:
Electron-nuclear two-spin order, 2SzIz
Start again…
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Davies ENDOR
Off-resonance RF
does nothing
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Davies ENDOR
Echo
height
RF frequency
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Davies ENDOR
Product operator notation:
Electron-nuclear two-spin order, 2SzIz
Start again again…
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Davies ENDOR
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Davies ENDOR
Davies ENDOR efficiency,
FDavies= 50%
Echo
height
RF frequency
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Davies ENDOR
Davies ENDOR disadvantage: selective
pulses on electron spins mean many spins
are ignored if the resonance is
inhomogeneously broadened
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Mims ENDOR
MW:
τ
τ
π
RF:
sweep RF frequency
Non-selective (short) MW pulses excite more spins  bigger signal.
However, ENDOR efficiency, FMims = ¼ (1 – cos (A τ))
so there are “blind spots” with no signal for some τ
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Mims ENDOR
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Beware Mims ENDOR blind spots
C Gemperle & A
Schweiger, Chem
Rev 91, 1481 (1991)
ENDOR efficiency,
FMims = ¼ (1 – cos (A τ))
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Beware short RF pulses in pulsed ENDOR
C Gemperle & A
Schweiger, Chem
Rev 91, 1481 (1991)
This problem is avoided by “time-domain pulsed ENDOR”,
instead of the standard frequency domain experiments.
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
Pulsed ENDOR
For more details including TRIPLE
(ENDOR with two RF frequencies) see:
Schweiger & Jeschke, Principles of pulse
electron paramagnetic resonance, OUP
2001
C Gemperle & A Schweiger, Chem Rev 91,
1481 (1991)
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013
ENDOR conclusions
- ENDOR is much more sensitive
than NMR and has much higher
resolution than EPR
- Continuous-wave ENDOR for
very sharp resonances
- Pulsed ENDOR with:
-
Selective pulses (Davies)
Non-selective pulses (Mims)
Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10th April 2013