introduction to endor, gavin w morley, imr cdt advanced workshop, 10 th april 2013 electron nuclear...

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

Electron nuclear double resonance (ENDOR)

Gavin W Morley,

Department of Physics,

University of Warwick

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th 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, 10 th 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, 10 th April 2013

George Feher (born 1924) Photo from AIP Emilio Segre Visual Archives

Image by Manuel Vögtli (UCL)

Birth of ENDOR

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

EPR-detected NMR: how?

Electron nuclear double resonance (ENDOR)

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

Energy of a spin system

Magnetic field, B

Photons reflected

S = ½ I = ½

Iz = ½ Iz = -½

Magnetic field, B

Electron paramagnetic resonance

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

Energy of a spin system

Magnetic field, B

Photons reflected

Iz = ½ Iz = -½

Magnetic field, B

Electron paramagnetic resonance

S = ½ I = ½

You need to record an EPR spectrum before trying ENDOR

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

Energy of a spin system

Magnetic field, B

Photons reflected

Iz = ½ Iz = -½

Magnetic field, B

ENDOR

RF in

S = ½ I = ½

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

Microwave photons reflected

RF frequency

Two ENDOR transition frequencies

For isotropic A

“Weak coupling”

Microwave photons reflected

RF frequency

“Strong coupling”

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

Energy of a spin system

Magnetic field, B

Photons reflected

Iz = ½ Iz = -½

Electron paramagnetic resonance

Magnetic field, B

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

Energy of a spin system

Magnetic field, B

Electron paramagnetic resonance

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

B0 is static magnetic fieldB1 is EPR magnetic fieldB2 is NMR magnetic field

• EPR-detected NMR: how?

Electron nuclear double resonance (ENDOR)

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

B0 is static magnetic fieldB1 is EPR magnetic fieldB2 is NMR magnetic field

• EPR-detected NMR: how?

Electron nuclear double resonance (ENDOR)

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

B0 is static magnetic fieldB1 is EPR magnetic fieldB2 is NMR magnetic field

• EPR-detected NMR: how?

Electron nuclear double resonance (ENDOR)

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

B0 is static magnetic fieldB1 is EPR magnetic fieldB2 is NMR magnetic field

• EPR-detected NMR: how?

Electron nuclear double resonance (ENDOR)

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

EPR-detected NMR

Continuous Wave ENDOR

George Feher Photo from AIP Emilio Segre Visual Archives

Image by Manuel Vögtli (UCL)

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

Continuous Wave ENDOR

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

Continuous Wave ENDOR

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th 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, 10 th April 2013

Continuous Wave ENDOR

Hale & Mieher, Phys Rev 184 739 (1969)(following Feher, Phys Rev 114, 1219 (1959))

νNMR (MHz)

CW ENDOR is the desaturation of a saturated EPR transition by providing an extra T1e relaxation route via NMR

(use FM and lock-in)

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

Continuous Wave ENDOR

νNMR (MHz)

Image by Manuel Vögtli (UCL)

B Koiller, R B Capaz, X Hu and S Das Sarma, PRB 70, 115207 (2004)

Hale & Mieher, Phys Rev 184 739 (1969)(following Feher, Phys Rev 114, 1219 (1959))

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

Continuous Wave ENDOR

νNMR (MHz)

CW ENDOR effect is typically a few % of the EPR signal

Hale & Mieher, Phys Rev 184 739 (1969)(following Feher, Phys Rev 114, 1219 (1959))

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

Continuous Wave ENDOR

George Feher Photo from UCSD

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.

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, 10 th 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, 10 th April 2013

Davies ENDOR

Roy Davies, Royal Holloway, University of London

π

π π/2

RF:

MW:

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.

π

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

Rotating frame

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

Rotating frame

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

Spin echo

In rotating frame

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

Spin echo

In rotating frame

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

Davies ENDOR

Product operator notation:Electron-nuclear two-spin order, 2SzIz

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

Davies ENDOR

RF pulse duration is an important parameter to set

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

Davies ENDOR

Echo height

RF frequency

Davies ENDOR efficiency, FDavies= 50%

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th 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, 10 th April 2013

Davies ENDOR

Off-resonance RF does nothing

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

Davies ENDOR

Echo height

RF frequency

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th 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, 10 th April 2013

Davies ENDOR

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

Davies ENDOR

Echo height

RF frequency

Davies ENDOR efficiency, FDavies= 50%

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th 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, 10 th April 2013

Mims ENDOR

τ τπ

RF: sweep RF frequency

MW:

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, 10 th April 2013

Mims ENDOR

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th April 2013

Beware Mims ENDOR blind spots

ENDOR efficiency,

FMims = ¼ (1 – cos (A τ))

C Gemperle & A Schweiger, Chem Rev 91, 1481 (1991)

Introduction to ENDOR, Gavin W Morley, iMR CDT Advanced Workshop, 10 th 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, 10 th 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, 10 th 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)