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



Overview-Why do ENDOR?-Continuous-wave ENDOR-Pulsed ENDOR with:

- Selective pulses- Non-selective pulses



-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


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

Image by Manuel Vögtli (UCL)

Birth of ENDOR


EPR-detected NMR: how?



Energy of a spin system

Magnetic field, B

Photons reflected

S = ½ I = ½

Iz = ½ Iz = -½

Magnetic field, B

Electron paramagnetic resonance



Magnetic field, B

Photons reflected

Iz = ½ Iz = -½

Magnetic field, B


S = ½ I = ½

You need to record an EPR spectrum before trying ENDOR



Magnetic field, B

Photons reflected

Iz = ½ Iz = -½

Magnetic field, B

ENDOR

RF in

S = ½ I = ½


Microwave photons reflected

RF frequency

Two ENDOR transition frequencies

For isotropic A

“Weak coupling”

Microwave photons reflected

RF frequency

“Strong coupling”



Magnetic field, B

Photons reflected

Iz = ½ Iz = -½


Magnetic field, B



Magnetic field, B



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

• EPR-detected NMR: how?



EPR-detected NMR

Continuous Wave ENDOR

George Feher Photo from AIP Emilio Segre Visual Archives




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



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)



νNMR (MHz)


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




νNMR (MHz)

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




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)


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


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


Rotating frame


Spin echo

In rotating frame


Davies ENDOR

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


Davies ENDOR

RF pulse duration is an important parameter to set


Davies ENDOR

Echo height

RF frequency

Davies ENDOR efficiency, FDavies= 50%


Davies ENDOR


Start again…


Davies ENDOR

Off-resonance RF does nothing


Davies ENDOR

Echo height

RF frequency


Davies ENDOR


Start again again…


Davies ENDOR


Davies ENDOR

Echo height

RF frequency

Davies ENDOR efficiency, FDavies= 50%


Davies ENDOR

Davies ENDOR disadvantage: selective pulses on electron spins mean many spins are ignored if the resonance is inhomogeneously broadened


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


Mims ENDOR


Beware Mims ENDOR blind spots

ENDOR efficiency,

FMims = ¼ (1 – cos (A τ))

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


Beware short RF pulses in pulsed ENDOR


This problem is avoided by “time-domain pulsed ENDOR”, instead of the standard frequency domain experiments.


Pulsed ENDOR

For more details including TRIPLE (ENDOR with two RF frequencies) see:

Schweiger & Jeschke, Principles of pulse electron paramagnetic resonance, OUP 2001



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, 10 th april 2013 electron nuclear...

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