Overview. Magnetism. Electron paramagnetic resonance (EPR) 28/02/2014. Electron Paramagnetic Resonance and Dynamic Nuclear Polarisation AS:MIT CH916

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1 Electron Paramagnetic Resonance and Dynamic Nuclear Polarisation AS:MIT CH916 Overview What it is Why it s useful Gavin W Morley, Department of Physics, University of Warwick Dynamic nuclear polarization Why it s useful What it is (EPR) Magnetism NMR for electrons: The crucial difference is that the electron magnetic moment is 660 times larger than that proton Also known as electron resonance (ESR) or electron magnetic resonance (EMR) or ferromagnetic resonance (FMR) Paramagnetism: Diamagnetism: Ferromagnetism: Electron s tend to: follow oppose ignore an applied magnetic field 1

2 Magnetic resonance Magnetic resonance Magnetic field Photon energy Isidor Isaac Rabi ( ) H = g µ B B S Pieter Zeeman ( ) Magnetic field I = ½ system H = g µ B B S- g N µ N B I Bruker J van Tol, L-C Brunel & RJ Wylde, Rev Sci Instrum 76, (2005) 2

3 I z = ½ I z = -½ A I z = ½ I z = -½ A Stable free radicals: BDPA Nitroxides DPPH system H = g µ B B S- g N µ N B I + A S I I = ½ J van Tol, L-C Brunel & RJ Wylde, Rev Sci Instrum 76, (2005) TEMPO 20/g from Aldrich absorbed I z = ½ I z = -½ A absorbed I z = 1 I z = 0 I z = -1 A TEMPO TEMPO J van Tol, L-C Brunel & RJ Wylde, Rev Sci Instrum 76, (2005) 20/g from Aldrich J van Tol, L-C Brunel & RJ Wylde, Rev Sci Instrum 76, (2005) 20/g from Aldrich 3

4 absorbed I z = 1 I z = 0 I z = -1 Differential absorption I z = 1 I z = 0 I z = -1 Magnetic field modulation (~10 to 100 khz) Advantage: noise only at modulation frequency A TEMPO Differential absorption J van Tol, L-C Brunel & RJ Wylde, Rev Sci Instrum 76, (2005) 20/g from Aldrich Motional narrowing occurs due to molecular rotation at higher temperature spectrometer Magic angle ning is not used in EPR because you can t fast enough TEMPO Microwave bridge S Console Computer J van Tol, L-C Brunel & RJ Wylde, Rev Sci Instrum 76, (2005) 20/g from Aldrich Electromagnet (up to 1.5 T) or superconducting magnet for higher field 4

5 spectrometer spectrometer Bridge source detector Modulation coils Continuous-wave (CW) EPR so far Pulsed EPR next S Microwave resonator Pulsed electron paramagnetic resonance spectrometer Pulsed electron paramagnetic resonance Bridge High power pulsed source Protected detector Modulation coils off S Microwave resonator Bruker probe Bloch sphere Felix Bloch ( ) Photo courtesy Stanford News Service 5

6 Pulsed electron paramagnetic resonance B res resonance condition g µ B B res = h f Pulsed EPR J van Tol, L-C Brunel & RJ Wylde, Rev Sci Instrum 76, (2005) f Spins polarize (thermalize) on timescale T 1 f ~100 MHz width Pulse width < 500 MHz Resonator ringing deadtime Short T 2 and T 2* compared to NMR π / 2 Free induction decay (FID) on timescale T 2 * π / 2 Homogeneous (T 2 ) can be much longer than inhomogeneous (T 2* ) so most pulsed EPR uses echo Rotating Frame Spin echo In rotating frame 6

7 Spin echo decay Pulsed EPR Double electron-electron resonance (DEER) allows distances between two electron s in the range 2 to 6 nm to be measured (cf < 1 nm by NMR for two nuclear s) Dipolar coupling α 1/r 3 Site directed labelling with one or more TEMPO Erwin L Hahn (born 1921) Photo: AIP Emilio Segre Visual Archives, Stephen Jacobs Collection Electron nuclear double resonance (ENDOR) I z = ½ I z = -½ A Electron nuclear double resonance (ENDOR) Why do ENDOR instead of NMR? only need ~10 10 s instead of ~10 15 NMR may be difficult near the electron system H = g µ B B S- g N µ N B I + A S I I = ½ system H = g µ B B 0 S- g N µ N B 0 I + A S I I = ½ 0 7

8 Electron nuclear double resonance (ENDOR) system EPR language: B 0 is static magnetic field B 1 is EPR magnetic field B 2 is NMR magnetic field (NMR language: B 1 is NMR magnetic field) H = g µ B B 0 S- g N µ N B 0 I + A S I I = ½ ENDOR for quantum computing Classical Quantum computer computer Bits Qubits 0 or 1 α 0 + β 1 0 Spin qubits in silicon Other EPR applications Quality control in beer, wine etc Radiation dose received by teeth Image by Manuel Vögtli (UCL) 8

9 Overview What it is Why it s useful Dynamic nuclear polarization Why it s useful What it is Dynamic nuclear polarization More signal Boltzmann polarization: 1 Boltzmann polarization: = n 2 n 1 E = + Polarization E-3 1E-4 1E-5 Electrons Protons Magnetic field = 10 T Temperature (K) = + So transfer electronic polarization to nuclei 9

10 Initial polarizations: Nuclear magnetic resonance (NMR) Electrons > 95% Nuclei < 0.1% For 8.6 T and 3 K (EPR) (EPR) Forbidden transitions: zero quantum and double quantum 10

11 Magnetic resonance Magnetic resonance I = ½ I z = ½ I z = -½ A Gregory Breit ( ) system H = g µ B B S- g N µ N B I system H = g µ B B S- g N µ N B I + A S I I = ½ Magnetic resonance Magnetic resonance Triplet, F = 1: ( + )/ 2 system Breit-Rabi diagram A S I = A / 2 (S + I - + S - I + ) + A S z I z system Singlet, F = 0: ( - )/ 2 S ± = S x ± i S y H = g µ B B S- g N µ N B I + A S I H = g µ B B S- g N µ N B I + A S I 11

12 (EPR) Solid effect DNP more Forbidden at high magnetic fields also gets weaker at high magnetic fields Solid effect DNP Cross effect e- n e- Thermal Mixing e- e- n n e- e- n also gets weaker at high magnetic fields also get weaker at high magnetic fields 12

13 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) EPR + NMR = Electron nuclear double resonance (ENDOR) EPR + NMR = Electron nuclear double resonance (ENDOR) ENDOR-DNP: A S Brill, PRA (2003) ENDOR-DNP: A S Brill, PRA (2003) 13

14 Background Our research EPR + NMR = Electron nuclear double resonance (ENDOR) GWM, J van Tol, A Ardavan, K Porfyrakis, J Zhang & GAD Briggs, PRL 98, (2007) GWM, K Porfyrakis, A Ardavan & J van Tol, AMR 34, 347 (2008) ENDOR-DNP: A S Brill, PRA (2003) ENDOR-DNP: A S Brill, PRA (2003) See also: B Epel, A Poppl, P Manikandan, S Vega & D Goldfarb, JMR (2001) Background Our research Dynamic nuclear polarization 240 GHz excitation, A/ħ = 16 MHz Dynamic nuclear polarization Temperature jump CW EPR signal Thermalized at 4 K After ENDOR-DNP ENDOR-DNP at 4 K Polarization E-3 1E-4 T DNP Magnetic field = 10 T T NMR gain x50 from DNP and x200 from temp x10,000 total See Ardenkjaer- Larsen et al, PNAS 100, (2003) Magnetic field (T) 1E Temperature (K) 14

15 Dynamic nuclear polarization Temperature jump with dissolution plus Arnaud Comment and Rolf Gruetter, Lausanne Conclusions 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 Dynamic nuclear polarization For a review see: T Maly et al., DNP at high magnetic fields, Journal of Chemical Physics, 128, (2008) 15