Dynamics Of Nuclear Spin Polarization

1. Dynamics Of Nuclear Spin Polarization investigated by simultaneous NMR and polarized neutron scattering J. Kohlbrecher Paul Scherrer Institute CH-5232 Villigen Switzerland B. van den Brandt, P. Hautle, J. Kohlbrecher, J.A. Konter, S. Mango (PSI) H. Glättli, E. Leymarie (CEA-Saclay) I. Grillo, R. May (ILL) H. Jouve, H.B. Stuhrmann (IBS) O. Zimmer (TU München) J. Kohlbrecher, Polarized Solid Targets, Honnef 2003

2. Dynamics Of Nuclear Spin Polarization investigated by simultaneous NMR and polarized neutron scattering Joachim Kohlbrecher Contents: • DNP: a two step process • How to measure clusters of polarized protons • scattering of polarized neutron on polarized nuclei • small angle neutron scattering (SANS) • model system: EHBA-Cr(V) • time-resolved SANS and NMR experiments J. Kohlbrecher, Polarized Solid Targets, Honnef 2003

3. proton electron 2. H0 DNP: a two step process 1. polarization diffuses to the bulk protons by flip-floptransitions time constant: tdiff» few seconds • protons close to electron polarize first • time constant: tpol < 1 s • microwave induced direct interaction • falls off like r6 paramagnetic centre generates strong local field close protons:● strongly coupled to the p.c. ● weakly coupled to the bulk ● “poorly visible" by cw-NMR bulk protons:● weakly coupled to thep.c ●"visible" by cw-NMR J. Kohlbrecher, Polarized Solid Targets, Honnef 2003

4. How to measure clusters of polarized protons what do we want to see: • time-dependent polarization built-up of the proton spins around the radical • and in the bulk requirements for experimental method: • sensitive on polarization of protons • sensitive on length scale in nm-range experimental method: • cw-NMR (bulk protons) • neutron scattering technique (close protons) J. Kohlbrecher, Polarized Solid Targets, Honnef 2003

5. Neutron scattering technique polarized neutron scattering on polarized nuclei cold neutrons and their interaction with matter wavelength: 4 Å frequency: 1.2 THz energy: 5meV wavevector: 1.55 1/Å velocity: 980 m/s temperature: 58 K interaction potential for a single atom refraction index l » range of V(r) scattering length density scattering length J. Kohlbrecher, Polarized Solid Targets, Honnef 2003

6. parallel antiparallel 1H 2H 12C 14N 16O neutron isotope neutron isotope s = ½ I = ½ s = ½ I = -½ Pp=-1 Pp=1 spin dependent scattering length b = b0 + bnI.s Polarized neutron scattering on polarized nuclei b0 = -0.374 bn = 5.8254 proton: J. Kohlbrecher, Polarized Solid Targets, Honnef 2003

7. How to measure clusters of polarized protons what do we want to see: • time-dependent polarization built-up of the proton spins around the radical • and in the bulk requirements for experimental method: • sensitive on polarization of protons • sensitive on length scale in nm-range experimental method: • cw-NMR (bulk protons) • neutron scattering technique (close protons) J. Kohlbrecher, Polarized Solid Targets, Honnef 2003

8. sample incidentneutrons, l scatteredneutrons detector d λ ≈ 0.5 nm θ≈ 3 deg d ≈ 10 nm Small Angle Neutron Scattering J. Kohlbrecher, Polarized Solid Targets, Honnef 2003

9. 2.0 R=0.45nm, b2 =1 1.8 R=0.45nm, b2 =1.2 1.6 R=0.5nm, b2 =1 1.4 R=0.5nm, b2 =0.53 1.2 I(Q) / a.u. 1.0 0.8 0.6 0.4 0.2 0.0 0 1 2 3 4 5 6 7 8 -1 Q / nm Formfactor in SANS shape height J. Kohlbrecher, Polarized Solid Targets, Honnef 2003

10. bulk proton electron close proton glycerol-water (80% ….. 98% deuterated) 0.1 0.1 P=1 P=1 0.05 [C2H5]4 0.05 scattering length density 0 P=0 [CrVO7C4]- scattering length density 0 P=0 -0.05 -0.05 P=-1 P=-1 -0.1 -0.1 R2 -10 -5 0 5 10 -10 -5 0 5 10 radius [Å] P=-1 P=0 P=1 R1 radius [Å] P=-1 P=0 P=1 close protons Shell of polarized protons: Model System: EHBA-Cr(V) sample E2 (solvent 88% D) sample E5 (solvent 98% D) J. Kohlbrecher, Polarized Solid Targets, Honnef 2003

11. positive dynamic nuclear polarization (DNP): 10 s time resolved acquisition switch to negative polarization frequency switch to positive polarization frequency • 200 spectra (in time frames of 0.1 s length) • several hundreds of 20 s long cycles are averaged •  precise scattering intensity for each time frame time resolved acquisition negative dynamic nuclear polarization (DNP): 10 s Time resolved data acquisition J. Kohlbrecher, Polarized Solid Targets, Honnef 2003

12. t = 10.1 s t = 10 s t = 20 s t = 10.9 s t = 0.9 s t = 3.4 s t = 13.4 s t = 0.1 s PH PH PH PH PH PH PH PH t [s] t [s] t [s] t [s] t [s] t [s] t [s] t [s] 20 20 20 20 20 20 20 20 0 0 0 0 0 0 0 0 10 10 10 10 10 10 10 10 Observation of polarized proton clusters J. Kohlbrecher, Polarized Solid Targets, Honnef 2003

13. Results Experimental results: 200 spectra of neutron scattering interpretation of the scattering curves close protons Fitting parameters : R1, R2, A, IInc and P Conditions:• R1 and R2 are constant and time independent, • Incoherent scattering independent of Q but dependent on time • homogeneous polarization of the protons in the shell, dependent on time IInc(t) P(t) R1 = 3.4 Å R2 = 4.9 Å J. Kohlbrecher, Polarized Solid Targets, Honnef 2003

14. 0.3 0.2 0.1 0.0 polarization -0.1 -0.2 close protons (fit) bulk protons (NMR) -0.3 exponential fit (t1= 1.1 s, t2=5.5s) -0.4 time dependent polarization gradient between the close and bulk protons 0 5 10 15 20 time [s] Time evolution of the polarization characterization: sum of two exponential t1 = 1.1s t2 = 5.5s close protons versus bulk protons time dependence of polarization close protons~ exponential (SANS) bulk protons~ linear (NMR) Europhys. Lett. 59 (2002) 62-67 J. Kohlbrecher, Polarized Solid Targets, Honnef 2003

15. Influence of the solvent deuteration Evolution of the close proton polarization for various solvent deuterations: 98%, 95%and92%D expected effect: faster diffusion of spin in solvent J. Kohlbrecher, Polarized Solid Targets, Honnef 2003