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J. Kohlbrecher, Polarized Solid Targets, Honnef 2003 Dynamics Of Nuclear Spin Polarization J. Kohlbrecher Paul Scherrer Institute CH-5232 Villigen Switzerland.

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Presentation on theme: "J. Kohlbrecher, Polarized Solid Targets, Honnef 2003 Dynamics Of Nuclear Spin Polarization J. Kohlbrecher Paul Scherrer Institute CH-5232 Villigen Switzerland."— Presentation transcript:

1 J. Kohlbrecher, Polarized Solid Targets, Honnef 2003 Dynamics Of Nuclear Spin Polarization 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) investigated by simultaneous NMR and polarized neutron scattering

2 J. Kohlbrecher, Polarized Solid Targets, Honnef 2003 DNP: a two step process How to measure clusters of polarized protons 1.scattering of polarized neutron on polarized nuclei 2.small angle neutron scattering (SANS) model system: EHBA-Cr(V) time-resolved SANS and NMR experiments Dynamics Of Nuclear Spin Polarization investigated by simultaneous NMR and polarized neutron scattering Contents: Joachim Kohlbrecher

3 J. Kohlbrecher, Polarized Solid Targets, Honnef 2003 protons close to electron polarize first t pol < 1 s time constant: t pol < 1 s electron 1. proton polarization diffuses to the bulk protons by flip-flop transitions time constant: t diff » few seconds 2. H0H0 DNP: a two step process close protons:● strongly coupled to the p.c. ● weakly coupled to the bulk ● “poorly visible" by cw-NMR bulk protons: ● weakly coupled to the p.c ● "visible" by cw-NMR paramagnetic centre generates strong local field microwave induced direct interaction falls off like r 6

4 J. Kohlbrecher, Polarized Solid Targets, Honnef 2003 How to measure clusters of polarized protons requirements for experimental method: 1.sensitive on polarization of protons 2.sensitive on length scale in nm-range what do we want to see: 1.time-dependent polarization built-up of the proton spins around the radical 2.and in the bulk experimental method: 1.cw-NMR (bulk protons) 2.neutron scattering technique (close protons)

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

6 J. Kohlbrecher, Polarized Solid Targets, Honnef 2003 parallel neutron isotope s = ½I = ½ b = b 0 + b n I. s spin dependent scattering length neutronisotope s = ½I = -½ antiparallel Polarized neutron scattering on polarized nuclei proton: b 0 = -0.374 b n = 5.8254 1 H 2 H 12 C 14 N 16 O Pp=-1 Pp=1

7 J. Kohlbrecher, Polarized Solid Targets, Honnef 2003 How to measure clusters of polarized protons requirements for experimental method: 1.s ensitive on polarization of protons 2.sensitive on length scale in nm-range what do we want to see: 1.time-dependent polarization built-up of the proton spins around the radical 2.and in the bulk experimental method: 1.cw-NMR (bulk protons) 2.neutron scattering technique (close protons)

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

9 J. Kohlbrecher, Polarized Solid Targets, Honnef 2003 Formfactor in SANS shape height 012345678 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 R=0.45nm, b 2 =1 R=0.45nm, b 2 =1.2 R=0.5nm, b 2 =1 R=0.5nm, b 2 =0.53 I(Q) / a.u. Q / nm

10 J. Kohlbrecher, Polarized Solid Targets, Honnef 2003 Shell of polarized protons: R1R1 R2R2 [Cr V O 7 C 4 ] - [C2H5]4[C2H5]4 glycerol-water (80% ….. 98% deuterated) bulk proton electron close proton Model System: EHBA-Cr(V) close protons sample E5 (solvent 98% D) sample E2 (solvent 88% D)

11 J. Kohlbrecher, Polarized Solid Targets, Honnef 2003 Time resolved data acquisition switch to negative polarization frequency negative dynamic nuclear polarization (DNP): 10 s time resolved acquisition time frames 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 switch to positive polarization frequency positive dynamic nuclear polarization (DNP): 10 s time resolved acquisition

12 J. Kohlbrecher, Polarized Solid Targets, Honnef 2003 t = 0.1 s t [s] 010 20 PHPH t = 0.9 s t [s] 010 20 PHPH Observation of polarized proton clusters t = 3.4 s t [s] 010 20 PHPH t = 10 s t [s] 010 20 PHPH t = 10.1 s t [s] 010 20 PHPH t = 10.9 s t [s] 010 20 PHPH t = 13.4 s t [s] 010 20 PHPH t = 20 s t [s] 010 20 PHPH

13 J. Kohlbrecher, Polarized Solid Targets, Honnef 2003 Results Experimental results: 200 spectra of neutron scattering interpretation of the scattering curves Fitting parameters : R 1, R 2, A, I Inc and P Conditions: R 1 and R 2 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 R 1 = 3.4 Å R 2 = 4.9 Å P(t) I Inc (t) close protons

14 J. Kohlbrecher, Polarized Solid Targets, Honnef 2003 time dependent polarization gradient between the close and bulk protons time dependence of polarization close protons ~ exponential (SANS) bulk protons ~ linear (NMR) close protons versus bulk protons Europhys. Lett. 59 (2002) 62-67 characterization: sum of two exponential t 1 = 1.1s t 2 = 5.5s Time evolution of the polarization

15 J. Kohlbrecher, Polarized Solid Targets, Honnef 2003 Influence of the solvent deuteration expected effect: faster diffusion of spin in solvent Evolution of the close proton polarization for various solvent deuterations: 98%, 95% and 92% D


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