Laserunterstützte magnetische Resonanz

Transcription

1 Laserunterstützte magnetische Resonanz Dieter Suter

2 Magnetische Resonanz Prinzip Die MR mißt Übergänge zwischen unterschiedlichen Spin-Zuständen. Diese werden durch ein magnetisches Wechselfeld im Radiofrequenzbereich angeregt. rf B Informationsgehalt Resonanzfrequenz erlaubt Rückschlüsse auf elektronische und geometrische Struktur Spin-Spin Kopplungen - direkte (dipolare) Kopplungen messen die geometrische Struktur - indirekte (skalare) Kopplungen sind abhängig von elektronischer und geometrischer Struktur Relaxationsraten Bewegungsprozesse CH 3 Resonanzfrequenz Struktur

3 Laser Spectroscopy Principle LS excites and detects transitions between different electronic states with laser light. Characteristics sensitivity: experiments with individual Mg+ ions single atoms and molecules Waki et al., PRL 68, 2007 (1992) time-resolution: t < sec frequency resolution: ν/ν > µm

4 Adv. Magn. Opt. Reson. 16, 1 (1991) R esonance M agnetic E nhanced O ptically Why? use lasers Sensitivity increase by > 10 orders of magnitude Selectivity spatial, temporal, chemical Excited States prepared with laser pulses Additional Information from the interaction with light Speed enhanced 'relaxation' Time-Resolution up to ~10 fsec (10-14 sec)

5 Sensitivity Resonance M agnetic of E nhanced O ptically Polarization NMR: optically pumped: n - n n + n n - n n + n 1 Signal Energy per Spin NMR hν rf Laser hν opt J

6 Optical Polarisation and Detection of Magnetic Resonance Transitions Optical Pumping A. Kastler, J. Phys. 11, 255 (1950) J' = 1/2 Polarisation-Selective Detection Adv. Magn. Opt. Reson. 16, 1 (1991). J' = 1/2 J = 1/2 J = 1/2

7 GaAs Quantum Wells Quantum well structures are used in semiconductor lasers experimental realisations of the 'particle in a box' AlGaAs GaAs 20 nm GaAs AlGaAs 15 AlGaAs GaAs GaAs AlGaAs GaAs AlGaAs GaAs AlGaAs Luminescence spectrum of multi quantum well sample 810 λ/nm 820

8 NMR of a single GaAs Quantum Well Twofold challenge: - number of spins in QW ~ sensitivity - number of spins in substrate ~ selectivity Solution: Optical excitation and detection of NMR: high sensitivity and selectivity Marcus Eickhoff Björn Lenzmann nm λ/nm Signal from 19 nm QW 75 As 2QT 71 Ga 69 Ga 75 As B / T

9 Hanle Effect magnetic field induces Larmor precession Resulting average spin orientation = polarisation of luminescence spin polarisation decays: emssion, relaxation S(τ) S z (B) = B 2 B 2 + B 2 S z S(0) ω L Width B: Larmor frequency = relaxation rate "Hanle curve" Nuclear field from hyperfine interaction adds to external field z 0 B B B S z -B N Laser θ B N B ext Polarisation B nuclear spin polarisation small nuclear spin polarisation large B / T

10 B-field scan during rf-irradiation: at resonance the nuclear spins are saturated and the Hanle curve shifts polarization of luminescence γb=ω rf before saturation after saturation magnetic field polarization / % As sweep time 20 s halfwidth of resonance line: 1.3 khz frequency / MHz

11 Adiabatic Fast Passage Single up/down Passage polarization / % Ga frequency scan sweep time 0.1 s 4.27 frequency / MHz 4.07 repetitive 3 Hz 10 sec time

12 Quadrupole Splitting data taken at a fixed frequency of 1 MHz by sweeping the magnetic field derivative of polarization 75 As Splitting ν=66 khz 150 Magnetic Field [mt] strain ε =

13 Metalloproteins Some 30% of all enzymes contain one or more metal ions that are essential for the biological function Example: electron transfer through membrane Electron Paramagnetic Resonance (EPR) is an important spectroscopic tool for the analysis of the geometrical and electronic structure of the active site Cytochrome Fe ion

14 Chemical Specificity in Metalloproteins Birgit Börger Cytochrome Jörg Gutschank Marc-Oliver Schweika with Andrew Thomson (UEA, Norwich, GB) and Stephen Bingham (Univ. Bath, GB) B µ-wave Polarisation selective detection Fe Laser Fe impurities ESR Signal z conv. EPR y theoretical Raman-Heterodyne EPR B-Field/T

15 Transverse MCD circular dichroism due to precessing magnetisation B Signal transverse magnetization resonant process Microwave Signal in phase out of phase ω mw ω mw Correlation with g-value: optical anisotropy tensor C ε x C z g z g 2 g 2 sin2 θ + C g ( g2 z g 2 cos2 θ +1) B 0 g B 0 g z C z C Laser Laser

16 Optical Bands from ODEPR λ = 810 nm ODEPR Dispersion Spectra at different wavelengths λ = 514 nm z C z γ C λ = 720 nm C x,y γ / rad 3π 2 γ C π π 2 MCD Energy / 1000 cm Energy / 1000 cm -1