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  • 1
    Digitale Medien
    Digitale Medien
    NMR at single crystal surfaces (1997)
    Springer
    Applied physics 65 (1997), S. 567-572 
    Details
    ISSN: 1432-0630
    Schlagwort(e): PACS: 68.35.Fx; 76.60.Es; 82.65.My
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: 15 sites on 1 cm2. To overcome this for the important class of alkali adsorbates on metals and semiconductors, two methods are presented. Common to both is the preparation of a highly nuclear spin-polarized atomic beam of 6Li in the one case and 8Li in the other. The latter isotope is radioactive and undergoes a β-decay with a half-life of 0.84 s. Li adsorbed on the close-packed Ru(001) surface is investigated. The longitudinal relaxation time, T1, is the main observable and is used to deduce the local electronic density of states [LDOS(EF,r=0)] and Li diffusion barriers. The second experiment uses 6Li as an adsorbate, also studied on Ru(001). The nuclear polarization is measured by beam foil spectroscopy. A novel particle detected (photon counting) Fourier transform NMR technique is demonstrated. This is done by observing the time-dependent flux of circularly polarized light emitted behind the foil after a 90° pulse has been employed at the surface. Electric field gradients and transverse relaxation times, T2, are thus determined. A large difference between T1 and T2 is traced to the dimensionality of the system.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/s003390050623
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  • 2
    Digitale Medien
    Digitale Medien
    Production of nitrogen-free, hyperpolarized 129Xe gas (1999)
    Springer
    Applied physics 68 (1999), S. 93-97 
    Details
    ISSN: 1432-0649
    Schlagwort(e): PACS: 76.60.-k; 68.35.Md; 32.80.Bx
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: 129 Xe with a nuclear polarization far above the thermal equilibrium value (hyperpolarized) is used in NMR studies to increase sensitivity. Gaseous, adsorbed, or dissolved xenon is utilized in physical, chemical, and medical applications. With the aim in mind to study single-crystal surfaces by NMR of adsorbed hyperpolarized 129Xe, three problems have to be solved. The reliable production of 129Xe with highest nuclear polarization possible, the separation of the xenon gas from the necessary quench gas nitrogen without polarization loss, and the dosing/delivery of small amounts of polarized xenon gas to a sample surface. Here we describe an optical pumping setup that regularly produces xenon gas with a 129Xe nuclear polarization of 0.7(±0.07). We show that a freeze–pump–thaw separation of xenon and nitrogen is feasible without a significant loss in xenon polarization. The nitrogen partial pressure can be suppressed by a factor of 400 in a single separation cycle. Dosing is achieved by using the low vapor pressure of a frozen hyperpolarized xenon sample.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/s003400050592
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  • 3
    Digitale Medien
    Digitale Medien
    Single crystal surfaces probed by polarized nuclei (1997)
    Springer
    Hyperfine interactions 106 (1997), S. 219-228 
    Details
    ISSN: 1572-9540
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract Nuclear magnetic resonance has developed into a very powerful technique to study the structure and dynamics of atomic and molecular systems, both in liquid and solid phase. However the investigation of single crystal surfaces with “conventional” NMR methods is essentially impossible due to the small sample size of less than 1015 sites on a cm2. To overcome this for the important class of alkali adsorbates on metals and semiconductors, two methods are presented. Common to both is the preparation of a highly nuclear spin polarized atomic beam of 6Li in the one case and 8Li in the other. The latter isotope is radioactive and undergoes a \beta‐decay with a halflife of 0.84 s. Li adsorbed on the close packed Ru(001) surface is investigated. The T{in1} relaxation rate is the main observable and is used to deduce the local electronic density of states (LDOS(EF,r=0)) and the Li diffusion barriers at low and high adsorbate coverage. The second experiment uses 6Li as an adsorbate, also studied on Ru(001). The nuclear polarization is measured by beam foil spectroscopy. A novel particle detected (photon counting) Fourier‐Transform NMR technique is demonstrated, by observing the time dependent flux of circularly polarized light emitted behind the foil after a 90\circ‐pulse has been employed at the surface. Development and prospects of the latter technique are presented.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1012658411960
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  • 4
    Digitale Medien
    Digitale Medien
    Nuclear spin relaxation of 8Li adsorbed on surfaces (1997)
    Springer
    Hyperfine interactions 110 (1997), S. 295-312 
    Details
    ISSN: 1572-9540
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract Nuclear spin relaxation experiments with 8Li adsorbed on various surfaces provide new information in surface science which is not obtainable othervise. Both dipolar (Korringa) and quadrupolar relaxation due to diffusion are observed. However, in addition, a fast and presently not understood spin relaxation mechanism is present while dosing during the first 0.5 s the surface with polarized 8Li. Most strange in this respect is the fact that those 8Li atoms which survive depolarization through this mechanism depolarize afterwards with modest spin lattice relaxation rates. The origin of the fast spin lattice relaxation mechanism is presently unknown.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1012656306817
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  • 5
    Digitale Medien
    Digitale Medien
    An apparatus for NMR of laser polarized 129Xe on single crystal surfaces (2000)
    Springer
    Hyperfine interactions 127 (2000), S. 469-474 
    Details
    ISSN: 1572-9540
    Schlagwort(e): NMR ; single crystal surfaces ; optical pumping of Rb ; hyperpolarized 129Xe ; laser polarized 129Xe
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract Due to a lack of at least 2 orders of magnitude in the amount of sample nuclei, single crystal surfaces are out of reach for conventional NMR measurements. Our aim is to prove that highly polarized 129Xe provides a technique to overcome this restriction. Therefore an apparatus for polarizing 129Xe up to 0.7 by spin transfer from optically pumped Rb has been designed as well as an NMR spectrometer in combination with a UHV chamber with sample cleaning, cooling and characterization abilities and a special manifold of glass stopcocks with a liquid nitrogen cooled trap for dosing nitrogen free polarized Xe into the chamber onto the surface.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1012630120408
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