SINGLE-CRYSTAL SURFACES PROBED BY POLARIZED NUCLEI

Nuclear magnetic resonance has developed into a very powerful techniqu e to study the structure and dynamics of atomic and molecular systems, both in liquid and solid phase. However the investigation of single c rystal surfaces with ''conventional'' NMR methods is essentially impos sible due to the small sample size of less than 10(15) sites on a cm(2 ). To overcome this for the important class of alkali adsorbates on me tals and semiconductors, two methods are presented. Common to both is the preparation of a highly nuclear spin polarized atomic beam of Li-6 in the one case and Li-8 in the other. The latter isotope is radioact ive 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-1 relaxatio n rate is the main observable and is used to deduce the local electron ic density of states (LDOS(E-F, r = 0)) and the Li diffusion barriers at low and high adsorbate coverage. The second experiment uses Li-6 as an adsorbate, also studied on Ru(001). The nuclear polarization is me asured by beam foil spectroscopy. A novel particle detected (photon co unting) Fourier-Transform NMR technique is demonstrated, by observing the time dependent flux of circularly polarized light emitted behind t he foil after a 90 degrees-pulse has been employed at the surface. Dev elopment and prospects of the latter technique are presented.