NMR at single crystal surfaces

Nuclear magnetic resonance has developed into a very powerful technique to study the structure and dynamics of atomic and molecular systems, in both the liquid and the solid phase. However, the investigation of single-crystal surfaces with the "conventional" NMR methods is essentially impossible due to the small sample size of less than 10(15) sites on 1 cm(2). 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 Li-6 in the one case and Li-8 in the other. The latter isotope is radioactive and undergoes a beta-decay with a half-life of 0.84 s. Li adsorbed on the close-packed Ru(001) surface is investigated. The longitudinal relaxation time, T-1, is the main observable and is used to deduce the local electronic density of states [LDOS(E-F, r = 0)] and Li diffusion barriers. The second experiment uses Li-6 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 the time-dependent flux of circularly polarized light emitted behind the foil after a 90 degrees pulse has been employed at the surface. Electric field gradients and transverse relaxation times, T-2, are thus determined. A large difference between T-1 and T-2 is traced to the dimensionality of the system.