NMR at single crystal surfaces

Abstract

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 6 Li in the one case and 8 Li 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, 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 6 Li 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, T 2 , are thus determined. A large difference between T 1 and T 2 is traced to the dimensionality of the system.