NMR at single crystal surfaces

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(E_F,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. Received: 21 March 1997/Accepted: 12 August 1997