NUCLEAR-SPIN RELAXATION OF LI ADSORBED ON SI(111) - LOCAL-DENSITY OF STATES AND DIFFUSION

Nuclear spin relaxation of isolated Li-6 and Li-7 atoms adsorbed on a hot Si(111) surface was investigated. The nuclear spin relaxation of L i-6 is caused by Fermi contact interaction of the Li-6 nuclear magneti c moment with the fluctuating magnetic moments of delocalized electron s at the Fermi energy. The local density of states at the Fermi energy was determined to be LDOS(E(F)) = 0.23/eV Angstrom(3). This finite va lue indicates that the adsorbed Li atoms cannot be completely ionized. Nuclear spin relaxation of Li-7 is due to the interaction of the nucl ear electric quadrupole moment with fluctuating electric field gradien ts, which are caused by the diffusion of the Li-7 adsorbate. Monte Car lo simulations show that the diffusion process is characterized by two extremely different correlation times. This is interpretable by two d ifferent adsorption sites on the surface, one of them with a very low concentration. It is tempting to interprete them with terrace and step s sites. The diffusion constant on the terrace has been estimated to b e around 10(-7) cm(2)/s at a surface temperature of 1200 K. This value is considerably smaller than found in other experiments with alkali m etal atoms adsorbed on a Si(111) surface.