ADSORPTION OF LI ON SI(111) INFLUENCED BY THE PHASE-TRANSITION (7X7)[--](1X1)

The adsorption of Li on Si(111) (coverage below 10(-3) ML) was investi gated around the (7x7) to (1x1) phase transition with nuclear magnetic resonance (NMR) and mean residence-time measurements. At the phase tr ansition the local density of states at the Fermi energy drops signifi cantly, indicating a transition from a ''metallic'' surface for the (7 x7) reconstruction to a semiconducting surface for the (1x1) one. At t he phase transition the activation energy for desorption changes from 2.5 to 2.85 eV. Furthermore, the electric field gradient at the adsorb ed Li nucleus, which reflects the surrounding charge distribution, has a discontinuity at the phase transition.