PAC INVESTIGATION OF BR-77-]SE-77 ON SILICON SURFACES

The perturbed gamma gamma-angular correlation (PAC) method has been ap plied to investigate the sites occupied by isolated Br-77-atoms on sil icon surfaces. The nuclear quadrupole interaction was used to determin e the electric field gradients (efg) acting at the 5/2(-) state in Se- 77, populated in the Br-77 decay. For the Si(100)2 x 1 surface two dif ferent field gradients have been observed with the parameters nu(Q1) = 583(5) MHz, eta(1) = 0.0(1) and nu(Q2) = 543(5) MHz, eta(2) = 0.40(3) . Typical values for the Si(111)7 x 7 surface from preliminary data ar e nu(Q) = 548(9) MHz, eta = 0.54(6). These electric field gradients ar e identified with adsorption sites of the selenium at dangling bonds o f the silicon surfaces.