CHEMISORPTION OF ISOLATED BR ATOMS ON SI(100)-(2X1) STUDIED BY PAC

Chemisorption and desorption of isolated bromine adatoms on the Si(100 )-(2 x 1) surface were investigated with nuclear methods. Br adsorptio n sites at low coverages of 10(-3) monolayers (ML) were characterised by measuring the nuclear quadrupole interaction with perturbed W-angul ar correlation (PAC) of Br-77 --> Se-77 probe atoms. Below room temper ature, two distinct adsorption sites for Br are revealed by PAC. One o f them disappears after isochronal annealing above 300 K. The more sta ble probe-atom state is associated with single Br atoms saturating a d angling bond of the surface, while the less stable state is attributed to adsorption of Br at a bridge site. The potential barrier between t he two adsorption sites is estimated to be 0.9(1) eV. At temperatures above 550 K, the fraction of atoms on distinct sites decreases, presum ably due to surface diffusion. By measuring the gamma-activity of the sample, complete desorption of the Br-77 atoms was observed above 620 K.