Adsorption-desorption properties and surface structural chemistry of bromine on clean and sodium-dosed Ag(111)

Abstract

The chemisorption of bromine and its coadsorption with sodium on Ag(111) have been studied by LEED, Auger spectroscopy, thermal desorption, and surface potential (Δφ) measurements. Adsorption is rapid at 300 K, the sticking probability remaining virtually constant at 0.75 ± 0.25 right up to the saturation coverage. Two ordered phases are formed during bromine uptake: a (√3 × √3)R30° structure being followed by a (3 × 3) structure which corresponds to the saturated overlayer. The Br-Br nearest-neighbour spacing at this point is 4.34 Å. ΔΦ measurements indicate that significant depolarisation of the adlayer occurs at higher coverages, and the value of ΔΦ(max) is +1.4 eV. Desorption occurs exclusively as atomic Br, the estimated low-coverage desorption energy being ~234 kJ mol −1. In the presence of surface Na, bromine adsorption again leads to the formation of √3 and (3 × 3) structures; however, much smaller Br 2 doses are called for than in the case of the clean surface. Desorption of NaBr occurs from the mixed adlayer with zero-order kinetics, and this is the lowest energy channel for loss of both Br and Na. The work function changes induced by mixed adsorption are uniformly small, and (in contrast with simple Br overlayers) electron impact desorption of Br becomes significant. The kinetic, electron impact, and structural data are interpreted in terms of the growth of two dimensional islands of NaBr “ionic crystal”, the structure of which corresponds to slightly distorted (111) layers of bulk crystalline NaBr.