Reaction and sticking probabilities using line of sight techniques: iodine on Al(111)

A technique, line of sight sticking probability (LOSSP) measurements, is described and applied to the reaction of iodine with Al(111) in ultra-high vacuum. The technique measures sticking probabilities for species adsorbed from a thermally randomised gas above a surface, by detecting a narrow beam of particles which has been reflected from the surface. The equipment comprises a cryoshield, fitted with a small aperture, surrounding a mass spectrometer such that only those species originating from a small patch on the sample surface, and flying by direct line of sight to the ionisation region of the mass spectrometer, are detected. Iodine reacts with Al(111) by the following reactions [GRAPHICS] The initial sticking probability of I-2 to form chemisorbed iodine, reactions (a)-(c), was found to be S-0=0.8+/-0.1 at 300 K. Under steady state etching conditions at 300 K the overall reaction probability of I-2 to form AlI3(gas) was R-ss = 0.36 +/- 0.07, while the surface iodine coverage, consisting of a majority of chemisorbed iodine and a minority of adsorbed AlI3, was similar to 9 x 10(18) iodine atoms m(-2) (similar to 0.6 ML). The sticking probability of I-2 to solid iodine at 103 K, reaction (a), was measured as S-phys=0.98+/-0.02, while the sticking probability on the halogenated surface at 300 K was measured as S-phys>0.8+/-0.1. Variable temperature experiments yielded the activation energy for desorption of AlI3 as similar to 57 kJ mol(-1), reaction (e). (C) 1999 Published by Elsevier Science B.V. All rights reserved.