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

• Published in: Surface science 1999-03, Vol.424 (1), p.127-138
• Main Authors: Jones, Robert G., Fisher, C.J.
• Format: Article
• Language: English
• Subjects: Adsorption kinetics; Aluminium; Chemistry; Etching; Exact sciences and technology; General and physical chemistry; Halides; Iodine; Low index single crystal surfaces; Solid-gas interface; Sticking; Surface chemical reaction; Surface physical chemistry; Thermal desorption; Thermal desorption spectroscopy
• ISSN: 0039-6028; 1879-2758
• DOI: 10.1016/S0039-6028(99)00021-7

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 (a) I 2(gas)→I 2(phys) (b) I 2(phys)→I 2(gas) (c) 2Al(clean)+I 2(phys)→2Al–I(chem) (d) Al(subsurface)+Al–I(chem)+I 2(phys)→AlI 3(ads)+Al(clean) (e) AlI 3(ads)→AlI 3(gas) . 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 AlI 3(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 AlI 3, was ∼9×10 18 iodine atoms m −2 (∼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 AlI 3 as ∼57 kJ mol −1, reaction (e).