Surface reactions of 2-iodopropane on GaAs(1 0 0)

The surface reactions of 2-iodopropane ((CH₃)₂CHI) on gallium-rich GaAs(1 0 0)-(4 × 1), was studied by temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). CH₃CHICH₃ adsorbs molecularly at 120 K but dissociates below room temperature to form chemisorbed 2-propyl ((CH₃)₂CH) and iodide (I) species. Thermal activation causes desorption of the molecular species at 240 K, and this occurs in competition with the further reactions of the (CH₃)₂CH and I chemisorbed species. Self-coupling of the (CH₃)₂CH results in the formation of 2,3-dimethylbutane ((CH₃)₂CH-CH(CH₃)₂) at 290 K. β-Hydride elimination in (CH₃)₂CH yields gaseous propene (CH₃CHCH₂) at 550 K while reductive elimination reactions of (CH₃)₂CH with surface hydrogen yields propane (CH₃CH₂CH₃) at 560 K. Recombinative desorption of the adsorbed hydrogen as H₂ also occurs at 560 K. We observe that the activation barrier to carbon-carbon bond formation with 2-propyls on GaAs(1 0 0) is much lower than that in our previous investigations involving ethyl and 1,1,1-trifluoroethyl species where the β-elimination process was more facile. The difference in the surface chemistry in the case of 2-propyl species is attributable to its rigid structure resulting from the bonding to the surface via the second carbon atom, which causes the methyl groups to be further away from the surface than in the case of linear ethyl and 1,1,1-trifluoroethyl species. The β-hydride and reductive elimination processes in the adsorbed 2-propyl species thus occurs at higher temperatures, and a consequence of this is that GaI desorption, which is expected to occur in the temperature range 550-560 K becomes suppressed, and the chemisorbed iodine leaves the surface as atomic iodine.