DESORPTION-KINETICS AND ADLAYER STICKING MODEL OF N-BUTANE, N-HEXANE,AND N-OCTANE ON AL2O3(0001)

Temperature-programmed desorption (TPD) was used to investigate the de sorption of butane, hexane, and octane from Al2O3(0001) in ultrahigh v acuum. At low coverages, TPD traces for butane and hexane displayed on e peak which was attributed to monolayer desorption. A second, multila yer peak, was observed at a lower temperature as the coverage was incr eased. However, the multilayer peak appeared at coverages well below t he saturation coverage of the monolayer peak implying that the multila yer was forming before the monolayer was completely full. A simple sta tistical adlayer sticking model was used fo simulate the relative numb er of molecules in the monolayer and multilayer as a function of total coverage giving good agreement with the TPD data. In addition, the va riation of ramp rates method was used to measure the desorption kineti cs at coverages well below one monolayer for each of the alkanes. All three alkanes displayed first-order desorption kinetics with activatio n barriers of butane E(d) = 8.4 +/- 1.2 kcal/mol; hexane E(d) = 10.4 /- 0.8 kcal/mol; octane E(d) = 14.6 +/- 0.8 kcal/mol. The first-order preexponentials were butane v(1) = 4 x 10(10+/-2) s(-1); hexane v(1) = 5.4 x 10(9+/-1.5) 5 s(-1); octane v(1) = 1.6 x 10(12+/-2) s(-1). The comparison of these desorption barriers to the bulk heats of sublimati on along with the separation between monolayer and multilayer peaks in the TPD as a function of chain length suggest that the relative magni tude of molecule-surface interactions compared to molecule-molecule in teractions decreases with increased chain length.