DESORPTION-KINETICS AND ADLAYER STRUCTURE OF N-PENTANE ON AL2O3(0001)

Temperature-programmed desorption (TPD) and Fourier transform infrared spectroscopy (FTIR) were used to investigate the desorption kinetics and adlayer structure of n-pentane adsorbed on Al2O3(0001) in ultrahigh vacuum. At low coverages, TPD traces displayed one peak that saturated with increasing exposure time and was attributed to monolayer desorption. A second, multilayer peak was observed at a lower temperature for coverages exceeding 1 ML. At coverages below one monolayer, pentane was found to display first-order desorption kinetics with an activation barrier for desorption of E(d) = 13.3+/-0.8 kcal/mol and a first-order preexponential of nu1 = 3 x 10(20+/-0.5) s-1. The multilayer displayed zero-order desorption kinetics with an activation barrier of E(d) = 9.2+/-0.6 kcal/mol and a zero-order preexponential of nu0 = 1 X 10(30+/-0.3) Molecules/(cm2 S). FTIR spectra revealed that pentane molecules orient primarily parallel to the surface at submonolayer coverages and perpendicular to the surface at multilayer coverages. Independent coverage measurements using laser interference techniques to calibrate infrared absorption cross sections revealed that the surface number density at one monolayer was (2.8+/-0.5) X 10(14) molecules/CM2, which is in agreement with molecules laying flat separated by their van der Waals radii.