OBSERVATION OF STERIC CONFORMATIONAL EFFECTS IN HYDROCARBON ADSORPTION AND DECOMPOSITION - CIS-BUTENE-2 AND TRANS-BUTENE-2 ON SI(100)-(2X1)

Adsorption, desorption and decomposition of the isomeric hydrocarbons, cis- and trans-butene-2, on Si(100)-(2 X 1) have been studied by means of a kinetic uptake method, temperature programmed desorption (TPD) and Auger electron spectroscopy (AES). Both butene-2 molecules adsorb molecularly on Si(100)-(2 X 1) at 120 K with an initial sticking coefficient of near unity. Both olefin isomers show very similar dependence of the adsorption rate on the coverage with an initial region of constant rate followed by a decrease above a certain critical coverage. This kinetic behavior is typical for adsorption of unsaturated hydrocarbons on Si(100)-(2 X 1) at low temperature and is explained by a precursor mediated adsorption mechanism. Observation of the same adsorption kinetics for cis- and trans-butene-2 indicates that for the precursor mediated chemisorption process, the conformational difference between the isomers is of minor importance. The saturated coverages at 120 K are 1.4 X 10(14) molecules cm(-2) for trans-butene and 1.6 X 10(14) molecules cm(-2) for cis-butene. TPD experiments show a difference in the stability of the adsorbed layers. In the limit of low coverage the activation energies of desorption are similar to 34 and similar to 30 kcal/mol for trans- and cis-butene-2, respectively, assuming a first-order preexponential factor of 10(13) s(-1). Di-sigma bonding is suggested for the chemisorption state where the molecule preserves its cis- or trans-structure. Upon heating of saturated butene-2 layers, 25% of trans-butene and 13% of cis-butene undergo dissociation. Thus steric conformational effects are likely responsible for the different thermal stability of the layers with respect to both desorption and decomposition. In contrast to the role of adsorbate conformation on desorption and decomposition, the lack of a significant effect of hydrocarbon conformation on adsorption kinetics indicates that adsorption and desorption processes occur by independent pathways. It is postulated that the intermediate surface species responsible for dissociation is a di-sigma bonded 1,3 butane which involves coupling of one methyl group with the substrate.