A THEORETICAL-STUDY OF THE STRUCTURES OF PROPYLENE ADSORBED ON THE SI(100)-(2X1) SURFACE

The adsorption of propylene, C3H6, on the Si(100)-(2 x 1) surface is e xamined. The adsorbed structure is obtained from minimising the total energy with respect to variations in the atomic coordinates of the ads orbed molecule and the first four silicon layers of the underlying sub strate. Two similar values for a local minimum in the total energy, ea ch corresponding to a different adsorbed structure, are found by this process. One structure maintains both the C3H6 molecule and the underl ying Si dimer, while the other structure is formed from the breaking o f one C-H bond and the underlying Si dimer. These two structures corre late with the experimentally observed characteristics of adsorbed C3H6 on Si(100) where 35% of the propylene can be desorbed on heating and the remaining 65% is dissociated. The two structures of adsorbed C3H6 can be compared with the previously determined structures of C2H2, C2H 4 and C6H6 on Si(100) to give a more general description of the struct ures derived from the adsorption of unsaturated hydrocarbons on the Si (100) surface.