Isomeric effects on room-temperature chemisorption and thermal evolution of iso-, cis- and trans-dichloroethylene on Si(1 1 1)7 × 7

The room-temperature adsorption and thermal evolution of iso-, cis- and trans-dichloroethylene (DCE) on Si(1 1 1)7 × 7 have been studied by vibrational electron energy loss spectroscopy and thermal desorption spectrometry (TDS). The presence of the Si-Cl stretch at 510 cm−1 suggests that, upon adsorption, all three isomers dissociate via C-Cl bond breakage on the 7 × 7 surface to form mono-σ bonded chlorovinyl (HC˙CHClorClC˙CH2), which could, in the case of iso-DCE, further dechlorinate to vinylidene (:CCH2) upon insertion into the back-bond. The higher saturation exposure for the Si-Cl stretch at 510 cm−1 observed for cis- and trans-DCE than iso-DCE suggests that Cl dissociation via the CHCl group in the cis and trans isomers is less readily than the CCl2 group in iso-DCE. Our TDS data show remarkable similarities in both molecular desorption near 360 K and thermal evolution of the respective adstructures for all three isomers on Si(1 1 1)7 × 7. In particular, upon annealing to 450 K, the mono-σ bonded chlorovinyl adspecies is found to further dechlorinate to either vinylene (HC˙C˙H) di-σ bonded to the Si surface or acetylene to be released from the surface. Above 580 K, vinylene could also become gaseous acetylene or undergo H abstraction to produce hydrocarbon or SiC fragments. All three DCE isomers also exhibit TDS features attributable to an etching product SiCl2 at 800-950 K and recombinative desorption products HCl at 700-900 K and H2 at 650-820 K. The stronger Cl-derived TDS signals and Si-Cl stretch at 510 cm−1 over 450-820 K for trans-DCE than those for cis-DCE indicate stronger dechlorination for trans-DCE than cis-DCE, which could be due to less steric hindrance resulting from the formation of the chlorovinyl adspecies for trans-DCE during the initial adsorption/dechlorination process. Finally, our density functional calculations qualitatively support the thermodynamic feasibility and relative stabilities of the proposed adstructures involving chlorovinyl, vinylidene, and vinylene adspecies.