Room-temperature chemisorption and thermal evolution of 1,1-dichloroethylene and monochloroethylene on Si(111)7 x 7: formation of vinylidene and vinylene adspecies.

The room-temperature (RT) adsorption and thermal evolution of 1,1-dichloroethylene (1,1-C2H2Cl2 or iso-DCE) and monochloroethylene (C2H3Cl or MCE) on Si(111)7 x 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 iso-DCE dissociates via C-Cl bond breakage on the 7x7 surface to form mono-sigma-bonded 1-chlorovinyl (ClC=CH2) and/or di-sigma-bonded vinylidene (: C=CH(2)) adspecies. Upon annealing to 450 K, the 1-chlorovinyl adspecies undergoes further dechlorination to vinylidene adspecies, which may be converted to di-sigma-bonded vinylene (HC=CH) before dehydrogenating to hydrocarbon fragments above 580 K. TDS studies reveal both molecular desorption of iso-DCE near 350 K and C2H2 fragments near 700 K, and the presence of the latter confirms the existence of the di-sigma-bonded vinylene adspecies. Like the other chlorinated ethylene homologues, iso-DCE also exhibits TDS features of an etching product SiCl2 at 800-950 K and a dehydrochlorination product HCl at 700-900 K. Unlike iso-DCE, MCE is found to adsorb on the 7 x 7 surface predominantly through a [2 + 2] cycloaddition mechanism at RT, with similar di-sigma bonding structure as ethylene. The thermal evolution of MCE however follows that of iso-DCE, with the formation of vinylene above 580 K. Despite the lack of TDS feature attributable to HCl, weaker SiCl2 TDS feature could be observed at 800-950 K. For both iso-DCE and MCE, strong recombinative desorption of H2 is observed near 780 K. The differences in the Cl content among iso-DCE, MCE, and ethylene therefore play a key role in the RT chemisorption and thermally driven chemical processes on Si(111)7 x 7.