Sequential adjacent Si dimer dechlorination mechanism of perchloroethylene adsorption on Si(1 0 0) with temperature evolution

To elucidate the peak shifting of C-1s in X-ray photoelectron spectrum (XPS) of perchloroethylene (PCE) adsorption on Si(1 0 0), the sequential adjacent Si dimer dechlorination mechanism was proposed based on first principles calculations. The highly-symmetric Cl atoms of PCE induce three possible initial di-dechlorination processes occurring on intra, inter-dimer and iso intra-dimer, respectively, to yield three tetra-σ states. These tetra-σ states are identified to coexist at room temperature (RT) due to the relatively low reaction barriers (<0.59 eV). However, their further di-dechlorination to form intra and inter-dimer hexa-σ states requires much higher activation barriers (>1.08 eV), which leads to hexa-σ states only exist at elevated temperatures although they are found to be the most stable in terms of energetics. The calculated ionization energies (IEs) of C-1s core electron and vibrational frequencies of various potential adspecies are well consistent with the experimental data observed by XPS and vibrational electron energy loss spectroscopy (EELS), which further corroborates the sequential dechlorination processes of PCE on Si(1 0 0).