Using a first-principles pseudopotential technique, we have investigated the adsorption of C 2 H 2 on the Si͑001͒ surface. We have found that, at low temperatures, the di-bond configuration is the most stable structure from the energetic point of view. According to our calculations C 2 H 2 adsorbs preferentially on the alternate dimer sites, corresponding to a coverage of 0.5 monolayer. Our calculated surface band structure suggests that the end-bridge configuration, recently pointed out as a more favorable configuration by firstprinciples calculations, presents a metallic character and thus is Peierls unstable. The di-adsorbed system is characterized by symmetric and slightly elongated Si-Si dimers, and by a symmetric CC bond with length close to the double carbon bond length of the ethylene molecule. Our total-energy calculations suggest that other metastable configurations, like the 1,2-hydrogen transfer, the p bridge and the tetra-model are also possible. Available high-resolution electron-energy-loss spectroscopy experimental data are reinterpreted to support the existence of the tetra-model.

Effect of the cluster size in modeling the H 2 desorption and dissociative adsorption on Si(001)Using a first-principles pseudopotential method we have studied the adsorption and dissociation of NH 3 , PH 3 , and AsH 3 on the Si͑001͒-͑2ϫ1͒ surface. Apart from the existence of a barrier for the adsorption of the precursor state for arsine, we observe that the global behavior for the chemisorption of the XH 3 molecules considered in this work is as follows: the gas phase XH 3 adsorbs molecularly to the electrophilic surface Si atom and then dissociates into XH 2 and H, bonded to the electrophilic and nucleophilic surface silicon dimer atoms, respectively. The energy barrier, corresponding to a thermal activation, is much smaller than the usual growth temperature, indicating that all three molecules will be observed in their dissociated states at room temperature. All adsorbed systems are characterized by elongated Si-Si dimers that are ͑almost͒ symmetric in the dissociative case but asymmetric in the molecular case. According to our first-principles calculations, all XH 3 and XH 2 systems retain the pyramidal geometry observed for the gas molecules. Our calculated vibrational spectra further support the dissociative model for the XH 3 molecules considered here.

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