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Density functional theory calculations are used to study the initial-stage adsorption of O 2 molecules on the Si͑111͒-͑7 3 7͒ surface. Contrary to experimental suggestions, we find no evidence of metastable O 2 molecular states on this surface, i.e., O 2 molecules dissociate spontaneously without any barrier. Our electronic and vibrational analysis reveals that the resulting atomic-oxygen products can explain the "molecular" features reported in previous experiments. [ S0031-9007(99) PACS numbers: 68.35.Bs, 68.35.Ja, 73.20.At Chemisorption of oxygen molecules on silicon surfaces has been a subject of both fundamental and technological importance as a model for the initial stage of silicon oxidation. In most cases, however, the adsorption-induced chemical and geometrical complexity has greatly hindered experimental or theoretical identifications of the atomic structure of the reaction products, and thus many structural and spectroscopic issues still remain unresolved.Of particular interest among such examples is the socalled "molecular precursor" model for the initial-stage chemisorption of O 2 molecules on the Si͑111͒-͑7 3 7͒ surface, which was proposed to explain the observed oxygen-induced metastable features: In their vibrational electron energy loss spectroscopy (EELS) studies, Ibach et al. [1] and Edamoto et al.[2] found a loss peak at 149-155 meV that was extinguished by 400 K annealing. Later Höfer et al. [3,4] reported in their ultraviolet photoelectron spectroscopy (UPS) studies two metastable spectra at 2.1 and 3.9 eV which were also quenched by ϳ400 K annealing. These metastable features have been ascribed to chemisorbed O 2 states (i.e., molecular precursors), and the reported lifetime at room temperature varies from ϳ10 min in early studies [3,4] to several hours in recent studies [5][6][7]. The concept of O 2 molecular precursors on Si͑111͒-͑7 3 7͒ has been put forward in a more concrete but controversial way in recent scanning tunneling microscopy (STM) studies: Oxygeninduced bright sites in STM topographs are the most numerous product at low O 2 exposures and are known to be quite stable at room temperature and to survive heating up to 600 K [8][9][10][11][12][13][14]. These bright sites have usually been assigned to atomic-oxygen species [8][9][10][11][12]. Recently, however, Dujardin et al.[13] emphasized their molecular origin by identifying the bright sites with the metastable state that gives a 3.9 eV UPS peak. Furthermore, initiated by the Dujardin et al.'s molecular assignment, Hwang et al. [14] interpreted even the motion of the bright sites at 300 365 ± C as hopping of single O 2 molecules. Despite these experimental proposals, however, the structural identification of the O 2 molecular precursors is yet to be done. Previous theoretical studies [15][16][17][18] were based either on the unreconstructed Si surface structure or on a qualitative theoretical scheme, and so more elaborate theoretical studies are needed.In this Letter, we report an atomistic identification of the initial-sta...

The adsorption of water on the Si͑001͒ surface is studied by using density-functional total-energy calculations within the generalized gradient approximation. We find that water can adsorb molecularly on the down atom of the Si dimer, but a dissociative adsorption wherein OH ͑H͒ forms a bond to the down ͑up͒ atom of the Si dimer is more favored over the molecular adsorption ͑by 1.8 eV͒. The decay of the molecular state to the dissociative state occurs via a transition state with the energy barrier of only 0.15 eV. While the interaction between water molecules is repulsive, that between dissociated OH species is attractive by hydrogen bonding.

The purpose of this study is to determine whether organ sparing and target coverage can be simultaneously maintained for pencil beam scanning (PBS) proton therapy treatment of thoracic tumors in the presence of motion, stopping power uncertainties, and patient setup variations. Ten consecutive patients that were previously treated with proton therapy to 66.6/1.8 Gy (RBE) using double scattering (DS) were replanned with PBS. Minimum and maximum intensity images from 4D CT were used to introduce flexible smearing in the determination of the beam specific PTV (BSPTV). Datasets from eight 4D CT phases, using ±3% uncertainty in stopping power and ±3 mm uncertainty in patient setup in each direction, were used to create 8×12×10=960 PBS plans for the evaluation of 10 patients. Plans were normalized to provide identical coverage between DS and PBS. The average lung V20, V5, and mean doses were reduced from 29.0%, 35.0%, and 16.4 Gy with DS to 24.6%, 30.6%, and 14.1 Gy with PBS, respectively. The average heart V30 and V45 were reduced from 10.4% and 7.5% in DS to 8.1% and 5.4% for PBS, respectively. Furthermore, the maximum spinal cord, esophagus, and heart doses were decreased from 37.1 Gy, 71.7 Gy, and 69.2 Gy with DS to 31.3 Gy, 67.9 Gy, and 64.6 Gy with PBS. The conformity index (CI), homogeneity index (HI), and global maximal dose were improved from 3.2, 0.08, 77.4 Gy with DS to 2.8, 0.04, and 72.1 Gy with PBS. All differences are statistically significant, with p‐values <0.05, with the exception of the heart V45 (p=0.146). PBS with BSPTV achieves better organ sparing and improves target coverage using a repainting method for the treatment of thoracic tumors. Incorporating motion‐related uncertainties is essential.PACS number: 87.55.D