3-fold Sites Research Articles

Coadsorption of Gold with Hydrogen or Potassium on TiO2(110) Surface

We performed periodic DFT calculations to study the influence of H or K on the gold adsorption on TiO2(110) rutile at low coverage. On the clean surface, using a p(3 × 1) cell, we find that Au interacts preferentially with surface oxygen ions. The most favorable adsorption site is the 3-fold site with Au interacting with one bridging oxygen, O2c, and two tricoordinated oxygen, O3c followed by on-top site of O2c. Next, we have considered Au adsorption on the surface in presence of coadsorbed hydrogen or potassium using a p(3 × 1) and p(4 × 1) unit cell, respectively. The coadsorbed species adsorb as on the clean surface: H is on-top O2c and K bridges two O2c. On the contrary, on both surfaces, Au prefers to interact with surface titanium cation. This is the consequence of a formal change in the oxidation state of the two coadsorbed species. A redox process occurs between the two coadsorbed species that allows electron pairing: the supersystem is closed shell. Formally, Au−I adsorbs on the surface acidic si...

Chemisorption and Reactivity of CHx (x = 0−4) on Fe−Co Alloy Surfaces

The present work reports results from density functional theory studies of the methanation on the FeCo(110) surface for the surface coverage below 0.25 ML. Except for CH3 which favors the adsorption at the long-bridge Fe sites, all CHx (x = 0−2) species are chemisorbed preferentially at the long-bridge Co and 3-fold Fe sites. On the other hand, CH4 is only physisorbed on FeCo(110) with the tripod configuration and does not exhibit an apparent site preference. The effects of surface coverage on the adsorption energies are not significant when the surface concentration of CHx increases from 0.125 to 0.250 ML; but geometrically, the less compact p(√2 × √2)R45 is more favorable than the p(1 × 2) configuration. Among all possible C1 species, CH is identified to be the most dominant on the surface, and the resulting kinetic potential energy surface, which includes the lateral interactions between coadsorbed species, shows that the formation of CH4 is exothermic. These observations are in good agreement with the experimental observations that FeCo catalysts, which are immune to carbon deposition in the Fischer−Tropsch (FT) synthesis, possess similar FT catalytic properties to the conventional Co catalysts.

Density functional theory study of H 2 adsorption on the (1 0 0), (0 0 1) and (0 1 0) surfaces of Fe 3C

Spin-polarized density functional theory has been used to characterize hydrogen adsorption on the Fe 3C(1 0 0), Fe 3C(0 0 1) and Fe 3C(0 1 0) surfaces. It is found that hydrogen adsorbs dissociatively on the three surfaces. On the Fe 3C(1 0 0) surface, the most stable surface species is CH at 1/3 or 2/3 monolayer, and CH 4 at 1 monolayer; in contrast on the Fe 3C(0 0 1) surface the most stable surface intermediates are 3-fold H at 1/3 monolayer, and CH at 2/3 monolayer, and CH 4 at 1 monolayer. On the metallic Fe 3C(0 1 0) surface, hydrogen adsorbs at 2-fold or 3-fold sites. The computed energetic order of hydrogen adsorption is Fe 3C(1 0 0) > Fe 3C(0 1 0) > Fe 3C(0 0 1), and this differs from that of CO adsorption (Fe 3C(0 1 0) > Fe 3C(1 0 0) > Fe 3C(0 0 1)).

Plane-Wave DFT Investigations of the Adsorption, Diffusion, and Activation of CO on Kinked Fe(710) and Fe(310) Surfaces

The adsorption, diffusion, and dissociation properties of CO on kinked Fe(710) and Fe(310) surfaces have been analyzed using spin-polarized plane-wave density functional theory (DFT) calculations within the generalized gradient approximation (GGA). Several one-, two-, three- and 4-fold binding configurations have been identified among which the preferential adsorption takes place at a 4-fold hollow site near the top of the step while the one with the smallest activation energy for dissociation is located at a 4-fold site near the bottom of the step. In the case of the individual atomic species, the adsorption takes place preferentially at the hollow site for the C atom and at the pseudo 3-fold site on the step for the O atom. By the increase in coverage, there is an overall decrease of the adsorption energies for either molecular (CO) or atomic (C,O) species. The diffusion barriers among different local minima at the steps or on the terraces of both surfaces have been determined, and their values were fou...

Atomic and electronic structures of Tl/Si(1 1 1)- [formula omitted

Ab initio calculations, based on pseudopotentials and density functional theory, have been performed to investigate the atomic and electronic structure of the Tl/Si(1 1 1)- ( 3 × 3 ) surface. In the ( 3 × 3 ) reconstruction of surface, the two possible adatom geometries were considered: adsorption in 3-fold symmetric hollow sites (H 3 model) and in 4-fold atop sites (T 4 model). We have found that the energy difference between these two models is indeed very small within the energy of 0.03 eV. By calculating the electronic band structure for the T 4 configuration, we have also identified two occupied and one unoccupied surface state. We have also determined the origin of these surface states. Our results are seen to be in agreement with the recent angle-resolved photoelectron spectroscopy.

Mutation-Selection Models of Codon Substitution and Their Use to Estimate Selective Strengths on Codon Usage

Current models of codon substitution are formulated at the levels of nucleotide substitution and do not explicitly consider the separate effects of mutation and selection. They are thus incapable of inferring whether mutation or selection is responsible for evolution at silent sites. Here we implement a few population genetics models of codon substitution that explicitly consider mutation bias and natural selection at the DNA level. Selection on codon usage is modeled by introducing codon-fitness parameters, which together with mutation-bias parameters, predict optimal codon frequencies for the gene. The selective pressure may be for translational efficiency and accuracy or for fine-tuning translational kinetics to produce correct protein folding. We apply the models to compare mitochondrial and nuclear genes from several mammalian species. Model assumptions concerning codon usage are found to affect the estimation of sequence distances (such as the synonymous rate d(S), the nonsynonymous rate d(N), and the rate at the 4-fold degenerate sites d(4)), as found in previous studies, but the new models produced very similar estimates to some old ones. We also develop a likelihood ratio test to examine the null hypothesis that codon usage is due to mutation bias alone, not influenced by natural selection. Application of the test to the mammalian data led to rejection of the null hypothesis in most genes, suggesting that natural selection may be a driving force in the evolution of synonymous codon usage in mammals. Estimates of selection coefficients nevertheless suggest that selection on codon usage is weak and most mutations are nearly neutral. The sensitivity of the analysis on the assumed mutation model is discussed.

Electronic Effects in CO Chemisorption on Pt−Pb Intermetallic Surfaces: A Theoretical Study

The chemisorption of CO on surfaces of Pt-Pb intermetallic compounds, found to be useful as fuel cell electrocatalysts, was analyzed theoretically. Specifically, density-functional theory and extended Huckel- based calculations on CO adsorption on Pt(111), Pt3Pb(111), and PtPb(0001) surfaces are reported. Binding energies on Pt3Pb(111) are computed to be generally smaller than binding energies on Pt(111). The binding energies at the 2- and 3-fold sites on Pt 3Pb(111) increase if there is a Pb atom underneath the site in the second surface layer. The binding energies on PtPb(0001) are much higher than those on the other surfaces. These trends have been analyzed with crystal overlap Hamilton population (COHP)-based energy partitioning. The most stabilizing interaction in chemisorption is the Pt -adsorbate bond formation; the surface and the adsorbate are internally destabilized. The major surface effects are pretty much restricted to the top two layers. The binding energy trend for the top site chemisorption follows the Pt-adsorbate interaction term (most stabilizing interaction term in chemisorption). This surface Pt -adsorbate interaction term, for top site chemisorption, has been analyzed further with a Frontier molecular orbital formalism based on the extended Huckel calculations. Electron donation from Pb atoms to Pt atoms plays an important role in distinguishing chemisorption on these surfaces. The higher Fermi energy of the Pt-Pb intermetallic surfaces, relative to Pt(111) surface, leads to a weaker Pt-adsorbate interaction, which correlates well with the lower binding energy on Pt-Pb intermetallic surfaces when compared to Pt(111). The variation of the binding energy within the 2- and 3-fold sites on Pt3Pb(111) cannot be explained by the Pt-adsorbate interaction term alone. From a detailed COHP analysis of the surface and adsorbate, we find that the adsorption site affects the electron movements (transfer of electrons) in the surface slab upon chemisorption and through them the overall binding energy of the adsorbate. The difference in binding energies between the Pt3Pb(111) hcp and fcc sites can be explained this way.

Purifying Selection Maintains Highly Conserved Noncoding Sequences in Drosophila

The majority of metazoan genomes consist of nonprotein-coding regions, although the functional significance of most noncoding DNA sequences remains unknown. Highly conserved noncoding sequences (CNSs) have proven to be reliable indicators of functionally constrained sequences such as cis-regulatory elements and noncoding RNA genes. However, CNSs may arise from nonselective evolutionary processes such as genomic regions with extremely low mutation rates known as mutation "cold spots." Here we combine comparative genomic data from recently completed insect genome projects with population genetic data in Drosophila melanogaster to test predictions of the mutational cold spot model of CNS evolution in the genus Drosophila. We find that point mutations in intronic and intergenic CNSs exhibit a significant reduction in levels of divergence relative to levels of polymorphism, as well as a significant excess of rare derived alleles, compared with either the nonconserved spacer regions between CNSs or with 4-fold silent sites in coding regions. Controlling for the effects of purifying selection, we find no evidence of positive selection acting on Drosophila CNSs, although we do find evidence for the action of recurrent positive selection in the spacer regions between CNSs. We estimate that approximately 85% of sites in Drosophila CNSs are under constraint with selection coefficients (N(e)s) on the order of 10-100, and thus, the estimated strength and number of sites under purifying selection is greater for Drosophila CNSs relative to those in the human genome. These patterns of nonneutral molecular evolution are incompatible with the mutational cold spot hypothesis to explain the existence of CNSs in Drosophila and, coupled with similar findings in mammals, argue against the general likelihood that CNSs are generated by mutational cold spots in any metazoan genome.

Order–disorder transition in the complex lithium spinel Li 2CoTi 3O 8

Li 2CoTi 3O 8 has an ordered Li 2BB′ 3O 8 spinel structure, space group P4 332, at room temperature with 3:1 ordering of Ti and Li on the octahedral sites, and Li, Co disordered over the tetrahedral site. Rietveld refinement of variable temperature neutron powder diffraction data has shown an order–disorder phase transition in Li 2CoTi 3O 8 which commences at ∼500 °C with Li and Co mixing on the tetrahedral and 4-fold octahedral sites and is complete at a first order structural discontinuity at ∼915 °C. The fraction of Ti on the 12-fold octahedral site exhibits a small decrease with increasing temperature, which may suggest that the disordering involves all three cations. Above 930 °C, the structure, space group Fd3¯ m, has Li, Co and Ti sharing a single-octahedral site and Li, Co sharing a tetrahedral site, although Co still exhibits a preference for tetrahedral coordination. A labelling scheme for ordered and partially ordered 3:1 spinels is devised which focuses on the occupancy of the Li, B cations.

Pb Deposition on I-Coated Au(111). UHV-EC and EC-STM Studies

This article concerns the growth of an atomic layer of Pb on the Au(111)( radical3 x radical3)R30 degrees -I structure. The importance of this study lies in the use of Pb underpotential deposition (UPD) as a sacrificial layer in surface-limited redox replacement (SLRR). SLRR reactions are being applied in the formation of metal nanofilms via electrochemical atomic layer deposition (ALD). Pb UPD is a surface-limited reaction, and if it is placed in a solution of ions of a more noble metal, redox replacement can occur, but limited by the amount of Pb present. Pb UPD is a candidate for use as a sacrificial layer for replacement by any more noble element. It has been used by this group for both Cu and Pt nanofilm formation using electrochemical ALD. The I atom layer was intended to facilitate electrochemical annealing during nanofilm growth. Two distinctly different Pb atomic layer structures are reported, studied using in situ scanning tunneling microscopy (STM) with an electrochemical flow cell and ultrahigh vacuum surface analysis combined directly with electrochemical reactions (UHV-EC). Starting with the initial Au(111)( radical3 x radical3)R30 degrees -I, 1/3 monolayer of I on the Au(111) surface, Pb deposition began at approximately 0.1 V. The first Pb UPD structure was observed just below -0.2 V and displayed a (2 x radical3)-rect unit cell, for a structure composed of 1/4 monolayer each of Pb and I. The I atoms fit in Pb 4-fold sites, on the Au(111) surface. The structure was present in domains rotated by 120 degrees. Deposition to -0.4 V resulted in complete loss of the I atoms and formation of a Pb monolayer on the Au(111), which produced a Moiré pattern, due to the Pb and Au lattice mismatch. These structures represent two well-defined starting points for the growth of nanofilms of other more noble elements. It is apparent from these studies that the adsorption of I- on Pb is weak, and it will rinse away. If Pb is used as a sacrificial metal in an electrochemical ALD cycle and adsorbed I atoms are employed for electrochemical annealing, I atoms will need to be applied each cycle.

Metal Core Bonding Motifs of Monodisperse Icosahedral Au13 and Larger Au Monolayer-Protected Clusters As Revealed by X-ray Absorption Spectroscopy and Transmission Electron Microscopy

The atomic metal core structures of the subnanometer clusters Au13[PPh3]4[S(CH2)11CH3]2Cl2 (1) and Au13[PPh3]4[S(CH2)11CH3]4 (2) were characterized using advanced methods of electron microscopy and X-ray absorption spectroscopy. The number of gold atoms in the cores of these two clusters was determined quantitatively using high-angle annular dark field scanning transmission electron microscopy. Multiple-scattering-path analyses of extended X-ray absorption fine structure (EXAFS) spectra suggest that the Au metal cores of each of these complexes adopt an icosahedral structure with a relaxation of the icosahedral strain. Data from microscopy and spectroscopy studies extended to larger thiolate-protected gold clusters showing a broader distribution in nanoparticle core sizes (183 +/- 116 Au atoms) reveal a bulklike fcc structure. These results further support a model for the monolayer-protected clusters (MPCs) in which the thiolate ligands bond preferentially at 3-fold atomic sites on the nanoparticle surface, establishing an average composition for the MPC of Au180[S(CH2)11CH3]40. Results from EXAFS measurements of a gold(I) dodecanethiolate polymer are presented that offer an alternative explanation for observations in previous reports that were interpreted as indicating Au MPC structures consisting of a Au core, Au2S shell, and thiolate monolayer.

Density Function Theory Study of CO Adsorption on Fe3O4(111) Surface

Density functional theory calculations have been carried out for CO adsorption on the Fe(oct2)- and Fe(tet1)-terminated Fe(3)O(4)(111) surfaces, which are considered as active catalysts in water-gas shift reaction. It is found that the on-top configurations are most stable on these two surfaces. Some bridge configurations are also stable in which the new C-O bond formed between the surface O atom and the C atom of CO. The adsorption on the Fe(oct2)-terminated surface is more stable than on the Fe(tet1)-terminated surface. The density of state reveals the binding mechanism of CO adsorption on the two surfaces. Our calculations have also shown that the absorbed CO can migrate from the on-top site to the bridge site or 3-fold site. The oxidation of CO via surface oxygen atoms is feasible, which is in good agreement with experimental results.

A periodic pattern of mRNA secondary structure created by the genetic code

Single-stranded mRNA molecules form secondary structures through complementary self-interactions. Several hypotheses have been proposed on the relationship between the nucleotide sequence, encoded amino acid sequence and mRNA secondary structure. We performed the first transcriptome-wide in silico analysis of the human and mouse mRNA foldings and found a pronounced periodic pattern of nucleotide involvement in mRNA secondary structure. We show that this pattern is created by the structure of the genetic code, and the dinucleotide relative abundances are important for the maintenance of mRNA secondary structure. Although synonymous codon usage contributes to this pattern, it is intrinsic to the structure of the genetic code and manifests itself even in the absence of synonymous codon usage bias at the 4-fold degenerate sites. While all codon sites are important for the maintenance of mRNA secondary structure, degeneracy of the code allows regulation of stability and periodicity of mRNA secondary structure. We demonstrate that the third degenerate codon sites contribute most strongly to mRNA stability. These results convincingly support the hypothesis that redundancies in the genetic code allow transcripts to satisfy requirements for both protein structure and RNA structure. Our data show that selection may be operating on synonymous codons to maintain a more stable and ordered mRNA secondary structure, which is likely to be important for transcript stability and translation. We also demonstrate that functional domains of the mRNA [5′-untranslated region (5′-UTR), CDS and 3′-UTR] preferentially fold onto themselves, while the start codon and stop codon regions are characterized by relaxed secondary structures, which may facilitate initiation and termination of translation.

Theoretical DFT Study on the Interaction of NO and Br2 with the Pt(111) Surface.

Density functional calculations were performed at the B3LYP level using combined basis sets for the NO and bromine interactions with the Pt(111) surface mimicked by the two-layer Pt10 cluster model. It explains well an attractive bonding interaction not only for bromine and Pt(111) but also for all three adsorption modes of NO on the Pt(111) surface. In accordance with the experimental observations, the calculations predict that the first peak in the IR spectra appears at around 1515 cm(-)(1) at the initial stage of low NO coverage, while it would shift to 1707 cm(-)(1) at high NO coverage. The bonding of NO on the 3-fold hollow fcc and hcp sites of Pt(111) proceeds via predominant back-donation interactions, while for the on-top adsorption, both the donation and back-donation interactions become equally important. Energetic criteria show also that the STM tip (made from Pt and Ir alloys) immersed into a bromine solution may contain only dissociated bromine atoms that bind strongly with the surface Pt atoms. As a result, the νBrBr stretching vibration mode for the bromine molecule may not be seen in the IR spectra because of its dissociation into adsorbed atoms. This leads to an appearance of a blue shifted band centered at ca. 202 cm(-)(1).

Formation and adsorption properties of the bridging sulfur vacancies at the ( [formula omitted]) edge of Mo 27S (54− x) : A theoretical study

The structure and adsorption of Mo 27S (54− x) ( x = 1–6) clusters have been investigated using density functional theory method. It was found that considerable relaxation occurred at the ( 1 ¯ 0 1 0 ) edge. The activity has been analyzed on the basis of frontier molecular orbital properties. The results suggest that the possible catalytic sites might be situated between two vicinal 4-fold coordinative unsaturated site CUS. The formation of Mo 27S (54− x) ( x = 4–6) from Mo 27S 54 is easy in the presence of atomic hydrogen, but difficult under molecular hydrogen. But the situation is different from the case of Mo 27S (54− x) formation ( x = 1–3) that is facile under both the atomic and molecular hydrogen reagent (under the normal HYD/HDS condition). The adsorption of thiophene at various vacancies on the ( 1 ¯ 0 1 0 ) edge of MoS 2 represents that thiophene is unstable to flatly adsorb at the vacancy of Mo 27S 50. Although uprightly adsorbed thiophene is adsorbed weakly, the flatly adsorbed thiohene is strongly activated when adsorbed between two vicinal 4-fold CUS ( 9, 10). On the basis of Hirshfeld charge analysis, the donation and back-donation between the thiophene and the substrate might account for the activation of thiophene.

Photoluminescence Enhancement of PEG-Modified YAG:Ce3+ Nanocrystal Phosphor Prepared by Glycothermal Method

Y3Al5O12:Ce3+ (YAG:Ce3+) nanocrystals were synthesized in 1,4-butylene glycol (BG) with and without poly(ethylene glycol) (PEG) by the glycothermal method. The internal quantum efficiency of the photoluminescence (PL) corresponding to the 5d --> 4f transition of Ce3+ in the YAG:Ce3+ nanocrystal increased from 21.3 to 37.9% by addition of PEG, while no appreciable change in the primary particle size, the crystallite size, and the lattice distortion was recognized by transmission electron microscopy and X-ray diffractometry. The thermogravimetry-differential thermal analysis, Fourier transform infrared absorption spectroscopy and 1H --> 13C cross-polarization magic angle spinning nuclear magnetic resonance (CP-MAS NMR) confirmed the preferential coordination of PEG to the YAG:Ce3+ nanocrystal. 27Al single-pulse excitation MAS NMR reveals that the ratio of the 4-fold coordination site to the 6-fold coordination site increased from 0.53 to 0.72 by addition of PEG. We conclude that the surface modification of the YAG:Ce3+ nanocrystal by PEG induces the surface passivation, the prevention of the oxidation of Ce3+ to Ce4+, the promotion of the incorporation of Ce3+ into YAG and the local structural rearrangement, resulting in the PL enhancement.

Properties of the CdSe(0001), (0001̄), and (112̄0) Single Crystal Surfaces: Relaxation, Reconstruction, and Adatom and Admolecule Adsorption

Details of the chemical mechanism underlying the growth of colloidal semiconductor nanocrystals remain poorly understood. To provide insight into the subject, we have preformed a comprehensive study of the polar (0001) and (0001) and nonpolar (1120) wurtzite CdSe surfaces that are exposed during crystal growth using first-principles density functional theory (DFT-GGA) calculations. Stabilization of these surfaces by relaxation and reconstruction was considered. Two particular reconstructions of the polar surfaces were examined: vacancy formation on a 2 x 2 unit cell and addition of Se and Cd atoms on the (0001) and (0001) surfaces, respectively. Calculation results indicate that the (1120) is the most stable surface when compared to the two polar surfaces. Furthermore, reconstructions of the (0001) surface are energetically favored when compared to reconstructions of the (0001) facet. Adsorption of Cd and Se atoms and the CdSe molecule on the three relaxed surfaces and two reconstructed (0001) surfaces were also investigated. Several binding sites were considered to determine the most stable binding geometries and energetics. Atomic species preferentially bind in either 2-fold or 3-fold sites, while the CdSe molecule binds parallel to the surface on all of the considered surfaces. Vibrational frequencies of the adspecies were calculated for the most stable binding configurations and were included in the zero point energy correction. Diffusion barriers for the atomic and molecular species were estimated where possible to be between 0.2 and 0.4 eV on the three relaxed surfaces. Thermochemistry of the CdSe molecule binding and dissociation was also investigated. On all considered surfaces, dissociation is preferred to desorption with dissociation only exothermic on the (0001) surface. Comparison of the three relaxed and two reconstructed surfaces indicates that CdSe molecule binding and dissociation is thermodynamically favored on the (0001) surface. This implies that under a reaction-controlled regime, the rate of homoepitaxy would be faster on the (0001) Se terminated surface than on the (0001) and (1120) surfaces, making the (0001) surface of a nanocrystal the primary direction of growth.

Surface Structure and Energetics of Hydrogen Adsorption on the Fe(111) Surface

Spin-polarized density functional theory calculations have been performed to characterize the hydrogen adsorption and diffusion on the Fe(111) surface at 2/3-, 1-, and 2-monolayer (ML) coverages. It is found that the most favored adsorption site for atomic hydrogen (H) is the top-shallow bridge site (tsb), followed by the quasi 4-fold site (qff) with the energy difference of about 0.1 eV, while the top site (t) is not competitive. Furthermore, the adsorbed atomic hydrogen (H) has a high mobility, as indicated by the small diffusion barriers. The local density of state (LDOS) analysis reveals that the Fe-H (tsb or qff) bond involves mainly the Fe 4s and 4p and H 1s orbitals with less contribution of the Fe 3d orbital, while the Fe 4s, 4p, and 3d orbitals all participate in the Fe-H (top) bond. In addition, the coverage effects on the adsorption configurations and adsorption energies are addressed.

Adsorption, Vibration, and Diffusion of O Atoms on Rh Low-Index and (711) Stepped Defective Surfaces

The adsorption, vibration, and diffusion of O atoms on Rh(100), Rh(111), Rh(110), and Rh(711) surfaces were studied using the 5-parameter Morse potential (5-MP) of interaction between an adatom and a metal surface cluster. Our theoretical calculations provide information about adsorption sites, adsorption geometry, binding energy, and eigenvibration. Our results agreed very well with experimental results. Four major results follow. First, the theoretical calculation showed that on the Rh(100) surface the 4-fold hollow site is the only adsorption site. Second, on the O-Rh(111) system, the 3-fold hollow site is the stable adsorption site. Third, on the Rh(110) surface at low coverage, the O atom is adsorbed preferably on the pseudo-3-fold site, while with increasing coverage, the O atom is adsorbed not only on the pseudo-3-fold site but also on the long bridge site. Last, as for the Rh(711) stepped surface, the 3-fold site on the (111) step is metastable, whereas the 4-fold sites on the (100) terrace are stable, which enables the O atoms to diffuse easily from the 3-fold to the 4-fold site at low coverage. Therefore, the O atoms are adsorbed preferrably on the stable 4-fold sites of the (100) terrace and then later as coverage increases on the metastable 3-fold site of the (110) step.

Theoretical Study of Adsorption Site and State for Hydrogen Atom on Pd(311)

The adsorption of hydrogen atom on open, rough Pd(311) stepped surface is investigated by the five-parameter Morse potential (5-MP) method in detail. Calculated results demonstrate that the frequency of 62.37 meV (56 meV in a HREELS experiment) is attributed to the vibration parallel to the fcc-like 3-fold site along the [233] direction. The other calculated results are in good accord with HREELS experiments. Meanwhile, we also obtain the critical characteristics of the subsurface adsorption sites.