A leed and AES study of the adsorption of chlorine on W(100) at room temperature

The surface structures formed on room temperature adsorption of chlorine on W(100) and subsequent annealing of the saturated surface have been characterised by LEED. The progress of gas adsorption was followed by AES which was also used to observe relative chlorine coverage on annealing. Room temperature adsorption was random up to the saturation exposure of 1.7 L. On annealing the chlorine adlayer ordering commenced at about 800 K. One-dimensional ordering into rows along the <1, 1> directions was followed by the ordering of these into two 2D structures: an interpenetrating $\text{7}\text{1}\text{1}\text{1}$ at 830 K and an interpenetrating $\text{5}\text{1}\text{6}\text{1}$ for 860 K and above. Desorption started after 1050 K annealing and was complete by 1440 K. Saturation chlorine coverage is inferred as 5 × 10¹⁴ atoms cm^?2 and the single desorption peak coupled with the LEED analysis suggests that chlorine is bridge bonded to the W(100) surface for the ordered overlayer.     

Characterization of oxygen,carbon, and sulfur adlayers on W(211)

Adlayers of oxygen, carbon, and sulfur on W(211) have been characterized by LEED, AES, TPD, and CO adsorption. Oxygen initially adsorbs on the W(211) surface forming p(2 × 1)O and p(1 × 1)O structures. Atomic oxygen is the only desorption product from these surfaces. This initial adsorption selectively inhibits CO dissociation in the CO(β₁) state. Increased oxidation leads to a p(1 × 1)O structure which totally inhibits CO dissociation. Volatile metal oxides desorb from the p(1 × 1)O surface at 1850 K. Oxidation of W(211) at 1200 K leads to reconstruction of the surface and formation of p(1 × n)O LEED patterns, 3 ? n ? 7. The reconstructed surface also inhibits CO dissociation and volatile metal oxides are observed to desorb at 1700 K, as well as at 1850 K. Carburization of the W(211) surface below 1000 K produced no ordered structures. Above 1000 K carburization produces a c(6 × 4)C which is suggested to result from a hexagonal tungsten carbide overlayer. CO dissociation is inhibited on the W(211)?c(6×4)C surface. Sulfur initially orders into a c(2 × 2)S structure on W(211). Increased coverage leads to a c(2×6)S structure and then a complex structure. Adsorbed sulfur reduces CO dissociation on W(211), but even at the highest sulfur coverages CO dissociation was observed. Sulfur was found to desorb as atomic S at 1850 K for sulfur coverages less than $\text{7}\text{6}$ monolayers. At higher sulfur coverages the dimer, S₂, was observed to desorb at 1700 K in addition to atomic sulfur desorption.     

Adsorption of CO on clean and oxygen covered Ni(111) surfaces

Adsorption of CO on Ni(111) surfaces was studied by means of LEED, UPS and thermal desorption spectroscopy. On an initially clean surface adsorbed CO forms a √3 × $\text{√3}\text{R30°}$ structure at θ = 0.33 whose unit cell is continuously compressed with increasing coverage leading to a c4 × 2-structure at θ = 0.5. Beyond this coverage a more weakly bound phase characterized by a $\text{√7}\text{2}$ × $\text{√7}\text{2R19°}$ LEED pattern is formed which is interpreted with a hexagonal close-packed arrangement (θ = 0.57) where all CO molecules are either in “bridge” or in single-site positions with a mutual distance of 3.3 Å. If CO is adsorbed on a surface precovered by oxygen (exhibiting an O 2 × 2 structure) a partially disordered coadsorbate 2 × 2 structure with θ_o = θ_co = 0.25 is formed where the CO adsorption energy is lowered by about 4 kcal/mole due to repulsive interactions. In this case the photoemission spectrum exhibits not a simple superposition of the features arising from the single-component adsorbates (i.e. maxima at 5.5 eV below the Fermi level with O_ad, and at 7.8 (5σ + 1π) and 10.6 eV (4σ) with CO_ad, respectively), but the peak derived from the CO 4σ level is shifted by about 0.3 eV towards higher ionization energies.     

Adsorption of oxygen on Pt(111)

Oxygen adsorbed on Pt(111) has been studied by means of temperature programmed thermal desorption spectroscopy (TPDS). high resolution electron energy loss spectroscopy (EELS) and LEED. At about 100 K oxygen is found to be adsorbed in a molecular form with the axis of the molecule parallel to the surface as a peroxo-like species, that is, the OO bond order is about 1. At saturation coverage (θ_mol= 0.44) a $\text{(}\text{3}\text{2}\text{×}\text{3}\text{2}\text{)R15°}$ diffraction pattern is observed. The sticking probability S at 100 K as a function of coverage passes through a maximum at θ = 0.11 with S = 0.68. The shape of the coverage dependence is characteristic for adsorption in islands. Two coexisting types of adsorbed oxygen molecules with different OO stretching vibrations are distinguished. At higher coverages units with v-OO = 875 cm^?1 are dominant. With decreasing oxygen coverages the concentration of a type with v-OO = 700 cm^?1 is increased. The dissociation energy of the OO bond in the speices with v-OO = 875 cm^?1 is estimated from the frequency shift of the first overtone to be ~ 0.5 eV. When the sample is annealed oxygen partially desorbs at ~ 160K, partially dissociates and orders into a p(2×2) overlayer. Below saturation coverage of molecular oxygen, dissociation takes place already at92 K. Atomically adsorbed oxygen occupies threefold hollow sites, with a fundamental stretching frequency of 480 cm^?1. In the non-fundamental spectrum of atomic oxygen the overtone of the E-type vibration is observed, which is “dipole forbidden” as a fundamental in EELS.     

Chlorine adsorption on copper: II. Photoemission from Cu(001)c(2 × 2)-C1 and Cu(111)(√3 × √3)R30°-Cl

We have studied high-resolution angle-resolved and photon-polarization dependent photoemission from chlorine adsorbed on Cu(OOl) and Cu(111). Chlorine forms a c(2 × 2) saturation overlayer on Cu(OO1) and adsorbs dissociatively as revealed by LEED and XPS. Several two-dimensional energy bands on Cu(001)c(2 × 2)-Cl could be iden along the $\text{}\text{?}$gG M? and $\text{}\text{?}$gG M? lines of the surface Brillouin zone, their respective mirror symmetry and their orbital character could be determined. An interpretation of these bands is given in terms of the interaction of the ordered overlayers with particular substrate bulk bands. Besides the appearance of adsorbate-induced two-dimensional bands drastic changes are resolved in the substrate d-band emission region. These can be explained almost exclusively by surface umklapp processes involving reciprocal lattice vectors of the ordered adsorbate mesh. Supplementary studies of the Cu(111) (√3 × √3)R30°-Cl system support our ideas. We discuss some important implications of our results for the interpretation of angle-resolved photoenussion spectra from ordered adsorbate layers.     

Halogen adsorption on Fe(100): I. The adsorption of Cl2 studied by AES,LEED, work function change and thermal desorption

The initial sticking probability of chlorine on Fe(100) at room temperature is calculated to be 0.13, and there is evidence to suggest that the chlorine adsorbs into a short lived mobile precursor state above the surface. The work function change, Δφ, is proportional to coverage and reaches a maximum value of 1.43 eV at saturation. At this coverage a c(2 × 4) LEED pattern is formed. On heating, chlorine is lost from the surface, but the mechanism is such that no detectable loss is incurred at a constant elevated temperature. The c(2 × 4) pattern is shown to be a coincidence structure formed from a $\text{(}\text{1}\text{2}\text{3}\text{?1}\text{2}\text{3}\text{)}$ net of chlorine atoms on the Fe(100) substrate. This structure is a special case of the more general $\text{(}\text{1}\text{2}\text{tan}\text{α}\text{?1}\text{2}\text{tan}\text{α}\text{)}$ structure formed at lower concentrations of chlorine. The c(2 × 4) is formed when α = 56.31°, which gives the chlorine atoms a hard sphere diameter of 0.345 nm and a concentration of 0.75 atoms per four-fold site.     

The chemisorption of carbon monoxide on a Co(0001) single crystal surface; studied by LEED,UPS, EELS,AES and work function measurements

The chemisorption of CO on Co(0001) has been investigated by LEED, UPS, EELS, Auger and sp measurements. CO is molecularly adsorbed on Co(0001) in the investigated temperature range from 100 to 450 K. This is deduced from the UPS and EELS results and the reversibility of the sp and LEED data. The isosteric heat of adsorption has a constant value of 128 kJ/mol up to a coverage of $\text{1}\text{3}$ and drops then to about 96 kJ/mol. This coincides with the completion of a (√3 × √3)R30° overlayer structure and the formation of a (2√3 × 2√3)R30° CO overlayer which is fully developed at 100 K.     

Angle-resolved photoemission of the c(2 × 2) and c(3 × 1) oxygen overlayers on Fe(110)

Angle-resolved photoemission spectroscopy utilizing synchrotron radiation has been used to study the band structure of the c(2×2) and (3×1) oxygen overlayers on Fe(110). The symmetries of the O-2p-derived states at the center of the surface Brillouin zone $\text{(}\text{Γ}\text{)}$ were identified using polarized light. At $\text{Γ}$ the p_xp_y- and p_z-derived levels are at about 5.5 and 7.0 eV below the Fermi level, respectively, for both ordered overlayers. The p-states of the c(2×2)-O structure show very little dispersion (?0.1 eV) with $\text{k}{}_{\mathrm{}}$. On the other hand, the c(3×1)-O overlayer exhibits considerable dispersion of ～1.6 eV. The essential features of the measured dispersion are reproduced well by the dispersion predicted in a qualitative way for an isolated c(3×1) oxygen monolayer.     

Hydrogen-induced reconstruction on a high step density W(001) surface

The hydrogen-induced reconstruction on a high step density W(001) crystal, ($\text{2}\text{×}\text{2}$)R45°-H, with steps oriented parallel to the [110] and ～ 28 Å average terrace width has been investigated using LEED symmetry, beam shape analyses, and EELS. The symmetry of the LEED pattern is observed to change from p2mg for the ($\text{2}\text{×}\text{2}$)R45° clean surface reconstruction to c2mm for the commensurate phase ($\text{2}\text{×}\text{2}$)R45°-H reconstruction. Correspondingly, the shapes of the half-order beams indicate that the hydrogen-induced reconstruction domains are much less elongated than the clean surface domains. A splitting of each half-order beam into four beams at higher exposures indicates the existence of two domains of the incommensurate phase. A commensurate phase v₁ vibrational loss peak centered at 160 meV in the EELS spectrum broadens on the low-energy side during the incommensurate phase and then shifts toward 130 meV and narrows as the (1×1)-H saturation structure develops. These observations imply that there is no long-range inhibition ( ～ 20 Å) to the formation of either commensurate or incommensurate phase; hydrogen induces a switching of the atomic displacements from 〈110〉 directions on a clean surface to 〈100〉 directions, even with steps oriented parallel to the [110]; and in the incommensurate phase there is a distribution of hydrogen site geometries with the most probable geometry more like the commensurate phase geometry than the saturation phase geometry.     

The condensation of gold onto tantalum (100) single crystal surfaces: LEED,AES analysis

The condensation of gold onto clean and contaminated, single crystal, tantalum (100) surfaces has been followed by using LEED and AES. On a contaminated surface gold condenses as crystallites in a (211) surface orientation with some degree of preferred, azimuthal orientation. On a clean surface gold condenses in an ordered overlayer. Up to approximately $\text{3}\text{4}$ monolayer the structure conforms to the (1 × 1) tantalum surface. Beyond this, the observed LEED structure may be interpreted initially in terms of a TaAu overlayer made up of 90° rotated domains with (001)TaAu//(100)Ta and 〈 10 〉 TaAu// 〈 11 〉 Ta, and then in terms of a gold overlayer in a “distorted (111)” orientation. Annealing of these gold films always results in the formation of a (1 × 1) TaAu overlayer of small crystallite size.     

The adsorption of acetylene on W(100) at room temperature: An electron spectroscopic study

The adsorption of acetylene on W(100) at room temperature has been studied by AES, ELS, thermal desorption, mass spectrometry, work function and LEED in one vacuum chamber. AES line profile analysis shows that there are at least two adsorption processes occurring at room temperature. Further, it is possible to explain all the AES results by assuming non-sequential adsorption into just two states, denoted by α and β. This picture was substantiated and embellished by comparison with other standard surface techniques. The α-state comprises either a C₂H₂ unit with an activation energy for desorption of $\text{2.3}\text{eV}\text{molecule}\text{(53}\text{kcal mole}{}^{\mathrm{?1}}\text{)}$ or CH units bounded through the carbon of the β-state. Saturation coverage for the α-state is 3 × 10¹⁴ molecules cm^?2. The β-state is dissociative at low acetylene exposures and comparison between a carbon covered surface and the β-state suggest the latter to be dissociative up to saturation. There also appears to be ca. 10¹⁴ hydrogen atoms cm^?2 on W(100) on room temperature acetylene saturation, the carbon content of the β-state being 9 × 10¹⁴ atoms cm^?2. The residual C?C bond from the molecule in the β-state remains unknown. No sign of ordering in the adsorbed species was detected, save the possibility of (1 × 1) in the β-state. Acetylene adsorption at 580 K showed hydrogen from the β-state to block acetylene adsorption by 15% at saturation. A two-site adsorption model for the β-state is proposed to explain the results. The α-state is bonded through the carbon of the β-state and it is speculated that the former adsorbs onto “β” domains where there is a critical minimum size for the latter.     

Angular resolved UPS of the c(2×2) overlayer of selenium on Ni (001) using polarised and unpolarised HeI radiation

The c(2×2) Se overlayer on Ni(001) was studied by angular resolved UPS using unpolarised and polarised HeI (21.2 eV) radiation. The polariser was designed in such a way that s-, p- and unpolarised light could be used without breaking vacuum. A good correlation between the photoelectron spectra and the surface Brillouin zone of the c(2×2) Se overlayer was found. The p_z- and the degenerate p_x-, p_y-derived Se levels are well separated at the M? point lying at 3.8 eV(p_z and 5.2 eV (p_x, p_y), respectively. The bands show dispersion and cross between M and $\text{Г}$ so that at $\text{Г}$ the p_x-, p_y -derived levels lie closer to the Fermi edge.     

Displacive surface phases formed by hydrogen chemisorption on W {001}

A detailed LEED study is reported of the surface phases stabilised by hydrogen chemisorption on W {001}, over the temperature range 170 to 400 K, correlated with absolute determinations of surface coverages and sticking probabilities. The saturation coverage at 300 K is 19(± 3) × 10¹⁴ atoms cm^?2, corresponding to a surface stoichiometry of WH₂, and the initial sticking probability for both H₂ and D₂ is 0.60 ± 0.03, independent of substrate temperature down to 170 K. Over the range 170 to 300 K six coverage-dependent temperature-independent phases are identified, and the transition coverages determined. As with the clean surface $\text{(}\text{2}\text{×}\text{2}\text{)R45°}$ displacive phase, the c(2 × 2)-H phase is inhibited by the presence of steps and impurities over large distances (～20 Å), again strongly indicative of CDW-PLD mechanisms for the formation of the H-stabilised phases. These phases are significantly more temperature stable than the clean $\text{(}\text{2}\text{×}\text{2}\text{)R45°}$, the most stable being a c(2 × 2)-H split half-order phase which is formed at domain stoichiometries between WH_0.3 and WH_0.5. LEED symmetry analysis, the dependence of half-order intensity and half-width on coverage, and I-V spectra indicate that the c(2 × 2)-H phase is a different displacive structure from that determined by Debe and King for the clean $\text{(}\text{2}\text{×}\text{2}\text{)R45°}$. LEED I-V spectra are consistent with an expansion of the surface-bulk interlayer spacing from 1.48 to 1.51 Å as the hydrogen coverage increases to ～4 × 10¹⁴ atoms cm^?2. The transition from the split half-order to a streaked half-order phase is found to be correlated with changes in a range of other physical properties previously reported for this system. As the surface stoichiometry increases from WH to WH₂ a gradual transition occurs between a phase devoid of long-range order to well-ordered (1 × 1)-H. Displacive structures are proposed for the various phases formed, based on the hypothesis that at any coverage the most stable phase is determined by the gain in stability produced by a combination of chemical bonding to form a local surface complex and electron-phonon coupling to produce a periodic lattice distortion. The sequence of commensurate, incommensurate and disordered structures are consistent with the wealth of data now available for this system. Finally, a simple structural model is suggested for the peak-splitting observed in desorption spectra.     

Superlattices formed by interaction of hydrogen bromide and hydrogen chloride with Pt(111) and Pt(100) studied by LEED,Auger and thermal desorption mass spectroscopy

HBr and HCl react with Pt(111) and Pt(100) surfaces to form adsorbed layers consisting of specific mixtures of halogen atoms and hydrogen halide molecules. Exposure of Pt(111) to HBr yielded a (3×3) LEED pattern beginning at $\text{Θ}{}_{\mathrm{Br}}\text{=}\text{2}\text{9}$ and persisting at the maximum coverage which consisted of $\text{Θ}{}_{\mathrm{<mtext>Br</mtext>}}\text{=}\text{1}\text{3}$ plus $\text{Θ}{}_{\mathrm{<mtext>HBr</mtext>}}\text{=}\text{1}\text{9}$. The most probable structure at maximum coverage, Pt(111)[c(3 × 3)]-(3 Br + HBr), nas a rhombic unit cell encompassing nine surface Pt atoms, and containing three Br atoms and one HBr molecule. On Pt(100) the structure at maximum coverage appears to be Pt(100)[c(2√2 × √2)]R45°-(Br + HBr), $\text{Θ}{}_{\mathrm{<mtext>Br</mtext>}}\text{= Θ}{}_{\mathrm{<mtext>HBr</mtext>}}\text{=}\text{1}\text{4}$; the rectangular unit cell involves four Pt atoms, one Br atom and one HBr molecule. Each of these structures consists of an hexagonal array of adsorbed atoms or molecules, excepting slight distortion for best fit with the substrate in the case of Pt(100). Treatment of Pt(100) with HCl produced a diffuse Pt(100)(2 × 2)-(Cl + HCl) structure at the maximum coverage of Θ_Cl = 0.13, Θ_HCl = 0.11. Exposure of Pt(111) to HCl produced a disordered overlayer. Thermal desorption, Auger spectroscopy and mass spectroscopy provided coverage data. Thermal desorption data reveal prominent rate maxima associated with the structural transitions observed by LEED. Br and HBr, Cl and HCl were the predominant thermal desorption products.     

A leed-aes study of the structure of sulfur monolayers on the Mo(100) crystal face

The formation of ordered phases of sulfur on the molybdenum (100) crystal face has been studied by Low Energy Electron Diffraction (LEED), Auger Electron Spectroscopy (AES) and Thermal Desorption Spectroscopies (TDS). Sulfur was deposited from a S₂ molecular flux streaming out of an Ag₂S containing electrochemical cell inside the UHV chamber. The use of a controlled flux of S₂ allowed the careful determination of saturation values for the monolayer, as well as the formation of multilayers of sulfur. This allowed the calibration of Auger intensities in terms of sulfur coverage. Various ordered structures, c(2 × 2), (1 × 2), $\text{2}\text{1}\text{?}\text{1}\text{1}$ and c(2 × 4), were observed by LEED for different values of the S coverage. Real space models for these structures are proposed that satisfy the coverage values observed and place sulfur atoms only on high symmetry four-fold sites on the (100) molybdenum surface.     

Adsorption of ethylene on the Pt(100) surface

Clean Pt(100) surfaces with bulk-like 1×1 structure, or the stable, reconstructed 5×20 structure and held at 200 or 330 K were exposed to ethylene. Ultraviolet photoemission spectroscopy identified the nature of the adsorbed species which depends on the structure and temperature of the clean surface and the amount adsorbed. It is ethylene on the 5 × 20 structure at 200 K, a vinyl radical on the same surface at 300 K up to half a monolayer, the remainder being added as acetylene; it is acetylene on the 1 × 1 surface at 330 K and a mixture of acetylene, vinyl and ethylene on the 1 × 1 surface at 200 K. Whatever the nature of the adsorbate, the surface coverage θ increased with exposure ? as $\text{(1 ? θ = C?}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>3</mtext>}}\text{)}$. By contrast, on a surface covered with any C₂ hydrocarbon acetylene adsorbs with Langmuir kinetics. The kinetics are explained in terms of the relationship between the attraction an approaching molecule experiences from the bare surface and its Van der Waals repulsion from preadsorbed molecules.     

Ionic transport in the lithium nitride bromides,Li6NBr3 and Li1 3N4Br

The ionic and electronic conductivities of the lithium nitride bromides Li₆NBr₃ and Li_{1 3}N₄Br have been studied in the temperature range from 50 to 220°C and 120 to 450°C, respectively. Both compounds are practically pure lithium ion conductors with negligible electronic contribution. Li₆NBr₃ has an ionic conductivity Ω of 2 × 10^-6Ω^-1cm^-1 at 100°C and an activation enthalpy for σT of 0.46 eV. Li_{1 3}N₄Br shows a phase transition at about 230°C. The activation enthalpy for σT is 0.73 eV below and 0.47 eV above this temperature. The conductivities at 150 and 300°C were found to be 3.5 × 10^-6 Ω^-1cm^-1 and 1.4 × 10^-3Ω^-1cm^-1, respectively. The crystal structure is hexagonal at room temperature with $\text{a = 7.415 (1)}\text{A}\text{?}$ and $\text{c = 3.865 (1)}\text{A}\text{?}$.     

Effects of adsorbates on electron spin polarization in low energy electron diffraction from tungsten (001): II. Ordered CO overlayer

Electron spin polarization and intensity profiles have been measured in low electron diffraction (LEED) for the (00) beam at θ = 13° and ø = 0° from a W(001) surface exposed to CO and annealed to obtain an ordered c(2 × 2) CO overlayer. The annealed surface with additional CO adsorbed was also studied. The polarization was found to be sensitive to the surface condition and the very distinct P?V profile corresponding to the c(2 × 2) overlayer is believed to be a very sensitive indicator of CO in the β₃ phase. The properties of the annealed surface exposed to further CO suggest the use of this surface as a low energy electron spin polarization analyzer.     

Reactive scattering of small molecules from platinum crystal surfaces: D2CO,CH3OH,HCOOH, and the nonanomalous kinetics of hydrogen atom recombination

The decomposition of D₂CO, CH₃OD and HCOOH on Pt(110) and of D₂CO on Pt(S)-[9(111) × (100)] was studied by molecular beam relaxation spectroscopy. D₂CO and CH₃OD evolved CO and H₂ via a desorption limited sequence of elementary steps. The rate constant for CO desorption from Pt(110) was 6 × 10¹⁴exp(? 35.5 $\text{kcal}\text{gmol}$ · RT) s^?1, and from Pt(S)-[9(111) × (100)] it was 1 × 10¹⁵ exp(?36.2 $\text{kcal}\text{gmol}\text{·RT}$) s^?1. On Pt(110) the rate constant for hydrogen formation was 10^{0 ± 1}exp(?24 $\text{kcal}\text{gmol}\text{·RT}$) $\text{m}\text{?}{}^2\text{atom}$ · s. On Pt(S)-[9(111) × (100)] two pathways for H₂ formation existed with rate constants of 8.7 × 10^?2exp( ?24.9 $\text{kcal}\text{gmol}\text{· RT}$) $\text{cm}{}^2\text{atom}$· s and 3.2 × 10^?3 exp(?19.5 $\text{kcal}\text{gmol}\text{·RT}$) $\text{cm}{}^2\text{atom}\text{·}$ s. These pre-exponential factors are in order of magnitude agreement with values typical of hydrogen recombination on other metals. When a small amount of sulfur ( ~ 0.1 ML) was adsorbed on the stepped Pt surface, only one pathway for H₂ formation existed due to blockage of stepped sites. A similar result was obtained when a beam of CO was impinged on the surface. Formic acid decomposed via a branched process to form primarily CO₂ and H₂.     

Composition,structure, surface states,and O2 sticking coefficient for differently prepared GaAs(1&#x0304;1&#x0304;1&#x0304;)As surfaces

The polar GaAs(1&#x0304;1&#x0304;1&#x0304;)As surface can be prepared in three stable and ordered states: two by molecular beam epitaxy (MBE), namely the As-stabilized and the Ga-stabilized states and one simply by ion bombardment and annealing at 770 K. The respective LEED structures are $\text{(2 × 2), (}\text{19}\text{×}\text{19}\text{)}\text{R}\text{23.4°}$, and (1 × 1) with a diffuse faint (3 × 3) superstructure. Auger measurements and the comparison with the stoichiometric cleaved (110) surface show that there are different As concentrations in the first atomic layer associated with each of these three surfaces. Whereas about 10 to 15% of the first As layer appears to be missing on the (2 × 2) surface, about 50% is missing on the $\text{19}$ surface. On the (1 × 1) surface the first As layer is removed completely. The intensity of emission from the surface sensitive states between 1 and 4 eV below the valence band edge, as seen by angular resolved UPS, roughly corresponds to the amount of As at the surface thus confirming their interpretation as As-derived surface states. The inital sticking coefficent for oxygen depends strongly on the surface structure: ～10^?8 for the (2 × 2), ～10^?7 for the $\text{19}$, and ～10^?4 for the (1 × 1) surface. The sticking coefficient does not depend on the surface concentration of As but rather on the degree of saturation of dangling bonds on Ga atoms.