Adsorption of HCl and HBr on Si(111): AES,ELS, and EID studies

The combined techniques in situ of Auger electron spectroscopy, electron energy loss spectroscopy, electron impact desorption, and work-function change measurement have been applied to the study of the adsorption of HCl and HBr on thermally cleaned Si(111) surfaces. Major results are summarized as follows: (1) HCl shows a fast adsorption to the saturation coverage of $\text{θ}{}_{\mathrm{s}}\text{? 0.3}$ (estimated using the continuum approximation) by the exposure of about 1 L at room temperature. (2) The average sticking probabilities for HCl and HBr are ～0.7. (3) Two adsorbed states of HCl or HBr at room temperature are discriminated. For HCl, the first state is characterized by the emission of ～1.2 eV ions and the electronic transition at 8.4 eV, which is subsequently converted to the second state characterized by the emission of ～3.2 eV ions and the electronic transitions at 7.0 and 8.4 eV. Heating the sample at ～800 K causes the desorption of hydrogen and the appearance of the Cl-related peaks at 6.0, 7.0, and 9.0 eV in the loss spectra. For HBr, the first and the second states are characterizied by the emission of ～1.2 and ～3.2 eV ions, respectively. The electronic transition is observed at 7.8 eV in both states. (4) It is proposed that HCl and HBr are adsorbed as molecules initially, which are subsequently dissociated into atoms spontaneously at room temperature.     

Co desorption and adsorption on Pt(111)

The kinetics of the desorption of CO from a Pt(111) crystal between 419 and 505 K is reported using a Low-Energy Molecular-Beam-Scattering (LEMS) technique with a helium probe beam and a CO dosing beam. The resulting first-order Arrhenius rate constant is $\text{k = 2.7 × 10}{}^{13}\text{exp(?31.1}\text{kcal}\text{mole}\text{· RT) s}{}^{\mathrm{?1}}$. We also report a study of the equilibriumadsorbed CO between 400 and 600 K using LEMS. These results, fitted to a Temkin isotherm model, indicate that the adsorption energy decreases linearly with surface coverage with the average value equal to $\text{31.1 + 1.2}\text{kcal}\text{mole}$ over the coverage range 0 < θ ? 0.5. The average harmonic oscillator frequency of the adsorbed CO molecules is 191 ± 76 cm^?1.     

The effect of sulfur on co adsorption/desorption on Pt(S)-[9(111) × (100)]

CO adsorption/desorption on clean and sulfur covered Pt(S)-[9(111) × (100)] surfaces was studied using AES, TPD, and modulated beam experiments. CO desorption occurred from two states on the clean surface — a low temperature state associated with the (111) terraces and a high temperature state associated with the steps/defects. Thermal desorption results indicated that above small CO coverages conversion from the low temperature state into the high temperature state was activated and that back conversion was slow. Sulfur preferentially adsorbed at step/defect sites and decreased the population of the high temperature desorption state. Modulated beam experiments were performed in order to determine CO adsorption/desorption parameters as a function of sulfur coverage on the Pt crystal. The sticking coefficient and binding energy of CO decreased as the sulfur concentration increased. Sulfur adsorption at step/defect sites decreased the CO sticking coefficient only slightly but increased the effective rate constant for CO desorption significantly. Sulfur adsorption on the terraces affected CO adosrption more than sulfur at step sites. On the clean surface the effective rate constant for CO desorption was

$\text{1 × 10}{}^{15}\text{s}{}^{\mathrm{?1}}\text{exp (}\text{?36.2 kcal/mole}\text{RT}\text{)}$

Desorption occurred from both terrace and step/defect sites, but the kinetics were characteristic of the step/defect sites. For the surface on which step/defect sites were blocked by sulfur the effective desorption rate constant was

$\text{k}{}_{\mathrm{eff}}\text{= 1 × 10}{}^{13}\text{s}{}^{\mathrm{?1}}\text{exp (?}\text{27.5 kcal/mole}\text{RT}\text{)}$

indicating an appreciable decrease in CO binding on the terraces, though sulfur-CO repulsive interactions had probably made k_eff larger than the true rate constant for desorption from clean (111) planes. The results showed clearly a compensation effect in activation energy and preexponential factor.     

Trapping probabilities and desorption energies of alkali atoms on a clean and an oxygen covered tungsten (110) surface

Alkali atoms were scattered with hyperthermal energies from a clean and an oxygen covered (θ ≈ 0.5 ML) W(110) surface. The trapping probability of K and Na atoms on oxygen covered W(110) has been measured as a function of incoming energy (0–30 eV) and incident angle. A considerable enhancement of trapping on the oxygen covered surface compared to a clean surface was observed. At energies above 25 eV there are still K and Na atoms being trapped by the oxygen covered surface. From the temperature dependence of the mean residence time τ of the initially trapped atoms the pre-exponential factor τ₀ and the desorption energy Q were derived using the relation: $\text{τ = τ}{}_0\text{exp}\text{(}\text{Q}\text{kT}{}_{\mathrm{<mtext>s</mtext>}}\text{)}$. On clean W(110) we obtained for Li: $\text{τ}{}_0\text{= (8 ±}\text{8}\text{4}\text{) × 10}{}^{\mathrm{?14}}\text{sec}$, Q = (2.78 ± 0.09) eV; for Na: τ₀ = (9 ± 3) × 10^?14 sec, Q = (2.55 ± 0.04) eV; and for K: τ₀ = (4 ± 1) × 10^?13 sec, Q = (2.05 ± 0.02) eV. Oxygen covered W(110) gave for Na: τ₀ = (7 ±3) × 10^?15 sec, Q = (2.88 ± 0.05) eV; and for K: $\text{τ}{}_0\text{= (1.3 ±}\text{0.9}\text{0.6}\text{) × 10}{}^{\mathrm{?14}}\text{sec}$, Q = (2.48 ±0.05) eV. The adsorption on clean W(110) has the features of a supermobile two-dimentional gas; on the oxygen covered W(110) adsorbed atoms have the partition function of a one-dimen-sional gas. The binding of the adatoms to the surface has a highly ionic character in the systems of the present experiment. An estimate is given for the screening length of the non-perfect conductor W(110):k_s^?1≈ 0.5 Å.     

Classical stochastic diffusion theory for desorption from solid surfaces

We present a theory of desorption of atoms and molecules from solid surfaces based on a classical stochastic diffusion formulation. We obtain a simple rate expression which has the form $\text{R = (}\text{Ω}{}_{\mathrm{o}}\text{2π}$f(T) exp($\text{?D}{}_{\mathrm{e}}\text{K}{}_{\mathrm{T}}$), where T is the temperature, k is Boltzmann's constant, the bond enthalpy, and Ω_o is the surface-adsorbate vibrational frequency. For atoms √(T) = 1, while for molecules f(T) depends on the parameters for the frustrated rotations at the surface. Application of this theory is reported for the desorption of atoms and molecules. We find that molecules lead to a greatly increased (factor of 100) Arrhenius preexponential factor in excellent agreement with experiment.     

Author index

The mean adsorption lifetimes of F, Cl and Br on (100) and (111) Mo surfaces have been obtained from the first order desorption kinetics observed in low converage conditions (θ < 10^?2 of a monolayer) using a pulsed ionic beam method. The mean adsorption lifetimes τ fit a general expression $\text{τ = τ}{}_0\text{exp}\text{(}\text{E}\text{kT}\text{)}$ in a large temperature range (1700–2400 K) allowing the determination of the binding energies E. The main results of this study are (1) the binding energies decrease from F through Br; (2) the binding energies on both (100) and (111) orientations are similar, E(F)～4.65 eV, E(Cl)～4.15 eV, and E(Br)～3.65 eV. These results are discussed and compared with those previously reported on (100) and (111) Nb and W surfaces. The close binding energies of F, Cl and Br on (100) and (111)^?Nb and Mo surfaces suggest that halogens have a different chemisorption behaviour with respect to O and N.     

X-ray photoelectron spectroscopic study of the physical adsorption of xenon and the chemisorption of oxygen on tungsten (111)

X-ray photoelectron spectroscopy (ESCA) has been used to study the physical adsorption of Xe and the chemisorption of oxygen by W (111). An ultrahigh vacuum ESCA spectrometer has been modified such that thermal desorption behavior from the W (111) crystal can be directly compared with ESCA spectra of the adsorbed species. In addition, since the work function of a W (111) crystal covered with one monolayer of Xe is accurately known from previous work, the binding energy of the $\text{Xe}\text{(3}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{)}$ adsorbate level can be accurately compared to the gaseous $\text{Xe}\text{(3}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{)}$ level.When Xe is physisorbed to 1 monolayer the $\text{Xe}\text{(3}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{)}$ level exhibits a binding energy (relative to the vacuum level) which is 2.1 eV below that found for Xe (g). At lower Xe coverages the shift becomes monotonically greater, approaching 2.6 eV at a Xe coverage of 0.05. This 0.5 eV shift downward is accompanied by an increase of only 0.05 eV in adsorption energy as coverage decreases, and may be partially caused by the presence of ~ 10–20 % of extraneous adsorption sites other than W (111) which adsorb Xe with higher adsorption energy. The adsorption energy of Xe may also be increased by coadsorption of oxygen and the $\text{Xe}\text{(3}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{)}$ binding energy exhibits a corresponding shift downward as adsorbed oxygen coverage is increased to θ_o = 0.5. Electronic relaxation processes affecting the final state are dominant factors in determining the magnitude of the chemical shift upon adsorption, in agreement with the predictions of Shirley. The magnitude of the relaxation effect seems to be very sensitive to small changes in Xe adsorption energy. Similar effects have been seen for chemisorption of CO.The adsorption of O₂ at 120 K by W (111) yields a single broad O(1s) peak whose line-width decreases with increasing coverage. The final spectra at θ_o = 1 monolayer are very similar to those obtained at temperatures of 300 K or above on polycrystalline tungsten.     

Evidence for triaxial shapes in Pt nuclei

Positive parity levels in ¹⁹¹Pt obtained from (α, xnγ) reactions and β-decay are presented as a first example of a rather complete $\text{i}{}_{\mathrm{<mtext>13</mtext><mtext>2</mtext>}}$ level family. The spectrum confirms triaxial shapes found before from $\text{h}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}$ and $\text{h}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ proton structures in this mass region. In addition to the usual decoupled yrast band, a second ΔI = 2 band within the $\text{i}{}_{\mathrm{<mtext>13</mtext><mtext>2</mtext>}}$ family, built on a low-lying $\text{j?1 =}\text{11}\text{2}$ state, is observed in agreement with theory.     

CO chemisorption on PtCu surfaces I. CO on (1 × 3) Pt0.98Cu0.02(110)

Thermal desorption and photoemission spectroscopy (PES) have been used to investigate the chemisorption of CO on an annealed Pt_0.98Cu_0.02(110) surface. The clean surface shows 9.1 ± 2.6% Cu within the top 4 Å, and is (1 × 3) reconstructed. Thermal desorption of CO has revealed the existence of various adsorption states with these respective heats of adsorption: (α) 35.2 to 37.8 kcal/mol and (β) 24.5 to 26.3 kcal/mol on Pt sites, (γ) 16.0 to 17.2 kcal/mol on PtCu “mixed” sited, and (δ) 12.9 to 13.9 kcal/mol on Cu sites. PES observation of Cu 3d-derived states (using hv = 150 eV) and the $\text{Cu 2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ core levels (using Mg Kα radiation) shows that the electronic structure of the Cu constituent is changed only when CO adsorbs on the Pt-Cu “mixed” sites or the Cu sites. Furthermore, the CO states associated with Pt sites reflect the structural difference between the (1 × 3) alloy surface and the (1 × 2) pure Pt(110) surface: α-CO on the alloy surface desorbs at a temperature 17 to 21 K. higher than the maximum desorption temperature of CO from pure Pt(110), and the ratio of β-CO to α-CO desorption from the alloy surface is larger than the ratio of low temperature to high temperature peaks in the desorption of CO from pure Pt(110).     

Thermal desorption of cyanogen from Pt(100)

Thermal desorption of cyanogen adsorbed on Pt(100) was studied by flash desorption mass spectrometry. By investigating the parent ion and all possible fragmentation products in the mass spectrometer during desorption it was concluded, that desorption takes place exclusively as molecular C₂N₂. Three desorption peaks were observed at 140, 410 and 480°C denoted as α, β₁ and β₂. The respective surface coverages at saturation were determined by quantitative evaluation of the flash desorption curves to be 2.0 ± 0.2 × 10¹⁴ and 5.5 ± 1.0 × 10¹⁴$\text{molecules}\text{cm}{}^2$ for the α and the β states, respectively. First order desorption kinetics was suggested by the coverage dependencé of the desorption spectra for both α and β states with desorption energies of 12 and 38–42 $\text{kcal}\text{mole}$, respectively. A large difference in the sticking probabilities of α and β states was observed with initial values of 0.06 (α) and 0.9 (β). Adsorption experiments at elevated temperatures led to the assumption, that α and β states coexist on the surface with no or very little interactions between them. The results are discussed in terms of different models for the adsorption states.     

Co chemisorption on PtCu surfaces II. (1 × 1)-CO/Pt0.98Cu0.02(110)

The chemisorption of CO on the Pt atoms of an initially (1 × 3) reconstructed Pt_0.98Cu_0.02(110) surface at ~ 373 K can lead to the formation of a (1 × 1) surface. Comparisons are made with (1 × 3)-CO surfaces formed by CO exposures at 293 or 155 K. Thermal desorption shows that the (1 × 1)-CO surface has an enhanced population of high temperature CO peak ( ~ 543 K) from Pt sites. The CO-induced structural conversion also leads to a decrease in the subsequent CO uptake on the low temperature Pt sites and on the Pt-Cu “mixed” sites, with a concomitant increase in adsorption on the Cu-like sites. Such a reduction in the number of the Pt-Cu “ mixed” sites is also reflected in the CO-induced changes of the Cu 3d-derived states and the $\text{Cu 2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ core levels. A dynamic interplay between chemisorption and surface structure is thus demonstrated.     

Polarization spectroscopy of the E0g+-B0u+ band system of Br2

The E-B (0_g⁺-0_u⁺) band system of Br₂ has been investigated at Doppler-limited resolution using polarization labeling spectroscopy. Merged E state data for the three naturally occurring isotopes in the range v_E = 0–16, expressed in terms of the constants for ⁷⁹Br₂, are (in cm^?1) Y_0,0 = 49 777.962(54), Y_1,0 = 150.834(22), Y_2,0 = ?0.4182(28), Y_3,0 = 6.6(11) × 10^?4, Y_0,1 = 4.1876(28) × 10^?2, Y_1,1 = ?1.607(16) × 10^?4, and Y_0,2 = 1.39(39) × 10^?8. The bond distance is $\text{r}{}_{\mathrm{<mtext>e</mtext>}}\text{= 3.194}\text{A}\text{?}$, and the diabatic dissociation energy to $\text{Br}{}^{\mathrm{+}}\text{(}{}^3\text{P}{}_2\text{) +}\text{Br}{}^{\mathrm{?}}\text{(}{}^1\text{S}{}_0\text{)}\text{is 34 700 cm}{}^{\mathrm{?1}}$.     

XPS studies of adatom-adatom interactions: I/Ag(111) and I/Cu(111)

We have studied submonolayer adsorption, at room temperature, of iodine on the (111) faces of silver and copper, using LEED and XPS. In both systems the √3 × √3 LEED pattern appears at ～0.2 monolayer (ML) coverage; no other superlattice pattern was observed. The $\text{I}\text{4}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ core electron binding energy in both cases decreases by ～0.15 eV between very dilute coverage and 0.33 ML. The leveling-off of the binding energy for I/Ag(111) for coverages >0.2 ML is shown to be a unique experimental manifestation of an indirect, substrate-mediated adatom-adatom interaction, an attraction of several meV between next-nearest neighbor iodine atoms. The more nearly linear decrease in the I binding energy on Cu(111) is shown to imply a significantly weaker next-nearest neighbor interaction on this surface. The appearance of the √3 × √3 LEED pattern at low coverages on Cu is shown to be consistent with short-range order produced merely by a size effect, that is, by nearest neighbor exclusion. These conclusions are reached with the help of Monte Carlo calculations of a triangular lattice gas.     

Interaction of neutral hydrogen atoms with KCl(OO1)

Neutral hydrogen atoms with velocity selected thermal energies, have been scattered from KCl(001) cleavage planes in UHV. For cold crystal surfaces (T_SF ? 170 K) even a small residual pressure of water resulted in an ordered monolayer coverage of the (001) plane. From this surface, diffraction has been observed with diffracted beams up to third order and characteristically varying intensities were observed and analysed in terms of a quantum mechanical rainbow scattering theory. Resonant transitions of atoms to bound states at the surface (“selective adsorption”) were observed and used to determine the interaction potential of hydrogen atoms with this water covered surface: first in terms of a Morse potential, second in terms of a $\text{C}{}_3\text{z}{}^3$-dependent attractive potential. The surface Debye-Waller factor has been measured for clean KCl(001), for the water covered (001) plane as well as for partially covered surfaces. The results are interpreted within a simple model.     

Planar channeling of helium ions along (100), (110) and (111) MgO

Planar channelling of 1, 1.5 and 2 MeV ⁴He⁺ ions along (100), (110) and (111) MgO have been studied experimentally using Rutherford backscattering. Values of the half angle $\text{ψ}{}^{\mathrm{P}}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, shoulder half angle $\text{ψ}{}^{\mathrm{s}}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and surface minimum yield x^P_min have been determined for channelling with respect to the two sublattices. Agreements and discrepancies with existing theories are discussed.     

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.     

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.     

Hole mass measurement in p-type InP and GaP by submillimetre cyclotron resonance in pulsed magnetic fields

The first observation of cyclotron resonance in p-type InP is reported. The holes were thermally excited at 110 K and the resonance was observed at 337μm wavelength (HCN laser) using a pulsed magnetic field of 0–350 kG. The effective masses of the light and heavy holes in the 〈111〉 direction were found to be $\text{m}{}^1{}_{\mathrm{<mtext>L</mtext>}}\text{= 0.12 ± 0.01 m}{}_0$, and in the 〈100〉 direction $\text{m}{}^1{}_{\mathrm{<mtext>L</mtext>}}\text{= 0.12 ± 0.01 m}{}_0$, $\text{m}{}^1{}_{\mathrm{<mtext>H</mtext>}}\text{= 0.56 ± 0.02 m}{}_0$. We obtain an estimate of the Dresselhaus parameters A = ?5.04, |B| = 3.12, C² = 6.57. We also report the effective masses for p-type GaP in the 〈111〉 direction as $\text{m}{}^1{}_{\mathrm{<mtext>L</mtext>}}\text{= 0.18 ± 0.02 m}{}_0$, $\text{m}{}^1{}_{\mathrm{<mtext>H</mtext>}}\text{= 0.56 ± 0.04 m}{}_0$.