Desorption kinetics and directional dependence of the surface diffusion of potassium on stepped W(100) surfaces

Using a surface ionisation ion microscope the desorption parameters and the diffusion constant of potassium were measured on stepped W(100) surfaces. The activation energy of ionic desorption as well as the corresponding prefactor do not depend on the step density; the mean adsorption lifetime τ can be expressed as τ=1.6×10^?14s exp(2.44 eV/kT).Whereas the surface diffusion of potassium on “flat” W(100) and on W(S)-[9(100)×(110)] was found to be isotropic, on W(S)- [5(100)×(110)] and W(S)-[3(100)×(110)] it occurs preferentially parallel to the step direction. The diffusion constant D_∥ for this direction has roughly the same value for all investigated surfaces: D_∥=7.8×10^?2 cm²s^?1 exp(?0.42 eV/kT). For the direction perpendicular to the steps D⊥ depends on the step density, whereby the activation energy as well as the prefactor increase with increasing step density.     

A molecular beam study of the adsorption and desorption of oxygen from a Pt(111) surface

The adsorption and desorption of O₂ on a Pt(111) surface have been studied using molecular beam/surface scattering techniques, in combination with AES and LEED for surface characterization. Dissociative adsorption occurs with an initial sticking probability which decreases from 0.06 at 300 K to 0.025 at 600 K. These results indicate that adsorption occurs through a weakly-held state, which is also supported by a diffuse fraction seen in the angular distribution of scattered O₂ flux. Predominately specular scattering, however, indicates that failure to stick is largely related to failure to accommodate in the molecular adsorption state. Thermal desorption results can be fit by a desorption rate constant with pre-exponential ν_d = 2.4 × 10^?2 cm² s^?1 and activation energy E_D which decreases from 51 to 42 kcal/mole^?1 with increasing coverage. A forward peaking of the angular distribution of desorbing O₂ flux suggests that part of the adsorbed oxygen atoms combine and are ejected from the surface without fully accomodating in the molecular adsorption state. A slight dependance of the dissociative sticking probability upon the angle of beam incidence further supports this contention.     

The adsorption of hydrogen and the reaction of hydrogen with oxygen on Pt (100)

The adsorption of hydrogen on Pt (100) was investigated by utilizing LEED, Auger electron spectroscopy and flash desorption mass spectrometry. No new LEED structures were found during the adsorption of hydrogen. One desorption peak was detected by flash desorption with a desorption maximum at 160 °C. Quantitative evaluation of the flash desorption spectra yields a saturation coverage of 4.6 × 10¹⁴ atoms/cm² at room temperature with an initial sticking probability of 0.17. Second order desorption kinetics was observed and a desorption energy of 15–16 kcal/mole has been deduced. The shapes of the flash desorption spectra are discussed in terms of lateral interactions in the adsorbate and of the existence of two substates at the surface. The reaction between hydrogen and oxygen on Pt (100) has been investigated by monitoring the reaction product H₂O in a mass spectrometer. The temperature dependence of the reaction proved to be complex and different reaction mechanisms might be dominant at different temperatures. Oxygen excess in the gas phase inhibits the reaction by blocking reactive surface sites. At least two adsorption states of H₂O have to be considered on Pt (100). Desorption from the prevailing low energy state occurs below room temperature. Flash desorption spectra of strongly bound H₂O coadsorbed with hydrogen and oxygen have been obtained with desorption maxima at 190 °C and 340 °C.     

The adsorption of CO,H2, H2, CO2 and H2O on carburized and graphitized Ni(110)

A study of the adsorption/desorption behavior of CO, H₂O, CO₂ and H₂ on Ni(110)(4 × 5)-C and Ni(110)-graphite was made in order to assess the importance of desorption as a rate-limiting step for the decomposition of formic acid and to identify available reaction channels for the decomposition. The carbide surface adsorbed CO and H₂O in amounts comparable to the clean surface, whereas this surface, unlike clean Ni(110), did not appreciably adsorb H₂. The binding energy of CO on the carbide was coverage sensitive, decreasing from 21 to 12 $\text{kcal}\text{mol}$ as the CO coverage approached 1.1 × 10¹⁵ molecules cm^?2 at 200K. The initial sticking probability and maximum coverage of CO on the carbide surface were close to that observed for clean Ni(110). The amount of H₂, CO, CO₂ and H₂O adsorbed on the graphitized surface was insignificant relative to the clean surface. The kinetics of adsorption/desorption of the states observed are discussed.     

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.     

Formation of bubbles and blisters in hydrogen ion implanted polycrystalline tungsten

ABSTRACT

Tungsten (W) has been regarded as one of the most promising plasma facing materials (PFMs) in fusion reactors. The formation of bubbles and blisters during hydrogen (H) irradiation will affect the properties of W. The dependence of implantation conditions, such as fluence and energy, is therefore of great interest. In this work, polycrystalline tungsten samples were separated into two groups for study. The thick samples were implanted by 18?keV H₃⁺ ions to fluences of 1?×?10¹⁸, 1?×?10¹⁹ and 1?×?10²⁰ H⁺/cm², respectively. Another thick sample was also implanted by 80?keV H₂⁺ ions to a fluence of 2?×?10¹⁷ H⁺/cm² for comparison. Moreover, the thin samples were implanted by 18?keV H₃⁺ ions to fluences of 9.38?×?10¹⁶, 1.88?×?10¹⁷ and 5.63?×?10¹⁷ H⁺/cm², respectively. Focused ion beam (FIB) combined with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used for micro-structure analysis, while time-of-flight ion mass spectrometry (ToF-SIMS) was used to characterize the H depth profile. It is indicated that bubbles and blisters could form successively with increasing H⁺ fluence. H bubbles are formed at a fluence of ～5.63?×?10¹⁷ H⁺/cm², and H blisters are formed at ～1?×?10¹⁹ H⁺/cm² for 18?keV H₃⁺ implantation. On the other hand, 80?keV H₂⁺ ions can create more trapping sites in a shallow projected range, and thus enhancing the blisters formation with a relatively lower fluence of 2?×?10^17?H⁺/cm². The crack-like microstructures beneath the blisters are also observed and prefer to form on the deep side of the implanted range.     

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.     

Diffusion of hydrogen and deuterium on the (111) plane of tungsten

The diffusion of ¹H and ²H on the (111) plane of a W field emitter has been studied by the fluctuation method at various coverages. Both activated and unactivated diffusion is observed; the latter shows very little isotope effect, suggesting that coupling to the substrate is so strong that mass renormalization makes the effective masses of ¹H and ²H nearly identical. Values of D in the tunneling, i.e. temperature independent, regime are 10^?13?5 × 10^?14 cm²/s depending on coverage. For activated diffusion at high coverages, corresponding to population of the β₁ state E = 2.4?3.2 kcal/mol and D₀ = 2 × 10^?8 ?5 × 10^?7 cm²/s, depending on coverage. For lower coverages, corresponding to β₂ population, E = 7–9 kcal/mol, D₀ = 9 × 10^?6 ?2 × 10^?3 cm²/s, again depending on coverage. Similar values are obtained for ²H, with E and D₀ values slightly reduced. An exponentially decaying correlation signal for clean W was also seen and interpreted in terms of flip-flop of W atoms.     

Exchange of injected hydrogen isotopes between diffusing and trapped states in Al

Abstract

Exchange of hydrogen isotopes between diffusing and trapped populations was found with successive injections of 25 keV D₂ ⁺ and 100 or 140 keV H₂ ⁺ into Al surface by simultaneously detecting elastically recoilled hydrogen and deuterium using 2 MeV He⁺.

When backward diffusion of injected hydrogen toward surface was supposed to be impeded or by-passed by structural changes in the subsurface layer due to ion bombardment, effective replacement cross-section of about 1.4 × 10^?18 cm² was observed. Otherwise, namely when the hydrogen backward diffusion is supposed to be rather free, the effective cross section of about 7.8 × 10^?18 was observed.

Due to the difference in energy of the first and second injections, this phenomenon is different from that so called “isotopic exchange”, which occurs through collisional process.     

Adsorption and desorption of ammonia,hydrogen, and nitrogen on ruthenium (0001)

The adsorption of ammonia, hydrogen, and nitrogen on a Ru(0001) surface have been investigated by Auger electron spectroscopy, low-energy electron diffraction, and thermal flash desorption. The adsorption of ammonia on Ru(0001) can be divided into a low temperature mode (100 K) and a higher temperature mode (300–500 K). For a crystal temperature of 100 K the ammonia adsorbs into two weakly bound molecular γ states with s = 0.2. The ammonia desorbs as NH₃ molecules with desorption energies of 0.32 and 0.46 eV. At 300–500 K adsorption occurs via an activated process with a low sticking probability (s ? 2 × 10^?4).This adsorption is accompanied by dissociation and formation of an apparent (2 × 2) LEED pattern. Hydrogen adsorbs readily (s = 0.4) on Ru(0001) at 100 K and desorbs with 2nd order kinetics in the temperature range 350–450 K. Nitrogen does not appreciably adsorb on Ru(0001) even at 100 K; maximum nitrogen coverage obtained was estimated to be <2% of a monolayer. Changes in the ammonia flash desorption spectra after hydrogen preadsorption at 100 K will be discussed.     

The ground state of molecular hydrogen

The v = 0?0 quadrupole spectrum of H₂ has been recorded using a 0.005-cm^?1 resolution Fourier transform spectrometer. The rotational lines S(1) through S(5) are observable in the spectra, in the region 587 to 1447 cm^?1. The spectral position for S(0) was also obtained from its v = 1-0 ground-state combination difference. The high accuracy of the H₂ measurements has permitted a determination of four rotational constants. These are (in cm^?1) B₀ = 59.33455(6); D₀ = 0.045682(4); H₀ = 4.854(12) × 10^?5; L₀ = ?5.41(12) × 10^?8. The hydrogen line positions will facilitate studies of structure and dynamics in astrophysical objects exhibiting infrared H₂ spectra. The absolute accuracy of frequency calibration over wide spectral ranges was verified using 10-μm CO₂ and 3.39-μm CH₄ laser frequencies. Standard frequencies for 5-μm CO were found to be high by 12 MHz (3.9 × 10^?4 cm^?1).     

Cross sections for slow ion production and charge transfer in H+3(D+3)−H2(D2) collisions

Slow ion production cross sections for collisions of H⁺₃ and D⁺₃ ions with H₂ and D₂ have been measured at collision energies between 100 eV and 500 eV. The values vary from 2 × 10^-17 cm² to 6 × 10^-17 cm². The smaller cross sections for D₃ projectiles may be explained as an internal energy effect.     

TDS and LEED studies of H2O adsorption on GaAs(110)

The UHV cleaved (110) face has been exposed to water in the range from 10 L to 2 × 10⁴ L. The main TDS peak in H₂O desorption appears at 350 K, independent of coverage. The low desorption energy of 0.7 eV (16 kcal/mol) is reasonable for oxygen atoms bound via the lone pair orbital to As as was earlier derived from UPS measurements. A broad spur between 450 and 600 K may be related to O-Ga bonds. The sticking probability shows values below 10^-4; only near 4.8 × 10³ L (6 × 10¹⁵ cm^-2 s^-1 H₂O molecules for 300 s) corresponding to a coverage of about 0.4 monolayes a steep maximum appears. At about one monolayer saturation is observed. Exposures to more than 10⁴ L of water quench the intensity of the (10) LEED spot considerably stronger than the intensity of the (11) spot. A comparison of the I(E) curves with existing model calculations suggests that the observed behaviour of the LEED spots is caused by a change in surface structure towards the unrelaxed configuration. The higher sticking coefficient observed near 0.4 monolayers may be connected with this rearrangement of surface atoms.     

Adsorption-desorption properties,coadsorption, and surface structural chemistry of chlorine and oxygen on Ag(331)

At 300 K oxygen chemisorbs on Ag(331) with a low sticking probability, and the surface eventually facets to form a (110)?(2 × 1) O structure with ΔΦ = +0.7 eV. This facetting is completely reversible upon O₂ desorption at ~570 K. The electron impact properties of the adlayer, together with the LEED and desorption data, suggest that the transition from the (110) facetted surface to the (331) surface occurs at an oxygen coverage of about two-thirds the saturation value. Chemisorbed oxygen reacts rapidly with gaseous CO at 300 K, the reaction probability per impinging CO molecule being ~0.1. At 300 K chlorine adsorbs via a mobile precursor state and with a sticking probability of unity. The surface saturates to form a (6 × 1) structure with ΔΦ = +1.6 eV. This is interpreted in terms of a buckled close-packed layer of Cl atoms whose interatomic spacing is similar to those for Cl overlayers on Ag(111) and Ag(100). Desorption occurs exclusively as Cl atoms with E_d ~ 213 kJ mol^?1; a comparison of the Auger, ΔΦ, and desorption data suggests that the Cl adlayer undergoes significant depolarisation at high coverages. The interaction of chlorine with the oxygen predosed surface, and the converse oxygen-chlorine reaction are examined.     

Adsorption and desorption of CO and H2 on Rh(111): Laser-induced desorption

Isosteric heats of adsorption ΔH_ad of CO and sticking coefficients S for CO and H₂ on Rh(111) are determined by laser-induced thermal desorption (LITD) in which a pulsed laser beam is focused onto the surface, and rapid local heating yields a desorption signal that is proportional to the adsorbate coverage θ. ΔH_ad for CO falls from 32.0±2 kcal/mol at low coverage to 14 kcal/mol at saturation, and the desorption pre-exponential factor v_d decreases from 10^14±0.5 to 10¹⁰ s^-1. ΔH_ad, v_d, and S of CO all decline sharply above θ = 0.2, corresponding to the occupation of a second binding state. Sticking coefficients for CO and hydrogen indicate precursor intermediates in adsorption.     

Molecular binding states on clean and oxidized surfaces: SiO(g) + W(clean) and SiO(g) + W(oxidized)

The interactions between a molecular beam of SiO(g) and a clean and an oxidized tungsten surface were examined in the surface temperature range 600 to 1700 K by mass spectrometrically determined sticking probabilities, by flash desorption mass spectrometry (FDMS) and by Auger electron spectroscopy (AES). The sticking probability, S, of SiO has been determined as a function of coverage and of surface temperature for the clean and the oxidized tungsten surface. Over the temperature range studied and at zero coverage S = 1.0 and 0.88 for the clean and oxidized tungsten surfaces respectively. The results are consistent with both FDMS and AES. For coverage up to one monolayer there is one major adsorption state of SiO on the clean tungsten surface. FDMS shows that T_m = constant (T_m is the surface temperature at which the desorption rate is maximum) and that desorption from this state is described by a simple first order desorption process with activation energy, E_d = 85.3 kcal mole^?1 and pre-exponential factor, ν = 2.1 × 10¹⁴ sec^?1. AES shows that the 92 eV peak characteristic of silicon dominates. In contrast on the oxidized tungsten surface, T_m shifts to higher temperatures with increasing coverage. The data indicate a first order desorption process with a coverage dependent activation energy. At low coverage (θ ? 0.14) there is an adsorption state with E_d = 120 kcal mole^?1 and ν = 7.6 × 10¹⁹, while at θ = 1.0, E_d = 141 kcal mole^?1. This variation is interpreted as due to complex formation on the surface. AES shows that on oxidized tungsten, in contrast to clean tungsten, the dominant peaks occur at 64 and 78 eV, and these peaks are characteristic of higher oxidation states of silicon. Thus, it is concluded that SiO exists in different binding states on clean and oxidized tungsten surfaces.     

The adsorption and desorption of oxygen on the Pt(110) surface; A thermal desorption and LEED/AES study

The interaction of oxygen with a Pt(110) crystal surface has been investigated by thermal desorption mass spectroscopy, LEED and AES. Adsorption at room temperature produces a β-state which desorbs at ~800 K. Complete isotopic mixing occurs in desorption from this state and it populates with a sticking probability which varies as (1 ? θ)², both observations consistent with dissociative adsorption. The desorption is second order at low coverage but becomes first order at high coverage. The saturationcoverage is 3.5 × 10¹⁴ mol cm^?2. The spectra have been computer analysed to determine the fraction desorbing by first (β₁) and second (β₂) order kinetics as a function of total fractional coverage θ using this fraction as the only adjustable parameter. The β₁ desorption commences at θ ～ 0.25 and β₁ and β₂ contribute equally to the desorption at saturation. The kinetic parameters for β₁ desorption were calculated from the variation of peak temperature with heating rate as ν₁ = 1.7 × 10⁹ s^?1 and E^≠₁ = 32 kcal mole^?1 whereas two different methods of analysis gave consistent parameters ν₂ = 6.5 × 10^?7 cm² mol^?1 s^?1 and E^≠₂ = 29 and 30 kcal mole^?1 for β₂ desorption. The kinetics of desorptior are discussed in terms of the statistics for occupation of near neighbour sites. While many fea tures of the results are consistent with this picture, it is concluded that simple models considering either completely mobile or immobile adlayers with either strong or zero adatom repulsion are not completely satisfactory. The thermal desorption surface coverage has been correlated with the AES measurements and it has been possible to use the AES data for PtO as an internal standard for calibration of the AES oxygen coverage determination. At low temperature (170 K) oxygen populates an additional molecular α-state. Adsorption into the α- and β-states is competitive for the same sites and pre-saturation of the β-state at 300 K excludes the α-state. This, together with the AES observation that the adsorption is enhanced and faster at 450 than 325 K suggests a low activation energy for adsorption into the β-state.     

Adsorption of oxygen on a Cu{110} surface with and without the influence of A keV Ne+ beam: Stage I: Coverages up to half a monolayer

Low Energy Ion Scattering has been used to study the interaction of molecular oxygen with a Cu{110} surface. The amount of adsorbed atomic oxygen was monitored by the 4 keV Ne⁺¦O reflection signal. In the first adsorption stage (coverage less than half a monolayer) the sticking probability varied proportional to the number of empty adsorption sites: S = S₀ (1 ? $\text{}\text{?}$gq). It turned out not to be influenced by the Ne⁺ bombardment. The initial sticking probability S₀ was found to be ≈ 0.24. In this first adsorption stage the oxygen-covered surface is reconstructed according to the “missing row” model, leading to a (2 × 1) LEED pattern.     

Chemisorption of nitrogen on platinum {111}: Reflection-absorption infrared spectroscopy

Reflection-absorption infrared spectroscopy has been combined with thermal desorption and surface coverage measurements to study nitrogen adsorption on a {111}-oriented platinum ribbon under ultrahigh vacuum conditions. Desorption spectra show a single peak (at 180 K) after adsorption at 120 K, giving a coverage-independent activation energy for desorption'of ～40 kJmol^?1. The initial sticking probability at this temperature is 0.15, and the maximum uptake was ～1.1 × 10¹⁴ molecule cm^?2. The adsorbed nitrogen was readily displaced by CO, h₂ and O₂. An infrared absorption band was observed with a peak located at 2238 ± 1 cm^?1, and a halfwidth of 9 cm^?1, with a molecular intensity comparable to that reported for CO on Pt{111}. The results are compared with data for chemisorption on other group VIII metals. An earlier assignment of infrared active nitrogen to B₅ sites on these metals is brought into question by the present results.     

Adsorption and desorption of hydrogen on bare and Xe-covered Cu(111)

Using polarization-modulated ellipsometry to monitor adsorbate coverage in-situ, we studied the activated adsorption of filament-heated molecular hydrogen on Cu(111) and subsequent isothermal desorption of hydrogen adatoms. The adsorption is characterized by a zeroth-order kinetic with a constant sticking probability of S₀=0.0062 up to θ=0.25, followed by a Langmuir kinetic until the saturation coverage θ_s=0.5 is reached. The desorption follows a second-order kinetic with an activation energy of 0.63 eV and a pre-exponential factor of 1×10⁹ /s. A pre-adsorbed monolayer of Xe atoms on Cu(111), with a desorption activation energy of 0.25 eV and a pre-exponential factor of 8×10¹⁴ /s, efficiently blocks the subsequent adsorption of hot molecular hydrogen, making physisorbedXe useful as templates for spatial patterning of hydrogen adatom density on Cu(111). PACS 68.43.Jk; 78.68.+m; 81.15.-z; 82.40.Np