Chlorinated ethenes at Cu(1 1 0): adsorption, dissociation and reactions of trans-1,2-dichloroethene

The adsorption and reactions of trans-1,2-dichloroethene on Cu(1 1 0) have been investigated using reflection absorption infrared spectroscopy, temperature programmed desorption and molecular beam adsorption reaction spectroscopy. These data reveal that the behaviour of the system is critically affected by temperature and can be described in terms of three main regimes: Regime I, which occurs over 85-165 K, represents molecular adsorption in the monolayer and the multilayer where the trans-1,2-dichloroethene adsorbs with its molecular plane oriented largely parallel to the metal surface; Regime II, extending over the temperature range 170-280 K, is dominated by desorption/dechlorination events which are first initiated at 171 K, then poisoned rapidly and re-initiated at the much higher temperature of 263 K, culminating in the creation of adsorbed acetylene and Cl atoms on the surface; Regime III, spanning the temperature range of 280-500 K, is governed by the behaviour of the C₂H₂ molecular intermediate at the surface which, if created at low temperatures, trimerises to form benzene that subsequently desorbs. However, when created at high temperature, direct desorption of C₂H₂ competes effectively with the trimerisation process.     

Temperature programmed desorption investigations of hydrogen and ammonia reactions on GaN

We report data for chemisorption and reaction of deuterium and isotopically labeled ammonia on single-crystalline GaN films grown on sapphire substrates. Temperature programmed desorption (TPD) and Auger electron spectroscopy (AES) studies, following exposure of the clean GaN film at room temperature to the probe reactant species, were conducted under UHV conditions. Deuterium desorption took place over a wide temperature range, 525–;800 K, with molecular deuterium as the only product. At low exposures, two distinct deuterium desorption peaks at ∼ 660 and 770 K were observed. The deuterium desorption peak at 660 K shifted to lower temperatures with increasing D adatom coverages. TPD experiments after ammonia adsorption on GaN revealed small amounts of hydrogen desorbed at ∼ 600 K and over a range 660–;770 K, suggesting partial decomposition of ammonia. Molecular ammonia desorption was observed at ∼ 560 and 600 K, with the low temperature desorption state growing with increasing ammonia exposures. Further studies on deuterium-precovered GaN films indicated that ammonia production resulted from recombination of NH_x species and hydrogen adatoms on the surface.     

Iodine etching of the GaAs(1̄1̄1̄)As surface studied by LEED,AES, and mass spectroscopy

The iodine interaction with the GaAs(1̄1̄1̄)As surface prepared by molecular beam epitaxy has been studied by LEED, LEED intensity measurements, Auger electron spectroscopy (AES) and computer controlled mass spectroscopic study of the whole desorption spectrum. It is shown that an iodine beam hitting the GaAs(1̄1̄1̄)As face at 300 K under UHV conditions etches the surface continuously. After this etching there remains an adsorbate of GaI_x where x is a number between 0 and 3. By thermal desorption of this GaI_x adsorbate an As stabilized GaAs(1̄1̄1̄)As surface showing a (2 × 2) structure can be prepared, which up to the present could be done only by molecular beam epitaxy.     

Thermal reaction and desorption behaviors of 2,5-diiodothiophene on clean and passivated Au surfaces

Thermal reactions and desorption behaviors of 2,5-diiodothiophene on Au were studied with temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES). Diiodo-substituted hetero-cyclic molecules are an important precursor molecule for photochemical production of conjugated polymers. This paper describes the surface reactions and multilayer structure of 2,5-diiodothiophene in the absence of photon irradiation. 2,5-Diiodothiophene adsorbs molecularly on Au at 100 K. At 200-300 K, the C-I bond of the molecule dissociates producing atomic iodine. The C-I bond cleavage appears to induce further dissociation of the thiophene ring structure. The iodine species desorb at 600-750 K from the surface. The dissociated carbon and sulfur remain on the Au surface even at 800 K. The desorption of thin multilayers occurs at ∼220 K. During the desorption of these layers, a clustering process seems to occur. The desorption of thick multilayer occurs at ∼235 K.     

The interactions of thiophene with polycrystalline UO2

We have studied the adsorption and desorption of thiophene on polycrystalline UO₂ as function of coverage, over the temperature range 100-640 K, using X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD) and electron stimulated desorption (ESD). Thiophene is found to adsorb molecularly on stoichiometric UO₂. C 1s and S 2p XPS spectra are measured at different thiophene exposures and at different temperatures; they show no evidence for the presence of dissociation fragments, confirming that thiophene adsorbs and desorbs molecularly on a polycrystalline stoichiometric UO₂ surface. The variation of the S 2p and C 1s intensity as function of exposure, together with ESD measurements of O⁺ as function of exposure, can be connected to the growth mode of a thiophene film on UO₂; the thiophene film converts from a flat-lying configuration to an inclined structure as coverage increases. The effects of X-rays, UV, and electron irradiation on thiophene films have been studied in two different coverage regimes, monolayer and multilayer. Irradiation leads to a modification of thiophene films, and appreciable concentrations of species stable to 640 K are present on the surface for both regimes. The XPS results suggest that irradiation induces polymerization and oligomerization, as well as formation of thiolates and dissociation fragments of thiophene. The adsorption and reactivity of thiophene on defective UO₂ surfaces have also been studied. The O vacancies and defects in the oxide surface cause cleavage of C-H and C-S bonds leading to the dissociation of thiophene at temperatures as low as 100 K. These results illustrate the important role played by O vacancies in the chemistry of thiophene over an oxide surface.     

Vibrational spectra of ammonia chemisorbed on platinum (111): I. Identification of chemisorbed states

Ammonia chemisorption on Pt(111) has been studied with high resolution electron energy loss spectroscopy (EELS), combined with thermal desorption spectroscopy (TDS). We detect two distinct molecular states of ammonia at different coverages. Near saturation monolayer coverage, ammonia is weakly chemisorbed (ΔH_ads ～ 9 kcal mol^?1) and coordinated to the metal surface via the nitrogen atom. The vibrational frequencies are shifted from the gas phase values, but not as strongly perturbed as in stable platinum-ammine complexes. Below 40% saturation coverage, a new molecular ammonia state is detected, which has a distinct vibrational and thermal desorption spectrum. This state has a considerably reduced ν(PtN) intensity, and the other frequencies are closer to those in solid ammonia, indicating a weaker interaction with the Pt surface. The thermal desorption spectrum of this lower coverage state is broad, from 160 to 450 K, and coverage dependent. Conversion between the two molecular states appears to be only a function of coverage. We propose that the two molecular states have different adsorption sites, and convert from one form to the other as the coverage is changed. No evidence is found for significant dissociation of ammonia on the Pt surface. At very low temperatures (100 K), solid ammonia multilayers may be grown.     

Interaction kinetics of As2 and Ga on {100} GaAs surfaces

A modulated molecular beam technique, using mass spectrometric detection of desorbed species, has been applied to a study of the kinetics of Ga and As₂ interactions on {100} GaAs surfaces. In the temperature range 300–600 K a surface association reaction occurs, leading to the desorption of As₄ by a first order process. Above 600 K, As₂ is lost from the substrate itself by a dissociation reaction, which gives rise to a temperature dependent Ga adatom population, and this in turn results in a temperature dependent As₂ sticking coefficient which can reach unity.     

Adsorption of H2O on Al(111)

The adsorption of H₂O on Al(111) has been studied by ESDIAD (electron stimulated desorption ion angular distributions), LEED (low energy electron diffraction), AES (Auger electron spectroscopy) and thermal desorption in the temperature range 80–700 K. At 80 K, H₂O is adsorbed predominantly in molecular form, and the ESDIAD patterns indicate that bonding occurs through the O atom, with the molecular axis tilted away from the surface normal. Some of the H₂O adsorbed at 80 K on clean Al(111) can be desorbed in molecular form, but a considerable fraction dissociates upon heating into OH_ads and hydrogen, which leaves the surface as H₂. Following adsorption of H₂O onto oxygen-precovered Al(111), additional OH_ads is formed upon heating (perhaps via a hydrogen abstraction reaction), and H₂ desorbs at temperatures considerably higher than that seen for H₂O on clean Al(111). The general behavior of H₂O adsorption on clean and oxygen-precovered Al(111) (θ_O ? monolayer) is rather similar at low temperature, but much higher reactivity for dissociative adsorption of H₂O to form OH _adsis noted on the oxygen-dosed surface around room temperature.     

An electron spectroscopic investigation of the adsorption of NO on Ni(111)

The adsorption, decomposition, and desorption of NO on the close packed Ni(111) surface have been investigated by XPS, XPS satellites, XAES, UPS, and LEED between 125 and 1000 K. At adsorption temperatures below 300 K a single molecular species (v) is formed with about unit sticking coefficient, which is interpreted as bridge-bonded; its saturation coverage is about 85% of that of CO, i.e. 0.5 relative to surface Ni atoms. Adsorption at 300 to 400 K yields dissociative adsorption (β) followed by molecular adsorption; above 400 K only dissociated species are formed. Upon heating, a full molecular layer dissociates only after some NO desorption (at 380–400 K), while dilute layers (below half coverage) dissociate already above 300 K without NO desorption. Together with quantitative findings this shows that for dissociation of one v-NO, the space of two is required. N₂ desorption from the β-layer occurs above 740 K; the oxygen staying behind diffuses into the crystal above 800 K. Readsorption of NO onto a β-layer or onto an oxygen precoverage at 125 K leads, besides to an α₁-state similar to v-NO, to another molecular state (α₂) which is interpreted as linearly bound. The resulting total coverage is considerably higher than in a virgin layer. This shows that the blocking of dissociation in a full v-layer is probably not due to β requiring the same sites, but to kinetic hindrance; an influence of β-induced surface reconstruction cannot be excluded, however. The LEED results agree with a previous report and are well compatible with the other results.     

Laser induced thermal desorption of carbon monoxide from Fe(110) surfaces

Thermal desorption of CO is induced by bombarding an Fe(110) surface with pulses of a neodymium glass laser. The maximum amplitude of the desorption signal is recorded by a mass spectrometer as a function of the laser pulse intensity and of the CO coverage for both single pulses and sequences of pulses. Since the half width of the laser pulses is only 30 ns the shape of the desorption signal is mainly determined by the time-of-flight of the desorbed particles. There is strong evidence that the latter obey a Maxwell-Boltzmann distribution of temperature T_d, identical in the low temperature range with the maximum surface temperature T_s. Above T_s = 600 K, however, T_d is smaller than T_s. The experimental observations are analyzed successfully with the first order rate equation for desorption.     

C2H4在清洁和有Cs覆盖的Ru(0001)表面吸附的TDS研究

用热脱附谱(TDS)方法研究了乙烯(C₂H₄)在Ru(0001)表面上的吸附.在低温下(200K以下)乙烯可以在清洁及有Cs的Ru(0001)表面上以分子状态稳定吸附,在衬底温度升高至200K以上时,乙烯发生了脱氢分解反应,乙烯分解后的主要产物为乙炔(C₂H₂).在清洁的Ru(0001)表面,乙烯有两种吸附状态,脱附温度分别为275K和360K.而乙炔的脱附温度为350K.在Ru(0001)表面有Cs的存在时,乙烯分解 关键词： 乙烯 钌(0001)表面 铯钌(0001)表面乙烯 钌(0001)表面 铯钌(0001)表面     

Space-charge-limited currents and photoconductive properties of Tl2InGaSe4 layered crystals

The extrinsic electronic parameters of Tl₂InGaSe₄ layered crystals were investigated through measurement of the temperature-dependent dark conductivity, space-charge-limited currents and photoconductivity. Analysis of the dark conductivity reveals the existence of two extrinsic energy levels at 0.40 and 0.51 eV below the conduction band edge, which are dominant above and below 260 K, respectively. Current–voltage characteristics show that the one at 0.51 eV is a trapping energy level with a concentration of (4.8–7.7) × 10¹⁰ cm^?3. Photoconductivity measurements reveal the existence of another energy level located at 0.16 eV. In the studied temperature range, the photocurrent increases with increasing temperature. The dependence of the photoconductivity on the incident light intensity exhibits a linear recombination character near room temperature and a supralinear character as the temperature decreases. The change in recombination mechanism is attributed to an exchange in the behavior of sensitizing and recombination centres.     

Interaction kinetics of As4 and Ga on {100} GaAs surfaces using a modulated molecular beam technique

A modulated molecular beam technique, using mass spectrometric detection of desorbed species, had been applied to a study of the kinetics of Ga and As₄ interactions on {100} GaAs surfaces. Time domain mass spectrometer signals were processed using fourier transform techniques to provide information on surface lifetimes, sticking coefficients, desorption energies and reaction orders. In the temperature range 300–450 K As₄ is nondissociatively chemisorbed on Ga atoms from a weakly bound precursor state, but above 450 K there is a pairwise dissociation-recombination reaction between As₄ molecules adsorbed on adjacent Ga lattice sites. At temperatures higher than 600 K a temperature dependent Ga adatom population is formed by the desorption of As₂ from the surface. Thus above 450 K it is possible to produce GaAs from beams of the elements, but below this temperature the compound does not form.     

Kinetics of evaporation of Cu beaded films on sapphire

-8 mbar were investigated by means of Auger electron spectroscopy in the temperature range 893–978 K. Measuring the time dependence of the normalized Auger intensity of Cu, we concluded that the process is controlled by a surface reaction at the perimeters of beads. From the temperature dependence of the initial and final parts of these curves we were able to determine the energy of activation of the desorption of Cu and of the surface reaction as well. Received: 9 June 1997/Accepted: 27 June 1997     

MgH2(110)表面Pd单原子催化氢脱附反应的第一性原理研究

本文采用第一性原理密度泛函理论计算研究了MgH₂(110)表面吸附单原子Pd后的氢脱附反应. 计算发现,在吸附一个Pd单原子后,MgH₂(110)表面氢脱附反应的能垒可以从1.802 eV显著地降低到1.154 eV,表明Pd单原子对于氢脱附具有很强的催化效应. 并且,Pd单原子催化还可以将氢脱附的温度从573 K显著地降低到了367 K,从而使MgH₂(110)表面的氢脱附反应更加容易和快速地发生. 此外,通过MgH₂(110)表面氢溢出机制的反向过程来讨论了氢脱附反应的微观过程. 该研究表明Pd/MgH₂薄膜在未来的实验中可作为良好的储氢材料.     

Thermal desorption of carbon monoxide from Mo(110)

Adsorption and desorption of carbon monoxide (CO) from Mo(110) have been investigated by temperature programmed desorption (TPD). The TPD spectra exhibit peaks in two temperature regions: 250-400 and 850-1100 K. The first region correspond to adsorption in molecular form, whereas the latter region corresponds to desorption of CO that has been dissociatively adsorbed. When the surface was saturated by CO, about 35% of the desorption intensity originates from the high-temperature region (recombinative desorption) and the remaining desorption signal is from the low-temperature region (molecular adsorption). Desorption parameters for molecular adsorbed CO were obtained using several different heating rates. Desorption energies were estimated to range from 0.9 eV for low coverages of CO to about 0.65 eV for high coverages. Corresponding prefactors were estimated to be in the range from 1 210¹¹ to 1 210⁹ s^{m 1}. The experimental data have been compared with Monte Carlo simulations of first-order TPD from a bcc (110) lattice, in which partial poisoning of adsorption sites by dissociated carbon and oxygen was modelled. Repulsive near-neighbour interactions were used.     

The formation of a surface iodide on Ni{100} and adsorption of I2 at low temperatures

Iodine adsorption on clean Ni[100] has been investigated using low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). At temperatures below 340 K. a saturated surface of adsorbed iodine atoms in a c(2 × 2) structure is observed. Adsorption of iodine on clean Ni{100} at temperatures in exces of 370 K forms a structure identified as a single layer of the layered compound NiI₂ on the metal substrate. Solid iodine is shown to grow epitaxially on both the c(2 × 2) chemisorbed surface and the surface iodide at temperatures less than 185 K. Heating to 185 < T < 226 K leaves a physisorbed molecular iodine layer, while on returning to room temperature the original c(2 × 2) or iodide is restored.     

Direct inelastic and trapping desorption scattering of N2 from polycrystalline W; elementary steps in the chemisorption of nitrogen

Scattering of N₂ from a clean polycrystalline W surface is studied with a time-of-flight molecular beam apparatus. The time-of-flight spectra are used to characterize the N₂-W energytransfer and condensation, allowing inferences to be made about the initial steps of N₂ chemisorption, thought to proceed via a molecular precursor state. The sticking coefficient on our sample for N₂ to chemisorb to an atomic nitrogen bound state was 0.5 ± 0.1 5 for a 600 K beam and a 450 K surface temperature. Unreacted N₂ scattered into direct and trapping-desorption channels. The direct channel is shown to be entirely inelastic with temperature independent differential energy accommodation coefficients that average 0.46 for normal and specular scattering at 45° incidence angle. The fraction of trapping-desorption scattering diminishes significantly with increasing surface and beam temperature. The observed decrease in sticking coefficient with increase in surface temperature is shown to be due to a diminution of the N₂ condensation coefficient as well as an increase in desorption of the N₂, recursor relative to its migration-chemisorption.     

Hydrogen-induced low temperature CO displacement from the Pt(111) surface

A new low temperature displacement mechanism for CO on the Pt(111) surface has been observed in the presence of high pressures of hydrogen (0.001 to 0.1 Torr H₂). Temperature-programmed fluorescence yield near-edge spectroscopy (TP FYNES) was used to continuously monitor the CO coverage as a function of temperature both with and without hydrogen. For hydrogen pressures above 0.01 Torr, removal of CO begins at 130 K (E_d = 10.6 kcal/mol) instead of near the desorption temperature of 400 K (E_d = 26 kcal/mol). The large decrease in CO desorption energy appears to be caused by substantial repulsive interactions in the compressed monolayer induced by coadsorbed hydrogen. The new low temperature CO desorption channel appears to be caused by displacement of the compressed CO adlayer by coadsorbed hydrogen. In addition, the desorption activation energy for the main desorption channel of CO near 400 K is lowered by ~ 1 kcal/mol for hydrogen pressures in the 0.001 to 0.1 Torr range. These new results clearly emphasize the importance of in-situ methods capable of performing kinetic experiments at high pressures on well characterized adsorbed monolayers on single crystal surfaces. High coverages of coadsorbed hydrogen resulting from substantial overpressures may substantially modify desorption activation energies and thus coverages and kinetic pathways available even for strongly chemisorbed species. These phenomena may play an important role in surface reactions which occur at high pressure.     

Surface chemistry of (2,4-dimethylpentadienyl)(ethylcyclopentadienyl)Ru on polycrystalline Ta

The adsorption and desorption of (2,4-dimethylpentadienyl)(ethylcyclopentadienyl)Ru [DER] on polycrystalline Ta have been studied by X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). DER exposures to Ta at 140 K result in primarily molecular adsorption and desorption, while a minor surface reaction occurs at defect sites. Monolayer DER desorbs between 278 and 297 K with increasing coverage, exhibiting a first order, zero coverage desorption energy of 2.3 eV. Multilayer DER desorbs between 272 and 263 K, most likely with fractional order kinetics, and exhibits a zero coverage desorption energy of 0.9 eV. XPS Ru 3d binding energies increase with increasing coverage due to core hole screening in the monolayer regime and increasing sample charging as the DER overlayer becomes thicker in the multilayer regime. DER exhibits a three-dimensional (3D) “hit and stick” growth mode in which random 3D structures form due to the lack of adsorbate mobility at 140 K. DER exposures to Ta between 298 and 773 K result in minor decomposition resulting primarily in adsorbed hydrocarbon species on the surface. When the Ta is pre-covered with atomic iodine, DER dissociation is significantly decreased while adsorption is increased.