Effects of potassium on the adsorption and dissociation of CH3Cl on Pd(100)

The effects of potassium on the adsorption and dissociation of CH₃Cl on a Pd(100) surface has been investigated by ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES), electron energy loss spectroscopy (in the electronic range EELS), temperature-programmed desorption (TPD) and work function change. In contrast to the clean surface, the adsorption of CH₃Cl caused a significant work function increase, 0.9-1.4 eV, of potassium-dosed Pd. Preadsorbed K enhanced the binding energy of CH₃C1 to the surface and induced the dissociation of adsorbed molecules. The extent of the dissociation increased almost linearly with the potassium content. The appearance of a new emission in the UPS spectrum at 9.2 eV, attributed to adsorbed CH₃ species, and the low-temperature formation of ethane suggest that a fraction of adsorbed CH₃Cl dissociates even at 115–125 K on potassium-dosed Pd(100). At the same time, a significant part of adsorbed CH₃ radical is stabilized, the reaction of which occurs only at 250–300 K. By means of TPD measurements, H₂, CH₄, C₂H₆, C₂H₄, KCl and K were detected in the desorbing gases. The results are interpreted by assuming a through-metal electronic interaction at low potassium coverage and by a direct interaction of the Cl in the adsorbed CH₃Cl with potassium at high potassium coverage. The latter proposal is supported by the electron excited Auger fine structure of the Cl signal and by the formation of KCl in the desorbing gases.     

Adsorption and reactions of CH2I2 on Ru(001) surface

The adsorption and dissociation of CH₂I₂ were studied at 110 K with the aim of generating CH₂ species on the Ru(001) surface. The methods used included X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), temperature programmed desorption (TPD), Auger electron spectroscopy (AES) and work function measurements. Adsorption of CH₂I₂ is characterized by a work function decrease (0.96 eV at monolayer), indicating that adsorbed CH₂I₂ has a positive outward dipole moment. Three adsorption states were distinguished: a multilayer (T_p=200 K), a weakly bonded state (T_p=220 K) and an irreversibly adsorbed state. A new feature is the formation of CH₃I, which desorbs with T_p=160 K. The adsorption of CH₂I₂ at 110 K is dissociative at submonolayer, but molecular at higher coverages. Dissociation of the monolayer to CH₂ and I proceeded at 198–230 K, as indicated by a shift in the I(3d_5/2) binding energy from 620.6 eV to 619.9 eV. A fraction of adsorbed CH₂ is self-hydrogenated into CH₄ (T_p=220 K), and another one is coupled to di-σ-bonded ethylene, which — instead of desorption — is converted to ethylidyne at 220–300 K. Illumination of the adsorbed CH₂I₂ initiated the dissociation of CH₂I₂ monolayer even at 110 K, and affected the reaction pathways of CH₂.     

TPD, HREELS and UPS study of the adsorption and reaction of methyl nitrite (CH3ONO) on Pt(111)

The adsorption and reaction of methyl nitrite (CH₃ONO, CD₃ONO) on Pt(111) was studied using HREELS, UPS, TPD, AES, and LEED. Adsorption of methyl nitrite on Pt(111) at 105 K forms a chemisorbed monolayer with a coverage of 0.25 ML, a physisorbed second layer with the same coverage that desorbs at 134 K, and a condensed multilayer that desorbs at 117 K. The Pt(111) surface is very reactive towards chemisorbed methyl nitrite; adsorption in the monolayer is completely irreversible. CH₃ONO dissociates to form NO and an intermediate which subsequently decomposes to yield CO and H₂ at low coverages and methanol for CH₃ONO coverages above one-half monolayer. We propose that a methoxy intermediate is formed. At least some C–O bond breaking occurs during decomposition to leave carbon on the surface after TPD. UPS and HREELS show that some methyl nitrite decomposition occurs below 110 K and all of the methyl nitrite in the monolayer is decomposed by 165 K. Intermediates from methyl nitrite decomposition are also relatively unstable on the Pt(111) surface since coadsorbed NO, CO and H are formed below 225 K.     

Amorphous SiC film formation on Si(100) using electron beam excitation

The effect of electron impact on methylsilane (CH₃SiH₃) conversion to amorphous-Si_0.5C_0.5:H (a-Si_0.5C_0.5:H) films on Si(100) has been studied by Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and low energy electron diffraction (LEED). It is found that electron impact greatly enhances CH₃SiH₃ decomposition on Si(100) at both 90 K and 300 K, resulting in a-Si_0.5C_0.5:H thin film formation. Thermal annealing of the film causes hydrogen desorption and amorphous silicon carbide (a-SiC) formation. Upon annealing to temperatures above 1200 K, the a-SiC film became covered by a thin silicon layer as indicated by AES studies. Ordered structures are not produced by annealing the a-SiC up to 1300 K.     

The role of co-adsorbed metal atoms in the chemistry of methyl species on Pt(111) formed by the decomposition of dimethylmercury and dimethylzinc

The chemistry of methyl species resulting from the decomposition of dimethylmercury (DMM) and dimethylzinc (DMZ) on Pt(111) in the range 300–400 K has been investigated by temperature prograrnmed desorption (TPD) and Auger electron spectroscopy (AES). In each case at 300 K, dissociative adsorption of the precursor results in the formation of an adlayer of methylmetal (CH₃M) moieties. These species are thermally stable to around 350 K before decomposing to yield mainly gaseous products, methane and hydrogen, and surface bound metal atoms. For DMM, subsequent heating to 400 K or direct dissociative adsorption at 400 K results in the formation of ethylidyne species. Ethylidyne formation is not observed in the thermal chemistry of DMZ at temperatures below 400 K and only transiently in the chemistry at 400 K. Complementary TPD and AES data indicate that, for DMM, desorption of the mercury atoms produced by CH₃Hg decomposition is the limiting factor in allowing the prevailing C₁ species to couple to form ethylidyne. In contrast, AES evidence indicates that zinc atoms remain on the surface to temperatures in excess of 750 K and hence prevent C---C coupling by blocking surface sites.     

Decomposition mechanism of triethylindium (TEI) on a GaP(0 0 1)-(2×1) surface studied by LEED, AES, TPD and HREELS

Adsorption and decomposition of triethylindium (TEI: (C₂H₅)₃In) on a GaP(0 0 1)-(2×1) surface have been studied by low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). It is found from the TPD result that ethyl radical and ethylene are evolved at about 300–400 and 450–550 K, respectively, as decomposition products of TEI on the surface. This result is quite different from that on the GaP(0 0 1)-(2×4) surface. The activation energy of desorption of ethyl radical is estimated to be about 93 kJ/mol. It is suggested that TEI is adsorbed molecularly on the surface at 100 K and that some of TEI molecules are dissociated into C₂H₅ to form P–C₂H₅ bonds at 300 K. The vibration modes related to ethyl group are decreased in intensity at about 300–400 and 450–550 K, which is consistent with the TPD result. The TEI molecules (including mono- and di-ethylindium) are not evolved from the surface. Based on the TPD and HREELS results, the decomposition mechanism of TEI on the GaP(0 0 1)-(2×1) surface is discussed and compared with that on the (2×4) surface.     

An infrared study of the chemistry of methyl species on Pt(111) formed by the decomposition of dimethylmercury

The chemistry of dimethyl mercury on a Pt(111) single crystal surface has been investigated by reflection-absorption infrared spectroscopy (RAIRS). Dimethyl mercury appears to be highly reactive on Pt(111) and readily decomposes on the surface at temperatures of 100 K and above. Adsorption at 100 K initially occurs in a dissociative manner to produce CH₃ and CH₃Hg species on the surface, both of which are identified as having C_3v local symmetry. At higher exposures, molecular adsorption dominates with the Hg---C---Hg axis initially oriented parallel to the surface. This preferred orientation, however, does not persist into the multilayer. Thermal treatment of the surface layer results in multilayer desorption between 130 and 135 K, and no parent molecular species are observed beyond 160 K. Adsorption at 200 and 300 K produces an overlayer consisting primarily of CH₃Hg species, which are thermally stable to about 350 K. Subsequent heating to 400 K results in the formation of ethylidyne species which are characterised by RAIRS. Adsorption at 400 K results in the direct formation of an ethylidyne layer estimated to be about 85% of saturated coverage.     

The interaction of C2Cl4 with fe(110)

Quantitative adsorption studies, temperature programmed desorption (TPD) and Auger spectroscopy have been used to study the interaction of C₂Cl₄ with Fe(110) at 90 and 325 K. At 90 K, multilayer C₂Cl₄ adsorption occurs. The following desorption products are observed in the temperature range of 90–1050 K: C₂Cl₄ from the multilayer and the monolayer, FeCl₂, and a high mass iron chloride species with mass spectrometer cracking products FeCl⁺₂, FeCl⁺, and Fe⁺. Irreversible dissociative C₂Cl₄ adsorption occurs at 325 K and the only desorption product which is observed is the high mass iron chloride species. Auger spectroscopy shows that surface carbon from C₂Cl₄ starts to diffuse into the bulk of the crystal at ˜ 480 K while small coverages of chlorine remain on the surface of the crystal even after heating to 1050 K. Comparison of the behavior of C₂Cl₄ and CCl₄ on Fe(110) indicates that radical products (·CCl₃ and :CCl₂) observed to be produced from CCl₄ adsorption are not produced from C₂Cl₄ adsorption. This difference is probably due to the enhanced surface reactivity of the C=C bond in C₂Cl₄. A special reactivity of iron defect sites for C₂Cl₄ is observed through the production of associated FeCl₂ species which desorb via zero-order kinetics with an activation energy of 44.8 ± 8.5 kcal/mol, the sublimation enthalpy of FeCl₂.     

NO adsorption and thermal behavior on Pd surfaces. A detailed comparative study

The adsorption and thermal behavior of NO on ‘flat’ Pd(111) and ‘stepped’ Pd(112) surfaces has been investigated by temperature programmed desorption (TPD), high resolution electron energy loss spectroscopy (HREELS), and electron stimulated desorption ion angular distribution (ESDIAD) techniques. NO is shown to molecularly adsorb on both Pd(111) and Pd(112) in the temperature range 100–373 K. NO thermally desorbs predominantly molecularly from Pd(111) near 500 K with an activation energy and pre-exponential factor of desorption which strongly depend on the initial NO surface coverage. In contrast, NO decomposes substantially on Pd(112) upon heating, with relatively large amounts of N₂ and N₂O desorbing near 500 K, in addition to NO. The fractional amount of NO dissociation on Pd(112) during heating is observed to be a strong function of the initial NO surface coverage. HREELS results indicate that the thermal dissociation of NO on both Pd(111) and Pd(112) occurs upon annealing to 490 K, forming surface-bound O on both surfaces. Evidence for the formation of sub-surface O via NO thermal dissociation is found only on Pd(112), and is verified by dissociative O₂ adsorption experiments. Both surface-bound O and sub-surface O dissolve into the Pd bulk upon annealing of both surfaces to 550 K. HREELS and ESDIAD data consistently indicate that NO preferentially adsorbs on the (111) terrace sites of Pd(112) at low coverages, filling the (001) step sites only at high coverage. This result was verified for adsorption temperatures in the range 100–373 K. In addition, the thermal dissociation of NO on Pd(112) is most prevalent at low coverages, where only terrace sites are occupied by NO. Thus, by direct comparison to NO/Pd(111), this study shows that the presence of steps on the Pd(112) surface enhances the thermal dissociation of NO, but that adsorption at the step sites is not the criterion for this decomposition.     

Evidence for bicarbonate formation on vacuum annealed TiO2(110) resulting from a precursor-mediated interaction between CO2 and H2O

The reaction of CO₂ and H₂O to form bicarbonate (HCO⁻₃) was examined on the nearly perfect and vacuum annealed surfaces of TiO₂(110) with temperature programmed desorption (TPD), static secondary ion mass spectrometry (SSIMS) and high resolution electron energy loss spectrometry (HREELS). The vacuum annealed TiO₂(110) surface possesses oxygen vacancy sites that are manifested in electronic EELS by a loss feature at 0.75 V. These oxygen vacancy sites bind CO₂ only slightly more strongly (TPD peak at 166 K) than do the five-coordinated Ti⁴⁺ sites (TPD peak at 137 K) typical of the nearly perfect TiO₂(110) surface. Vibrational HREELS indicates that CO₂ is linearly bound at the latter sites with a ν_a(OCO) frequency similar to the gas phase value. In contrast, oxygen vacancies dissociate H₂O to bridging OH groups which recombine to liberate H₂O in TPD at 490 K. No evidence for a reaction between CO₂ and H₂O is detected on the nearly perfect surface. In sequentially dosed experiments on the vacuum annealed surface at 110 K, CO₂ adsorption is blocked by the presence of preadsorbed H₂O, adsorbed CO₂ is displaced by postdosed H₂O, and there is little or no evidence for bicarbonate formation in either case. However, when CO₂ and H₂O are simultaneously dosed, a new CO₂ TPD state is observed at 213 K, and the 166 K state associated with CO₂ at the vacancies is absent. SSIMS was used to tentatively assign the 213 K CO₂ TPD state to a bicarbonate species. The 213 K CO₂ TPD state is not formed if the vacancy sites are filled with OH groups prior to simultaneous CO₂+H₂O exposure. Sticking coefficient measurements suggest that CO₂ adsorption at 110 K is precursor-mediated, as is known to be the case for H₂O adsorption on TiO₂(110). A model explaining the circumstances under which the proposed bicarbonate species is formed involves the surface catalyzed conversion of a precursor-bound H₂O–CO₂ van der Waals complex to carbonic acid, which then reacts at unoccupied oxygen vacancies to generate bicarbonate, but falls apart to CO₂ and H₂O in the absence of these sites. This model is consistent with the conditions under which bicarbonate is formed on powdered TiO₂, and is similar to the mechanism by which water catalyzes carbonic acid formation in aqueous solution.     

A NEXAFS and UPS study of the adsorption of tetrachloroethylene, trichloroethylene, iso-, cis-and trans-dichloroethylene on platinum surfaces at 95 K: multilayers and monolayers

UPS spectra of multilayers and monolayers of chlorinated ethylene molecules condensed or adsorbed on Pt(111) and Pt(110)(1 × 2) surfaces at 95 K are in close agreement. It shows that adsorption of these molecules is not dissociative and that they are weakly bonded to the surfaces in the monolayer range. NEXAFS has also been used in the case of Pt(111) samples. The multilayer spectra compare well with the analogous spectra of the fluorinated molecules, and the spacings between the various C 1s levels agree with the corresponding values for the π^* transitions. This shows that these resonances involve C 1s core level transitions to unoccupied π^* levels, which are located at almost the same energy in the excited state. The polarization dependence of the synchrotron light indicates that for monolayers the C=C axis and molecular planes arc parallel to the surface. The π^* resonance widths confirm that the adsorption of all chlorinated molecules is not dissociative at low temperatures, as suggested from the catalytic destruction of unsaturated chlorinated C2 compounds which occurs only at high temperatures. Above 120–130 K and under vacuum, desorption depletes the surface without conversion to a more firmly bonded species, a process occurring under catalytic conditions.     

Sodium adsorption on a Ge(100)-(2 × 1) surface

The adsorption of Na on a Ge(100)-(2 × 1) surface has been studied by means of AES, LEED, EELS, TPD and work-function measurements. In the submonolayer coverage region the coverage dependencies of the desorption activation energy E(Θ) and desorption frequency v(Θ) have been determined using the threshold TPD method. Our experimental data show that after the completion of the first Na layer, 3D crystallites develop on the Na/Ge(100) surface (Stranski-Krastanov growth mode). For Θ > 1 ML, formation, followed by decomposition of a certain Na---Ge surface compound occurs in the temperature range 410–550 K.     

Adsorption and reaction of oxygen and deuterium on a Pd(110) surface, studied by Δφ and TDS

The co-adsorption of oxygen and deuterium at 100 K on a Pd(110) surface has been studied by measurements of the change in work function (Δφ) and by thermal desorption spectroscopy (TDS). When the surface with co-adsorbed species is heated, the adsorbates O and D react to form D₂O which desorbs from the surface at T > 200 K. The D₂O desorption peaks shift continuously to lower temperatures as the surface D coverage (θ_D) increases. The maximum production of D₂O is estimated to be 0.26 ML (1 ML = 9.5 × 10¹⁴ atoms cm⁻²), resulting from reaction in a layer containing 0.65 ML D and 0.3 ML O. The maximum work function increase caused by adsorption of D to saturation onto oxygen precovered Pd(110) decreases almost linearly with Δφ_O of the oxygen precovered surface. On a surface with pre-adsorbed D however, the maximum Δφ increase contributed by oxygen adsorption decreases abruptly at Δφ_D > 200 mV. This sharp change occurs at θ_D > 1 ML and is believed to be associated with the development of the reconstructed (1 × 2) phase of D/Pd(110).     

Potassium adsorption on graphite(0001)

Potassium adsorption on graphite has been studied with emphasis on the two-dimensional K adlayer below one monolayer. Data are presented for the work function versus coverage, high-resolution electron energy loss spectroscopy (HREELS) vibrational spectra of K-adlayers, low energy electron diffraction and ultraviolet photoemission spectroscopy (UPS) spectra at different coverages. The data provide information regarding the vibrational properties of the K-adlayer, the metallization of the adlayer at submonolayer coverages, and the charge transfer from the K adatoms to the graphite substrate. Analysis of the work function, HREELS, and UPS data provides a qualitatively consistent picture of the charge state of the K adatoms, where at low coverages, below a critical coverage θ_c (θ_c=0.2–0.3), the K adatoms are dispersed and (partially) ionized, whereas at θ>θ_c islands of a metallic 2×2 K phase develops that coexist with the dispersed a K adatoms up to θ=1. We show that it is possible to understand the variation of the work function data based on a two-phase model without invoking a depolarization mechanism of adjacent dipoles, as is normally done for alkali-metal adsorption on metal surfaces. Similarly, the intensity variation as a function of coverage of the energy loss peak at 17 meV observed in HREELS, and the photoemission peak at E_b=0.5 eV seen in UPS can be understood from a two-phase model. A tentative explanation is presented that connects apparent discrepancies in the literature concerning the electronic structure of the K adlayer. In particular, a new assignment of the K-induced states near the Fermi level is proposed.     

TPD study of the adsorption and reaction of nitromethane and methyl nitrite on ordered Pt–Sn surface alloys

The adsorption and reaction of the isomers nitromethane (CH₃NO₂) and methyl nitrite (CH₃ONO) on two ordered Sn/Pt(111) surface alloys were studied using TPD, AES, and LEED. Even though the Sn–O bond is stronger than the Pt–O bond and Sn is more easily oxidized than Pt, alloying with Sn reduces the reactivity of the Pt(111) surface for both of these oxygen-containing molecules. This is because of kinetic limitations due to a weaker chemisorption bond and an increased activation energy for dissociation for these molecules on the alloys compared to Pt(111). Nitromethane only weakly adsorbs on the Sn/Pt(111) surface alloys, shows no thermal reaction during TPD, and undergoes completely reversible adsorption under UHV conditions. Methyl nitrite is a much more reactive molecule due to the weak CH₃O–NO bond, and most of the chemisorbed methyl nitrite decomposes below 240 K on the alloy surfaces to produce NO and a methoxy species. Surface methoxy is a stable intermediate until 300 K on the alloys, and then it dehydrogenates to evolve gas phase formaldehyde with high selectivity against complete dehydrogenation to form CO on both alloy surfaces.     

Adsorption and reactions of ethanol on Mo2C/Mo(1 0 0)

The adsorption, desorption and dissociation of ethanol have been investigated by work function, thermal desorption (TPD) and high resolution electron energy loss (HREELS) spectroscopic measurements on Mo₂C/Mo(1 0 0). Adsorption of ethanol on this sample at 100 K led to a work function decrease suggesting that the adsorbed layer has a positive outward dipole moment By means of TPD we distinguished three adsorption states, condensed layer with a Tp = 162 K, chemisorbed ethanol with Tp = 346 K and irreversibly bonded species which decomposes to different compounds. These are hydrogen, acetaldehyde, methane, ethylene and CO. From the comparison of the Tp values with those obtained following their adsorption on Mo₂C it was inferred that the desorption of methane and ethylene is reaction limited, while that of hydrogen is desorption limited process. HREEL spectra obtained at 100 K indicated that at lower exposure ethanol undergoes dissociation to give ethoxy species, whereas at high exposure molecularly adsorbed ethanol also exists on the surface. Analysis of the spectral changes in HREELS observed for annealed surface assisted to ascertain the reaction pathways of the decomposition of adsorbed ethanol.     

Adsorption of hydrogen and deuterium on potassium-promoted Pd(100) surfaces

The adsorption of H₂ and D₂ has been studied on clean and K-promoted Pd(100) surfaces using thermal desorption, work function changes, ultraviolet photoelectron and Auger spectroscopy. The potassium adlayer significantly lowers the sticking coefficient (from 0.6 to 0.06 at θ_k = 0.2), and the uptake of hydrogen, but increases the desorption energy for H₂ desorption. Calculation showed that each potassium adatom blocks approximately 4–5 adsorption sites for H₂ adsorption. Atomization of hydrogen led to an increase of hydrogen uptake. The adsorption of potassium on the H-covered surface caused a significant decrease in the amount of hydrogen adsorbed on the surface (as indicated by less desorbing hydrogen below 500 K) and promoted the dissolution of H atoms into the bulk of Pd. The dissolved hydrogen was released only above 600–650 K. In the interpetation of the results the extended charge transfer from K-dosed Pd to the adsorbed H atoms and the direct interaction between adsorbed H and K adatoms are taken into account.     

Adsorption studies of trichloroethylene (TCE) on MgO(100)/Mo(100)

Employing ultraviolet photoelectron spectroscopy (UPS, He I), the more surface sensitive metastable impact electron spectroscopy (MIES) and temperature programmed desorption (TPD) measurements of the adsorption properties of the pollutant trichloroethylene (TCE) on thin MgO(100) films, grown on a Mo(100) single crystal, have been investigated. From TPD spectra of different coverages it is concluded that TCE interacts only weakly with MgO, which is attributed to physisorption. For increasing coverages a change from one peak to two peaks in the TPD spectra, one at higher, the second at lower temperatures with respect to the single peak is detected. Additionally, the observation of a local minimum for the work function (WF) for both MIES and UPS spectra is presented. Such a local minimum has been reported previously for the adsorption of metals with outer s valence electrons on transition metal substrates and adsorption of metals with outer s valence electrons on metal oxide films. Herein, we present the first WF minimum observed for a system of organic molecules adsorbed on an insulating surface. Two different models are discussed in order to understand the presented results.     

Methane mobility in carbon nanotubes

Quasi-elastic neutron scattering has been used to characterize the diffusivity of CH₄ molecules condensed in single-wall carbon nanotubes. It is shown that the two sites of adsorption, previously observed by adsorption volumetry and calorimetry measurements, correspond to a solid-like phase for the more strongly bound site at T<120 K and to a liquid-like component for the more weakly bound site at 70<T<120 K. The diffusion coefficients of the mobile molecules range between 3×10⁻⁷ to 15×10⁻⁷ cm² s⁻¹. The fraction of this viscous liquid diminishes as the temperature is decreased; the adsorbate is fully solidified at 50 K and below.     

Adsorption of oxygen on Au(111) by exposure to ozone

Atomic oxygen coverages of up to 1.2 ML may be cleanly adsorbed on the Au(111) surface by exposure to O₃ at 300 K. We have studied the adsorbed oxygen layer by AES, XPS, HREELS, LEED, work function measurements and TPD. A plot of the O(519 eV)/Au(239 eV) AES ratio versus coverage is nearly linear, but a small change in slope occurs at Θ_O=0.9 ML. LEED observations show no ordered superlattice for the oxygen overlayer for any coverage studied. One-dimensional ordering of the adlayer occurs at low coverages, and disordering of the substrate occurs at higher coverages. Adsorption of 1.0 ML of oxygen on Au(111) increases the work function by +0.80 eV, indicating electron transfer from the Au substrate into an oxygen adlayer. The O(1s) peak in XPS has a binding energy of 530.1 eV, showing only a small (0.3 eV) shift to a higher binding energy with increasing oxygen coverage. No shift was detected for the Au 4f_7/2 peak due to adsorption. All oxygen is removed by thermal desorption of O₂ to leave a clean Au(111) surface after heating to 600 K. TPD spectra initially show an O₂ desorption peak at 520 K at low Θ_O, and the peak shifts to higher temperatures for increasing oxygen coverages up to Θ_O=0.22 ML. Above this coverage, the peak shifts very slightly to higher temperatures, resulting in a peak at 550 K at Θ_O=1.2 ML. Analysis of the TPD data indicates that the desorption of O₂ from Au(111) can be described by first-order kinetics with an activation energy for O₂ desorption of 30 kcal mol⁻¹ near saturation coverage. We estimate a value for the Au–O bond dissociation energy D(Au–O) to be 56 kcal mol⁻¹.