Thermal desorption of interacting molecules from mixed adlayers

A Monte Carlo simulation method is used to study thermal desorption of gas molecules from mixed adlayers containing two species. The effects of lateral interactions among adatoms and initial surface coverage on the desorption spectra are examined. It is shown that attractive lateral interactions lead to sharper peaks and that desorption occurs at high temperatures, whereas repulsive interactions lead to multiple peak spectra. It is found that interactions between unlike molecules affect the spectra for species with lower desorption energy only, and that the two species desorb together only in certain cases. Lateral interactions also affect the desorption kinetics significantly and very different behavior for the two species may be obtained.     

Monte Carlo simulations of controlled rate thermal analysis spectra: The influence of surface energetic heterogeneity and lateral interactions between adsorbed molecules

Monte Carlo simulations of controlled rate thermal analysis (CRTA) spectra are presented. Calculations are performed for a few kinds of energetically heterogeneous surface, characterized by symmetrical and unsymmetrical desorption energy distributions. It is shown that the shape of the CRTA spectra corresponds with the shape of integral form of desorption energy distribution on the surface. A simple way of estimation of the upper limit of desorption energy is proposed. The influence of lateral interactions on the shape of the CRTA spectra is also discussed. It is stated that strong attractive interactions cause the desorption temperature to be constant. As a result of repulsive interactions two steps on the desorption temperature curve are observed. For the case of repulsive interactions between the nearest-neighbors and next-nearest-neighbors the CRTA spectra do not reveal the existence of multiple steps, contrary to the classical thermodesorption spectra (TPD) where, in such a case, multiple peaks are observed.     

Effect of diffusion of adsorbed particles into the solid on thermal desorption spectra

The effect of diffusion of adsorbed particles into the solid on thermal desorption spectra is investigated theoretically. The thermal desorption spectra are calculated for associative desorption. Adsorbed particles are considered as a Langmuir gas or as a lattice gas with lateral interactions.     

Sequential desorption of dimers from square lattices: A novel mechanism for phase transitions

This work describes a novel mechanism for phase transitions during desorption, involving the formation of lattice size dependent intermediate states when there is enough adsorbate mobility. Monte Carlo simulations are performed to analyze the mechanism of the thermal desorption for adsorbed homonuclear dimers on two-dimensional square lattices. The lattice-gas model with nearest-neighbor repulsive interactions between particles is implemented to study the cases of mobile (with diffusion) and immobile desorption. The number of peaks for the immobile desorption spectra is related to the connectivity of the adsorbed species for both monomer and dimer molecules. However, for the case of mobile desorption, the spectra give information about the desorption mechanism, which differs significantly for monomers and dimers, particularly when the initial temperatures correspond to the critical region.     

The compensation effect and the manifestation of lateral interactions in thermal desorption spectroscopy

If thermal desorption spectra are analysed in terms of the Polanyi-Wigner equation lateral interactions between the adsorbates may lead to coverage (θ) dependent pre-exponential factors, v, and activation energies of desorption, E. Evidence from the literature shows that E and v often satisfy the well-known compensation effect 1n v(θ) = bE(θ) + c, with constants b and c. Here we insert this compensation effect into the rate equation of desorption and simulate spectra which illustrate the influence of the compensation effect in thermal desorption spectra of adsorbate systems where pairwise lateral interactions prevail.     

Theoretical analysis of thermal desorption

The physical causes of multiple peaks obversed in thermal desorption spectra are investigated using chemical rate equations and a two binding site model. An exclusion term is introduced which adds the repulsive adatom-adatom hard sphere interactions into the rate equations. Finally, using a fundamental statistical mechanical approach it is demonstrated how general adatom-adatom interactions can be incorporated into the expression for the surface delay time.     

Lateral interactions and nonequilibrium in adsorption and desorption. Part 1. Experimental results for (2 × 2)-(3O + NO)/Ru(0 0 1)

Extending earlier vibrational spectroscopy and thermal desorption measurements on this system, its geometrical structure and its adsorption/desorption kinetics have been investigated in detail. The adsorption and desorption of NO proceed, with little influence on the 3O template, on its (2 × 2) lattice of empty hcp sites. The desorption kinetics, with splitting of the TPD spectra into two peaks, are far from the expected behavior for independent sites. Also, the vibrational band structure shows dispersion beyond dipole-dipole interactions. So, despite the quite large NO-NO distance and their screening by the O atoms, there is clear evidence for static and dynamic lateral interactions which should be extractable from the data. A qualitative analysis suggests that these interactions are due to elastic coupling between the positions and vibrations, respectively, of the O and NO adsorbates. However, quantitative conclusions cannot be drawn directly as the kinetic data cannot be interpreted in a quasiequilibrium approach, as would be the normal procedure, due to the presence of strong nonequilibrium effects. The lack of internal equilibration is presumably caused by slow diffusion. The results are sufficiently complete and detailed to justify the effort of theoretical modeling with the aim to quantitatively describe both the lateral interactions and the nonequilibrium effects.     

Rate processes on a square lattice with top and bridge sites for adsorption

Thermal desorption and surface diffusion on a square lattice are analyzed in the framework of the lattice-gas model taking into account top and bridge sites for adsorption (with the difference in the partition functions for frustrated translations on these sites) and the nearest-neighbour and next-nearest-neighbour adsorbate-adsorbate interactions. For physically reasonable sets of parameters, the calculated thermal desorption spectra are rather insensitive with respect to the relative population of top and bridge sites. The same conclusion is applicable to the coverage dependence of the chemical diffusion coefficient. General results and discussion are accompanied by simulation of CO adsorption on Ni(001).     

A photoemission and thermal desorption study of carbon monoxide and oxygen adsorbed on platinum

We have studied the changes in the photoelectron spectra of platinum (for photon energies of 21.2 and ? 10.2 eV), in conjunction with thermal desorption experiments, for coverages of carbon monoxide and oxygen of up to ~0.25 monolayer (saturation coverage at room temperature). Based on a comparison of the photoemission and thermal desorption results, we suggest that the less tightly bound of the two adsorption states observed in the thermal desorption data is due to adsorbate-adsorbate interactions. We further suggest that a relatively delocalized chemisorption bond plays an important role in this interaction.     

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.     

Interactions between NO and CO on Pt(111)

Temperature programmed desorption (TPD) of coadsorbed NO and CO on Pt(111) shows that no reaction occurs (less than 2%) up to the desorption temperature of NO. At 100 K, adsorption is competitive, but neither gas displaces the other from the surface. Coadsorbed CO causes the NO desorption temperature to be lowered by as much as 100 K, but NO does not affect the CO desorption temperature. TPD spectra for NO depend on which gas is adsorbed first, indicating that equilibrium between species is not established on the surface during desorption. Electron energy loss spectra show that the vibrational spectrum of each gas is only weakly affected by the other. When NO is adsorbed first, CO does not affect the ratio of bridged and terminal NO but lowers the frequencies of the bridged NO by approximately 50 cm^?1 and lowers the intensities of vibrational peaks of both species by a factor of about four. When CO is adsorbed first, the ratio of terminal to bridged NO increases for given coverage of NO, and the frequency of the bridged NO remains at the pure NO value. These results are explained in terms of CO island formation, repulsive interactions between NO and CO, and low adsorbate mobilities.     

The effect of ferroelectric polarization on the interaction of water and methanol with the surface of LiNbO3(0 0 0 1)

Water and methanol temperature programmed desorption (TPD) measurements were performed on the positive (c⁺) and negative (c⁻) surfaces of poled ferroelectric lithium niobate (LiNbO₃) single crystals. The results indicate that the molecule-surface interactions are both coverage and polarization-dependent. From a comparison of the TPD spectra for the positive and negative surfaces, it is shown that the desorption temperatures of water and methanol are consistently lower on the negative surface by 15 K and 20 K, respectively. The TPD spectra were simulated using the Polanyi-Wigner equation with a coverage-dependent energy term. These calculations show that the polarization dependence of the desorption temperature is due to a difference in the zero-coverage desorption energies on the two surfaces equal to a few kJ per mole. The mechanism for the polarization effect is explored with in situ pyroelectric voltage measurements, which indicate that a surface voltage of ±2 mV develops in the LiNbO₃(0 0 0 1) samples during TPD measurements. The magnitude of the pyroelectric-induced surface charge is heating rate dependent.     

Zero-order desorption kinetics based on phase equilibrium

The zero-order desorption kinetics is described for adsorbate systems in which three phases are in equilibrium and first-order desorption kinetics is assumed for the desorption from the topmost phase. The calculated results represent typical features of the observed zero-order desorption spectra. The possibility of specifying the phase boundaries from the thermal or isothermal desorption spectra is proposed. The relationship between the thermal or isothermal desorption processes and trajectories in the phase diagram is also discussed.     

Thermal desorption of deuterium from MPG-8 and NB31 carbon materials after plasma irradiation

Deuterium retention in fine-grained graphite MPG-8 and carbon fiber composite NB31 after irradiation in a magnetron discharge and plasma-beam discharge was investigated by thermal desorption spectroscopy. The thermal desorption spectra reveal deuterium yield in a wide temperature range: from 400 to 1400 K. The spectra of the materials studied are qualitatively similar; however, the relative amplitudes of the thermal desorption peaks are different.     

Nonequilibrium thermal desorption spectra of dissociatively adsorbed molecules

Thermal desorption spectra of dissociatively adsorbed molecules with a low surface mobility and with lateral interaction between adspecies are studied. A method for the solution of a system of master equations for a nonequilibrium system is developed. It is shown that the model reproduces qualitative features of the thermal desorption spectra of CO/W(100).     

Coadsorption of oxygen and carbon monoxide on tungsten: Desorption spectra,electron stimulated desorption and field emission microscopy

CO/W desorption spectra are characterized by an α state and multiple β states; using electron stimulated desorption (ESD) the α state was shown to comprise two sub-states, α₁ and α₂. In this paper the consecutive interactions of O₂ and CO on W are investigated using ESD, flash desorption and field emission microscopy (FEM).Desorption spectra show that the α-CO state is displaced by O₂, in two stages. The ESD probe provides an identification of the first stage with the removal of the α₁-CO state, and energy analysis of ESD ions reveals a large energy shift (～ ? 1.5 eV) during O₂ coadsorption which can be attributed to an incresae in the α₁-CO WC bond length of ～ 0.15 Å. During this O₂-induced displacement, the two β peaks converge into a single peak at the β₁ position; this is ascribed to adatom interactions in the mixed O and C adlayer. Isotope exchange experiments with ²⁸CO and ³⁶O₂ reveal (i) no exchange in the α-CO states, and (ii) complete exchange in the β-CO states, which is consistent with dissociative adsorption in the latter. The amount of coadsorbed O₂ is estimated from these results, and from FEM data: a full monolayer of O adatoms can be coadsorbed on CO-saturated W, but CO pre-adsorption inhibits the formation of W oxides. The β₁-O₂ (ESD active) state also forms on the CO-covered surface: this state is identical in population, ESD cross section and ion energy distribution to β₁-O₂ on clean W, and retains its identity in the mixed layer (it does not undergo isotopic exchange). CO₂ desorption spectra from the mixed layer were also characterised, complete isotopic scrambling being observed.Pre-exposure of tungsten to O₂ inhibits CO adsorption: a monolayer of O₂ is sufficient to prevent CO adsorption, and at low O₂ coverages, every O₂ molecule preadsorbed prevents one CO molecule from adsorbing. Isotopic exchange is again complete in the β states, and a lateral interaction model for desorption kinetics, based on dissociative adsorption in the β-CO state, quantitatively describes the CO desorption spectra.     

A method for assessing the coverage dependence of kinetic parameters: Application to carbon monoxide desorption from iridium (110)

A method for analyzing thermal desorption mass spectra has been developed for determining the coverage dependence of the pre-exponential factor and the desorption energy in the Arrhenius equation for a one-step desorption process. The method, which involves variable heating schedules, is applicable to spectra in which several features appear. However, if the desorption process involves multiple steps, or if substantial desorption from multiple sites occurs for any one coverage, this method cannot be used. The method is applied to CO desorption from the (110) surface of Ir. Three features can be resolved in the desorption spectrum. Both the preexponential factor and the desorption energy vary strongly with coverage, and a compensation effect occurs.     

Thermal desorption of surface phosphorus on Si(100) surfaces

Thermal desorption of phosphorus on Si(100) surfaces has been investigated by varying the phosphorus coverage from zero to one monolayer (ML). The reaction path of phosphorus desorption is complicated and strongly dependent upon the phosphorus coverage. In the thermal desorption spectra, there are three apparent desorption peaks at 750, 850 and 1000°C. The entire phosphorus atoms on the surface desorb as P₂ through recombinative reactions irrespective of the desorption temperature and the coverage. In the lower coverages below 0.2 ML, the thermal desorption spectra are characterized by a single peak at 900°C which is considered to be the desorption from Si---P heterodimers. At higher coverages exceeding 0.2 ML, it is considered that three desorption schemes from P---P, Si---P dimers and defects coexist in the reaction stage.     

Chemisorption and reaction of NH3 on Ni(111)

The chemisorption of ammonia on Ni(111) has been investigated using LEED, thermal desorption, and angle-resolved photoemission. For exposures at 200 K, thermal desorption shows a coverage-dependent binding energy associated with dipole-dipole interactions. A (2 × 2) LEED pattern occurs at 2–4 L exposure. Time dependence of the LEED pattern and changes in the thermal desorption induced by the LEED beam indicate that the (2 × 2) pattern is due to a stable intermediate decomposition species. Using synchrotron radiation photoemission all three valence orbitals of ammonia have been observed for the first time. The energies of the ammonia-induced features in the photoemission (?22.0, ?11.0 and ?6.7 eV below the Fermi energy) and the observed symmatries positively identify the absorbed species as molecular ammonia. Additional structure observed in the photoemission spectra after electron bombardment is associated with the stable adsorbed intermediate.