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.     

The effect of sample readsorption on thermal desorption experiments

The influence of sample readsorption on the desorption transients measured in thermal desorption experiments is discussed. In particular, shifts in the temperature of the maximum of the rate of change of the number of sorbed atoms for first and second order desorption transients are calculated in the approximation of small (βτ/ΔW where β is the sweep rate, τ the vacuum time constant and ΔW the width (K) of the desorption transient. Evidence of significant distortion when sample readsorption is moderate is given from numerical solutions of the appropriate equation. A framework is proposed in which activation energies for desorption can be calculated in the presence of non-negligible sample readsorption.     

Thermal desorption of metals from tungsten single crystal surfaces

After a short review of the experimental methods used to determine desorption energies E and frequencies v the assumptions underlying the theoretical analysis of the experimental data are discussed. Then recent experimental results on the flash desorption of Cu, Ag and Au from clean, well characterized W {110} and {100} surfaces are presented and analysed in detail, in particular with respect to the coverage dependence. The results obtained this way clearly reveal the limitations of previous analysis methods and of the experimental technique per se (such as structure and phase changes below and in the temperature region in which desorption occurs). Furthermore the need for more theoretical work to understand the large changes of v and E with coverage and the so-called “compensation effect”, i.e. the relation between In v and E, becomes evident.     

A comparison of desorption rate parameters obtained by the variation of heating rate and the threshold temperature programmed desorption methods

Coverage dependent activation energies and preexponential factors for desorption of CO on Ni(110) have been obtained by the variation of heating rate and the threshold temperature programmed desorption (TTPD) methods. With the TTPD technique the initial coverage was varied by an adsorption and a desorption method. All three measurements yield nearly identical rate parameters at low coverages, but significant deviations are observed for higher coverages. In particular, abnormally low binding energies are observed in TTPD analysis of high coverages surfaces, where coverage is controlled by adsorption. These anamolous binding energies are attributed to desorption of CO which has not equilibrated to the tight binding sites on the surface. Better agreement is obtained between the variation of heating rate results and those obtained from TTPD analysis of surfaces where coverage is controlled by desorption. Remaining differences are considered to be a result of fundamental limitations in the TTPD technique as shown by comparison of the results with a calculated correction factor.     

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.     

Detailed balancing and quasi-equilibrium in the adsorption of hydrogen on copper

The apparent equilibrium relationship between adsorption and desorption distributions measured in recent molecular beam experiments is discussed. Through an equilibrium synthesis and the application of detailed balancing, the energy and angle dependence of the dissociative adsorption probability of hydrogen on copper is shown to predict the non-cosine angular distributions of desorption. In a similar construction, the velocity distribution of H₂ desorbing from nickel predicts the peaked desorption angle distributions observed. The implications of the apparent local equilibrium and detailed balancing at low pressure on the mechanism of surface catalyzed isotopic exchange and on the dynamics of chemical processes on surfaces are discussed.     

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.     

Influence of ultrasound on adsorption processes.

One of the most popular means for removal of organic water pollutants found in small concentrations is by adsorption. An important step in adsorption processes is the regeneration of the adsorbent as it does not only affect the adsorption-desorption cycle but also the expenses of the following process steps. State of the art regeneration of adsorbent resins is done by chemical methods. These require the use of organic solvents or inorganic chemicals and involve a difficult secondary separation step. Therefore, industry seeks for alternative methods to regenerate exhausted adsorbents. One of the discussed regeneration methods is desorption by ultrasound. Ultrasound does not only promote desorption but also enhances the mass transfer of sorption processes. We discuss the arising problems and basic effects when applying ultrasound during sorption processes in order to show the potentials of this desorption process. The focus is laid in particular on the influence of ultrasound frequency and intensity.     

Transient studies of carbon monoxide oxidation over platinum catalyst

The applicability of transient techniques to the study of the catalytic oxidation of carbon monoxide is discussed. It is shown that at 100–150°C adsorption and desorption equilibria between the gas phase and catalyst cannot be assumed, and an elementary step formulation may be used to predict both multiple steady states and transient behaviour.     

Vibrational excitation dynamics in photodesorption

Quantum state specific detection of photodesorbed molecules enables measuring their final state distributions in the translational and internal degrees of freedom, which contain a wealth of information about the desorption mechanism and dynamics. Vibrational state populations are of particular interest because of the information they contain about the lifetime and nature of the electronic excited states responsible for desorption. The measured vibrational distributions for nondissociative photodesorption of diatomic molecules tend to resemble Boltzmann distributions with temperatures of 600–1200 K for desorption from metal surfaces, and 1700–2000 K for semiconductors and oxides. Two-dimensional quantum dynamics calculations of the desorption process show that these vibrational distributions can be reproduced only if the intramolecular equilibrium bondlength in the electronic excited state is remarkably similar to that of the ground state. In particular, the results are inconsistent with a desorption mechanism in which the intramolecular bondlength change upon excitation is similar to that of electron capture in the gas phase.     

Adsorption of CO on W(001) by high resolution electron spectroscopy

The vibrational modes induced by CO on W(001) at temperatures ? 350 K are detected by means of electron energy loss spectroscopy with resolution in the 6–7 meV range. Two β adsorption regimes are identified depending on coverage. Heating at various increasing temperatures reveals coverage dependant irreversible surface structure modifications. The β spectra after adsorption or desorption are discussed in terms of the usual questions of multiple β states, dissociation, and reconstruction. The α₁ and α₂ states are detected both by their WC and CO frequencies. A small signal is assigned to a new a-state, named α₃, which may explain some thermal desorption results.     

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 states of C<Subscript>60</Subscript> fullerene molecules in polymer matrices

Polymer-C₆₀ fullerene composite coatings are studied using thermal desorption mass spectrometry. It is found that thermal desorption spectra of C₆₀ fullerene molecules can exhibit several resolved peaks (at a specified heating rate) corresponding to thermal desorption states. The relative intensity of the thermal desorption peaks depends on the procedure used for preparing the composite coatings, in particular, on the time of sedimentation of the polymer-fullerene suspension. The occurrence of different stages in thermally stimulated desorption of C₆₀ fullerene molecules is explained by the fact that the fullerene molecules can exist in several phase states characterized by different densities and degrees of ordering in the polymer matrix.     

Adsorption/desorption behavior of protein on nanosized hydroxyapatite coatings: A quartz crystal microbalance study

Protein separation relates closely to the interactions between proteins and various kinds of adsorbents. To obtain a direct and comprehensive understanding of the protein interaction at the solid/solution interface, quartz crystal microbalance (QCM) technique was employed to in situ investigate the adsorption process of bovine serum albumin (BSA) on nanosized hydroxyapatite coatings, and factors affecting its adsorption such as pH, solution ionic strength and temperature were discussed in detail. The adsorption kinetic parameter and the desorption of adsorbed BSA caused by phosphate buffer solution (PBS) introduction were investigated and discussed as well, and an adsorption/desorption mechanism has been proposed. The obtained information suggests that QCM is a useful method for monitoring the adsorption/desorption behavior of BSA on nanosized hydroxyapatite coating.     

The coadsorption of carbon monoxide and deuterium on potassium predosed Ni(100)

The effect of potassium on the coadsorption of carbon monoxide and deuterium on Ni(100), with particular attention given to the low temperature Σ-CO and Σ-D₂ desorption states, has been studied by thermal desorption spectroscopy. Potassium shifts the Σ state peak temperatures to higher values and attenuates their intensities. The effects of potassium on the higher temperature carbon monoxide and deuterium desorption states are similar to the effects observed when one or the other is absorbed alone. A model is presented for the conversion of carbide to graphite on transition metal surfaces and for a possible role which potassium may play.     

Local bond breaking via STM-induced excitations: the role of temperature

We discuss the influence of temperature on local bond breaking through multiple vibrational excitations induced by inelastic tunneling in the STM. We focus on hydrogen desorption from the H---Si(111) and H---Si(100) systems, but the results are general. The substrate temperature affects the desorption yield in two important ways: first, lowering the temperature increases the H---Si vibrational energy relaxation time, resulting in a higher effective adsorbate temperature and an increased desorption yield. Second, lowering the substrate temperature decreases the dephasing rate of the H---Si modes (manifested by a decrease of the infrared absorption linewidth), which then reduces the rate of incoherent (Förster) vibrational energy transfer away from the Stark-shifted H---Si mode under the tip. This increases the localization of the vibrational energy and enhances the probability for multiple vibrational excitation and desorption. Finally, we discuss the possible implications of our findings on the mechanism of MOS device degradation by hot electrons.     

Desorption rates and mechanisms for dissociatively chemisorbed molecules

The desorption kinetics of dissociatively chemisorbed diatomic molecules are examined from a kinetic-modeling point of view. A comparison is made between a one-step process, resulting in the usual second-order kinetics, and a two-step process which takes into account explicity recombination of atoms and subsequent desorption of molecules. The kinetics from the two-step mechanism are found to be equivalent to second-order desorption with a coverage-dependent activation energy which, in many cases, is a linear function of coverage. The two-step process leads to second-order kinetics with a constant activation energy only for special values of the model rate parameters, or if chemisorption is activated. The steady onate approximation for the intermediate in the two-step process is often adequate, but the transient period during which a steady state is reached also contains important kinetic information. The implications of these results for desorption kinetics and molecular beam reaction experiments are discussed. Work performed under the auspices of the Office of Basic Energy Sciences of the Department of Energy     

Statistical analysis and model determination for thermal desorption spectra: Nitrogen on tungsten

Thermal desorption spectra of nitrogen chemisorbed on tungsten have been examined in order to discriminate between two simple alternative kinetic models for the desorption process. Both these models consider the β₂ desorption as a second order process, while for the β₁ peak they assume first order and second order kinetics, respectively. The maximum likelihood method has been used for fitting the spectra and the resulting kinetic parameters have been discussed through the analysis of the correlation matrix. The strict correlation which exists between activation energies and frequency factors prevents effective discrimination and should diminish confidence in too simple and rigid models.     

Desorption stimulated by adsorption

Acceleration of desorption is possible due to the transfer of vibration energy in the adsorption well from adsorbing to desorbing particles. This effect is named “desorption stimulated by adsorption”. In the present paper, criteria for the existence of stimulated desorption are derived and the qualitative features of stimulated desorption are discussed.