Analysis Of Thermal Desorption Spectra Research Articles

Revealing Hydrogen States in Carbon Structures by Analyzing the Thermal Desorption Spectra

An effective methodology for the detailed analysis of thermal desorption spectra (TDS) of hydrogen in carbon structures at micro- and nanoscale was further developed and applied for a number of TDS data of one heating rate, in particular, for graphite materials irradiated with atomic hydrogen. The technique is based on a preliminary description of hydrogen desorption spectra by symmetric Gaussians with their special processing in the approximation of the first- and the second-order reactions. As a result, the activation energies and the pre-exponential factors of the rate constants of the hydrogen desorption processes are determined, analyzed and interpreted. Some final verification of the results was completed using methods of numerical simulation of thermal desorption peaks (non-Gaussians) corresponding to the first- and the second-order reactions. The main research finding of this work is a further refinement and/or disclosure of poorly studied characteristics and physics of various states of hydrogen in microscale graphite structures after irradiation with atomic hydrogen, and comparison with the related results for nanoscale carbon structures. This is important for understanding the behavior and relationship of hydrogen in a number of cases of high energy carbon-based materials and nanomaterials.

Methodology and results of studying the states of hydrogen in graphene structures

In this work, an effective methodology of analysis of thermal desorption spectra (TDS) of hydrogen in carbon materials was further developed and applied. The methodology makes it possible, from TDS data of one heating rate, to determine the activation energies and pre-exponential factors of the rate constants of desorption processes. The TDS data of Rajasekaran et al. for hydrogenated epitaxial graphenes with a diamond-like structure (as in graphane) were analyzed. Some new information was obtained about the states of hydrogen in such unusual structures. It was shown that about half of the hydrogen contained in the samples is localized in the interfacial regions (between graphene and the substrate), while the desorption of such hydrogen is limited by diffusion (with three activation energies (34 kJ/mol(H2), 45 kJ/mol(H2), and 68 kJ/mol(H2)) of hydrogen molecules in these regions to the lateral surfaces of the samples. It was also shown that the rest of the hydrogen is localized on the top surface of the samples, while desorption of hydrogen atoms occurs with two activation energies and frequency factors (174 kJ/mol(H), 2 × 1013 s−1, and 256 kJ/mol(H), 3 × 1018 s−1); the obtained quantities are comparable to the related characteristics for graphane.

Driving the Diffusion of Cesium Atoms of Graphite Monolayers on Metal Surfaces into Rhenium (1010)

It has been shown experimentally for the first time that diffusion of cesium n rhenium takes place at irradiation of atoms flux heated up to Т ≤ 1800 K -C. Analysis of thermal desorption spectra showed that cesium forms two high-temperature phases during adsorption of cesium on graphite monolayer in rhenium heated up to 1800 K. The first, intercalated phase starts filling immediately and grows up to maximum concentration. The second, diffusion phase appears with slight delay and grows incessantly during adsorption. It is for the first time that the Fick Equation has been applied for such kind of diffusion. Correspondences and average displacement L = f(Т,t) have been obtained at diffusion of surface cesium in rhenium in the interval of temperatures 1600 K Т ν = 5 × 1013 cm-2·s-1, U = +2000 V, T = 1700 K, effectiveness of cesium diffusion δCs in the system of Re(1010)-C was identified with application of suggested methods, which showed δCs = 0.45. Activation energies of Сs atoms diffusion in rhenium from rhenium were found and values Е1p = 5.65 ± 0.05 eV and Еp1 = 5.85 ± 0.05 eV were obtained respectively. Effect of electrical field on intercalation and cesium diffusion in Re-C was observed.

Reassessment of the molecular mechanisms for H2 thermal desorption pathways from Si(1−x)Gex(001)-(2×1) surfaces

One of the aims of temperature-programmed desorption experiments is to facilitate identification of molecular pathways for desorption. The authors provide a rigorous assessment of the difficulty of doing this for H(2)/Si((1-x))Ge(x)(100)-(2x1). An extensive series of density functional calculations using both cluster and slab methods is performed. The resulting desorption barriers are used to compute thermal desorption spectra. A mean-field approximation is used to treat the populations of the various adsites present on the surface. The authors find a number of significant results. First, slab and cluster calculations do not appear to predict consistent differences in desorption barriers between intradimer and interdimer channels. Second, they find that a germanium atom affects the desorption barrier significantly only if it is present at the adsite. A germanium atom adjacent to an adsite or in the second layer influences the desorption barrier negligibly. Both cluster and slab calculations consistently predict a decrease of approximately 0.3-0.4 eV per germanium atom at the adsite. Third, current analysis of thermal desorption spectra in the literature, although yielding good fits to experimental data, is not rigorous. The authors' calculated spectra can be fitted rather well by assuming, as in current analysis of experimental data, three independent second-order channels, even though the underlying molecular pathways used to calculate the spectra are considerably different. Fourth, the authors' results highlight the importance of treating the rearrangement of hydrogen and germanium atoms at the surface during the thermal desorption process. This is generally not taken into account in kinetics modeling of desorption spectra.

Application for Tsallis thermostatistics to half-width method used in thermoluminescence glow peaks in analysis of thermal desorption spectra

Several methods have been developed in the analysis of the thermoluminescence glow peaks, and in particular when applied to thermoluminescence (TL) dosimetry and TL dating of archaeological and geological minerals. Some of these methods are identical to those developed for the study of thermal desorption. Such a method is called half-width method where a shape factor is introduced. Recently TL decay and thermal desorption process have been studied using a nonextensive formalism, namely Tsallis thermostatistics. In the present work, we consider the half-width method because it is the easiest one to estimate the activation energy. Importantly, we give the generalized form of the activation energy for all order of kinetics. We also investigate the influence of the nonextensivity on the shape factor parameter and on the activation energy.

Application for Tsallis thermostatistics to half-width method used in thermoluminescence glow peaks in analysis of thermal desorption spectra

Several methods have been developed in the analysis of the thermoluminescence glow peaks, and in particular when applied to thermoluminescence (TL) dosimetry and TL dating of archaeological and geological minerals. Some of these methods are identical to those developed for the study of thermal desorption. Such a method is called half-width method where a shape factor is introduced. Recently TL decay and thermal desorption process have been studied using a nonextensive formalism, namely Tsallis thermostatistics. In the present work, we consider the half-width method because it is the easiest one to estimate the activation energy. Importantly, we give the generalized form of the activation energy for all order of kinetics. We also investigate the influence of the nonextensivity on the shape factor parameter and on the activation energy.

Chemical reactivity of the V–Pd(1 1 1) subsurface alloy: adsorption of CO

The reactivity of a V–Pd(111)(√3×√3)R30° subsurface alloy has been investigated, using the adsorption of CO as a test case, by photoelectron spectroscopy with synchrotron radiation of the valence and core states and by kinetic analysis of temperature-programmed thermal desorption spectra. The overall conclusion from this study is that the chemical reactivity towards CO is lowered on the subsurface alloy surface as compared to unmodified Pd(111). The photoemission data indicate that the room-temperature CO saturation coverage is reduced from 0.5 monolayer (ML) on the clean Pd(111) surface to ∼0.28–0.30 ML on the alloy surface, in excellent agreement with the thermal desorption results. This is explained in terms of a reduced CO adsorption energy, as revealed by the kinetic analysis of the thermal desorption spectra. The balance between threefold hollow and bridge CO adsorption sites at saturation coverage is changed in favour of the preferential occupation of hollow sites on the alloy surface. The observed modifications of the electronic structure of valence states upon alloying and on CO adsorption are well reproduced by a recent ab initio DFT study of CO on V–Pd alloys by Hirschl and Hafner [Surf. Sci. 498 (2002) 37].

Characterization of energetic topography of heterogeneous surfaces through the analysis of thermal desorption spectra

Monte Carlo simulations of thermal programmed desorption spectra (TPDS) are obtained and analyzed for general heterogeneous surfaces characterized by a dual-site-bond model, where the adsorptive energy topography is described in terms of a correlation length l 0. It is shown that the behavior of TPDS is such that the ambiguity arising in the characterization of the energetic topography through adsorption isotherms, where adsorbate–adsorbate interactions and topography effects compete with each other, is resolved in such a way that TPDS provide a more powerful characterization method.

Analysis of Thermal Desorption Spectra to Understand the Migration of Hydrogen in Tungsten

Tungsten and tungsten alloys are candidate plasma-facing materials for future fusion reactors due to their excellent thermal properties and sputtering characteristics. A Sandia National Laboratories experimental program investigated the retention and release characteristics of hydrogen isotopes in tungsten and tungsten doped with 1% lanthanum oxide. A single model based on a high recombination rate coefficient, enhanced diffusivity in the implant zone for high flux experiments, and a 1.4 eV trap was capable of simulating all of the data accumulated in the experimental program. In this report, the model is now applied to data obtained by other researchers examining hydrogen migration in tungsten. Almost without exception, the model was able to accurately duplicate the hydrogen isotope retention and release with the single variable of trap density.

Dynamic Effects of Promoters on Associative Desorption

The kinetics of the associative desorption of A(2) in the presence of coadsorbed promoter species B is studied under conditions of nonequilibrium by the analysis of thermal desorption spectra obtained by Monte Carlo simulation. A rich variety of behaviors is found and discussed with regard to the interplay between the concentrations of promoter species, the surface mobility of A and B, and the interparticle interactions.

Analysis of Thermal Desorption Spectra of Heterogeneous Surfaces

A proper analysis of thermal desorption spectra (TDS) can give important information about molecular properties of the adsorption system studied (bond energies of heterogeneous adsorption sites and their composition and structure, strengths of adspecies lateral interactions, role of adspecies mobility, etc.); however, this is complicated because all these factors are distributed over the total spectrum. To solve this problem, it is convenient to use a lattice model since this enables us to extend an approach of traditional chemical kinetics for nonideal reactionary systems in such a way that it jointly takes care of surface heterogeneity and adspecies lateral interactions. In this paper, this model is taken as a base to consider fundamental issues of TDS interpretation dealing with a heterogeneous distribution function of adsorption centers over bond energy, on one hand, and the number of peaks and shape of TDS, on the other hand. Moreover, we discuss the validity of phenomenological approaches to interpr...

Study of Tritium Retention in Graphite by Simulating First Wall Circumstances

Hydrogen retention property on isotropic graphite was studied by exposure with a high flux atomized D/T particles. From the analysis of thermal desorption spectra it is clarified that deuterium implanted to graphite existed in two different states, one was that in a trap site the other was that of C-D bond. The amount of deuterium retained in graphite was in proportion to a half power of total incident fluence and no saturation was observed up to 1026 atoms·m−2. The total hydrogen isotope retention in the sample exposed to atomized D/T particles with total incident fluence of 1025 – 1026 atoms·m−2 were in the range of 1 × 1022−4 × 1022 atoms·m−2.

Parameter determination of a thermal desorption model

A new method for the analysis of thermal desorption spectra is presented, based on the experimental peak maximum functions for temperatureT m(β) and pressureP m(β) and a rigorous mathematical treatment. The resonant heating rate βr is determined, satisfyingT m(βr)=T r, whereT r is the resonant temperature defined byA exp(−E d/(RT r))=1. Desorption energyE d and frequency factorA can be determined simultaneously with relatively high robustness towards statistical experimental errors as demonstrated by computer-simulated thermal desorption spectra.

Thermal desorption of strained monoatomic Ag and Au layers from Ru(001)

Model-independent or complete analysis of thermal desorption spectra suggests, in agreement with low-energy electron diffraction, that attractive lateral interactions between Ag atoms on a Ru(001) substrate lead to island formation for Ag coverages above 0.15 ML, with zeroth-order desorption kinetics and coverage-independent activation energy of desorption E, and preexponential factor ν. For the system Au/Ru(001), on the other hand, E and ν decrease monotonically with coverage, indicating that Au–Au interactions on Ru(001) are repulsive. For ΘAg<0.15 and ΘAu<1 the systems exhibit a clear compensation effect between E and ν.

The influence of CO on hydrogen sorption by Pd(111) single crystals

A novel method of calibration based on the solubility data of hydrogen in palladium is applied to both the analysis of thermal desorption spectra of hydrogen from Pd(111) and to studies on the influence of adsorbed carbon monoxide on the behaviour of adsorbed and absorbed hydrogen, at room temperature. For clean Pd(111) ( θ CO < 0.04 ML) the comparison of experimental and theoretical spectra shows that part of the hydrogen originally adsorbed on the surface dissolves. This hydrogen appears at higher temperatures as a diffusion tail. A strong influence of coadsorbed CO on the maximum hydrogen coverage was observed in the low-coverage region where usually the existence of isolated CO molecules on the surface can be expected. For hydrogen sorption comparison of experimental data with those calculated on the basis of the solution of Fick's second law for a plane sheet indicates that this process is solely diffusion controlled. On the other hand, a strong barrier for desorption of dissolved hydrogen is observed at θ CO = 0.33 ML. It is concluded that the influence of CO on the behaviour of dissolved hydrogen may be consistently explained if there is a CO-induced change in the polarization state of adsorbed hydrogen atoms.

Analysis of thermal desorption mass spectra obtained with exponential heating

An analysis of thermal desorption spectra obtained with non-linear healing of the sample is proposed. The paper is divided in two parts. Firstly, the calculation is performed for any heating function T( t) and, secondly, the results are applied to an exponential heating: T( t) = T a−( T a− T 0) exp( −α t). As in the linear case, in the first order of desorption, the temperature T p of the peak in the desorption rate is found to be independent of T 0 and of the initial coverage n 0, this second property being true for any continuous temperature change; while in the second order, T p depends on both T 0, and T 0.

“Physical” adsorption on single crystal zinc oxide

Weak “physical” adsorption of CO 2, CO, O 2, Xe and H 2 was studied on ZnO(101̄0) in a molecular flow system (UHV conditions, 95 ⩽ T ⩽ 241 K, 1.3 × 10 −6 ⩽ 1.3 × 10 −4 Pa). Freundlich type of adsorption isotherms is found. Surprisingly broad, distributions of adsorption energies are determined in a thermodynamic analysis of equilibrium coverages as function of pressure and temperature. From an additional peak shape analysis of thermal desorption spectra, distributions of activation energies of thermal desorption are also estimated.

An application of interactive computer graphics in experimental physics: the analysis of thermal desorption spectra

A modest computer graphics facility has been applied to the analysis of thermal desorption spectra. An outline is given of the procedure which permits the generation of a synthesized spectrum and its visual comparison with an experimental spectrum. Those aspects of the thermal desorption investigation which required a highly interactive analysis procedure and the essential characteristics of the interactive graphics system for this type of application are discussed. Incomplete mathematical models and the judgment and accumulated experience of the experimenter have been combined to extend the understanding of a complex phenomenon.

Analysis of thermal-desorption spectra using a computer graphics system

A computer graphics system has been programmed to allow experimental thermal-desorption spectra to be matched visually with the sum of up to twelve theoretical first-order desorption peaks. The system allows the resulting theoretical spectrum to be stored for later recovery or plotted along with the experimental spectrum using an X–Y plotter. The temperatures and amplitudes of the chosen theoretical peaks and the rms difference between the theoretical and experimental spectra are available for either display or printout. Analysis of a series of helium desorption spectra showed that peak temperatures could be determined to within ±0.8%, and that the main limitations were attributable to temperature uncertainties and noise in the experimental spectra.