Atomization, and Dissociation

On vaporizing MSe (M = La to Eu) from Knudsen cells (Mo and W cells) the solid phase in equilibrium with the vapor is essentially invariant in composition over the complete vaporization. The MSe composition is nearly stoichiometric based on X-ray and chemical analysis. Mass spectrometric analysis reveals that the vaporization process can be described by the following reactions, Nagai et al. [1]  

Third-law values for the sublimation enthalpy AH (sub) and the atomization enthalpy AH (at) both for 0 K and higher temperatures, all in kcal/mol, are  

Intensity ratios of the ion current, l(M)/I(MSe), were 15 to 20 for M = Nd, 90 to 95 for M = Sm, and 1000 for M = Eu, Gordienko et al. [2]. The reasons for the disagreements of the AH°(at) values for SmSe and NdSe are not clear. Along the series, the atomization enthalpy changes regularly with the atomic number of the rare earth metal whereas the sublimation enthalpy is insensitive to the number of f electrons [1]. 

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The coverage of water influences the mechanism for water dissociation. At low coverage, a single H2O molecule binds atop a single metal atom and dissociates homolytically to form adsorbed OH and H species ... 

The energy changes occurring at each step are shown alongside the arrows AHg andZliT are heats of sublimation (sodium to free atoms) and dissociation (chlorine molecules to free atoms) and, with the ionisation potential and electron affinity and 4. ), refer to one mole of material. By Hess s law (p. 168) the change of H is independent of the path and the heat of formation is thus ... 

Orthophosphoric acid has only one strongly ionising hydrogen atom and dissociates according to the scheme... 

Thennal dissociation is not suitable for the generation of beams of oxygen atoms, and RF [18] and microwave [19] discharges have been employed in this case. The first excited electronic state, 0( D), has a different spin multiplicity than the ground 0( P) state and is electronically metastable. The collision dynamics of this very reactive state have also been studied in crossed-beam reactions with a RF discharge source which has been... 

The CO oxidation occurring in automobile exhaust converters is one of the best understood catalytic reactions, taking place on Pt surfaces by dissociative chemisoriDtion of to give O atoms and chemisoriDtion of CO, which reacts with chemisorbed O to give CO, which is immediately released into the gas phase. Details are evident from STM observations focused on the reaction between adsorbed O and adsorbed CO [12]. 

McFeely and co-workers used soft x-ray photoelectron spectroscopy (SXPS) to measure the changes in binding energies of Si(2p) levels after slight exposure to fluorine atoms via dissociative chemisoriDtion of XeF2 [39]. Using synclirotron radiation at 130 eV as the source enabled extreme surface sensitivity. Since this level is split into a... 

Fast transient studies are largely focused on elementary kinetic processes in atoms and molecules, i.e., on unimolecular and bimolecular reactions with first and second order kinetics, respectively (although confonnational heterogeneity in macromolecules may lead to the observation of more complicated unimolecular kinetics). Examples of fast thennally activated unimolecular processes include dissociation reactions in molecules as simple as diatomics, and isomerization and tautomerization reactions in polyatomic molecules. A very rough estimate of the minimum time scale required for an elementary unimolecular reaction may be obtained from the Arrhenius expression for the reaction rate constant, k = A. The quantity /cg T//i from transition state theory provides... 

Chemical removal of surface material is produced through standard bond-breaking reactions. Typically chlorofluorocarbons (CECs) have been used, eg, CECl, CE2CI2, CE Cl, CE4, CHE, C2C1E. Eor example, CE dissociates into E atoms and fluorinated fragments of CE in a plasma ... 

A number of chemiluminescent reactions have been studied by producing key reactants through pulsed electric discharge, by microwave dissociation, or by observing the reactions of atoms and free radicals produced in the inner cone of a laminar flame as they diffuse into the flame s cool outer cone (182,183). These are either combination reactions or atom-transfer reactions involving transfer of chlorine (184) or oxygen atoms (181,185—187), the latter giving excited oxides. 

The oxidation of HC and CO must proceed in balance with the reduction of NO by CO, HC, or H2. For the NO removal reaction, a reductant is required. First NO is adsorbed on the catalyst surface and dissociates forming N2 which leaves the surface, but the O atoms remain. CO is required to remove the O atoms to complete the reaction cycle (53). 

The predominant species observed in SIMS spectra are singly charged atomic and molecular ions [51], However, inorganic and organic cluster ions can also be formed. If the sample consists of a simple single-component metal, then clusters such as M, M, etc., are observed in addition to M+ [52], Oxidation of the metal results in formation of MO ", MO/, M Oll", etc. The relative yield of MO+ to M+ depends on the bond dissociation energy of the oxide [52], For a two-component, oxidized metal, clusters of the type M/", M N, MjO, and M N O/ are observed [51]. 

In a recent paper [11] this approach has been generalized to deal with reactions at surfaces, notably dissociation of molecules. A lattice gas model is employed for homonuclear molecules with both atoms and molecules present on the surface, also accounting for lateral interactions between all species. In a series of model calculations equilibrium properties, such as heats of adsorption, are discussed, and the role of dissociation disequilibrium on the time evolution of an adsorbate during temperature-programmed desorption is examined. This approach is adaptable to more complicated systems, provided the individual species remain in local equilibrium, allowing of course for dissociation and reaction disequilibria. 

Eqs. (1,4,5) show that to determine the equilibrium properties of an adsorbate and also the adsorption-desorption and dissociation kinetics under quasi-equilibrium conditions we need to calculate the chemical potential as a function of coverage and temperature. We illustrate this by considering a single-component adsorbate. The case of dissociative equilibrium with both atoms and molecules present on the surface has recently been given elsewhere [11]. 

Coordination results in a lengthening of the S-N bond involving the coordinated nitrogen atom by ca. 0.1 A this N atom is also displaced out of the PN2S2 plane by 0.63 A. The Pt-N bond in 13.6 is weak and dissociation of the adduct occurs in solution. 

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