Atomic phase data

Ozin et al. 107,108) performed matrix, optical experiments that resulted in the identification of the dimers of these first-row, transition metals. For Sc and Ti (4s 3d and 4s 3d, respectively), a facile dimerization process was observed in argon. It was found that, for Sc, the atomic absorptions were blue-shifted 500-1000 cm with respect to gas-phase data, whereas the extinction coefficients for both Sc and Scj were of the same order of magnitude, a feature also deduced for Ti and Ti2. The optical transitions and tentative assignments (based on EHMO calculations) are summarized in Table I. 

The substrate in these studies was restricted to be rigid, and Morse functions were used for the hydrogen-surface and two-body interactions. The parameters in the Morse functions were determined for single hydrogen atoms adsorbed on the tungsten surface by fitting to extended Huckel molecular orbital (EHMO) results, and the H2 Morse parameters were fit to gas-phase data. The Sato parameter, which enters the many-body LEPS prescription, was varied to produce a potential barrier for the desorption of H2 from the surface which matched experimental results. 

Silicide layers can also be grown on Pt surfaces.247 The preferential facets of epitaxial growth are the 111. Atom-probe data reveal that the stoichiometry of the silicide phase is Pt2Si, and the Pt-Pt2Si interface is also very sharp. However, a small fraction of silicon atoms can diffuse into the Pt matrix. Formation of silicide layers on a nickel emitter surface is much more complicated where silicide layers of varying stoichiometries are formed.246,247 Owing to the statistical nature of the atom-probe data, identification of all the silicide phases in a nickel silicide layer is at best uncertain. 

Before we discuss some field ion microscope and atom-probe studies of partitioning in alloys, let us touch briefly here upon some of the methods commonly used for analyzing atom-probe data. Obviously if a precipitate or an alloy phase can be distinguished from the matrix from the field ion image, the size can easily be found. For the composition the probe-hole may be aimed at the precipitate, or the alloy phase, and analyze the composition of this volume. If the precipitate is very small and the probe-hole is large, matrix atoms will be detected concurrently regardless of where the probe-hole is aimed. Another problem is aiming... 

Williams83 has recently considered the possibility of ion molecule reactions in alcohols, ketones, ethers, and esters. He postulates that the primary reactions of the parent ion are the inter- or intra-molecular abstraction of a hydrogen atom and the formation of a x-bond between oxygen and the adjacent carbon atom after homolytic scission of a bond to that adjacent carbon atom. After examining some of the liquid phase data on the radiolysis of these oxygen compounds it was concluded that such ion molecule reactions may be of importance in these systems also. 

Predictive equations based on literature values were determined by correlating sets of aqueous-phase data with either gas-phase data or o constants for the same compounds (Haag and Yao, 1992). A correlation of hydroxyl radical H-atom abstraction rate constants for substituented alkanes in water vs. the gas phase was developed. The 19 compounds were predominantly (82%) straight chained and contained four or fewer carbon atoms 18% were C5-C8 and a few were cyclic or branched hydrocarbons. Some chemicals deviated noticeably from the best-fit line and were then omitted from the correlation. Most of the rate constants lie within a factor of three of the regression line given by ... 

A detailed study of the oxidation of alkenes by O on MgO at 300 K indicated a stoichiometry of one alkene reacted for each O ion (114). With all three alkenes, the initial reaction appears to be the abstraction of a hydrogen atom by the O ion in line with the gas-phase data (100). The reaction of ethylene and propylene with O" gave no gaseous products at 25°C, but heating the sample above 450°C gave mainly methane. Reaction of 1-butene with O gives butadiene as the main product on thermal desorption, and the formation of alkoxide ions was proposed as the intermediate step. The reaction of ethylene is assumed to go through the intermediate H2C=C HO which reacts further with surface oxide ions to form carboxylate ions in Eq. (23),... 

The crystal structure of iodine azide is polymeric with longer N-I distances than in the gas phase, becanse the IN3 units are connected by hnear coordinated iodine atoms. Structural data of the halogen azides are suimnarized in... 

Other methods. Vaporization processes and thermochemical properties of condensed phases evaluated from gas phase data are also considered. Investigations by Knudsen effusion mass spectrometry generally mean the study of equilibrium reactions involving neutral molecules and/or atoms. This meaning has also been adopted for this article. The reader is referred to Sect. 5.5 for the study of ion-ion and ion-molecule equilibrium reactions by Knudsen effusion mass spectrometry. 

An account of the gaseous species observed by Knudsen effusion mass spectrometry in the eqilibrium vapor of metals, alloys, oxides, halides, and technical systems is given. The fundamentals and recent developments of this method are briefly reported. Dissociation and atomization enthalpies of selected gaseous species are tabulated. Accounts of the equilibrium studies by Knudsen effusion mass spectrometry in order to obtain thermodynamic properties for condensed phases from gas phase data are additionally given for the aforementioned materials. Table 8 shows as an example the enthalpies and Gibbs energies of formation for different solid intermetallic compounds. A special section (Sect. 

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