Energy effusion mass spectrometry

The most reliable values for the bond energies of the majority of these molecules have been obtained by using Knudsen effusion mass spectrometry ( ). For some of them, notably the alkali dimers, and Group IIA and Group IIB dimers, as well as for some of their intermetallic diatomic molecules, optical spectroscopic methods have provided the best or only determination of dissociation energies ( 3, ). 

The examples given in this chapter illustrate the significance of recent experimental and theoretical molecular parameters on the reported dissociation energies that had previously been based on equilibrium measurements and on estimated molecular parameters. This additional information will permit the calculation of improved dissociation energies on the basis of the same experimental equilibrium data as typically obtained from Knudsen effusion mass spectrometry. 

Much theoretical work has been carried out on the lithium hydride molecule, which has become the workbench of the theoretical chemist (J ). Browne ( ), and Fraga and Ransil ( 3) have given the binding energy for the LiH ion by ab initio calculation Com-panion(j4) has applied the diatomic-in-molecule theory to the Li H and LiH. molecules and predicted the stabilities of these molecules. We have intensively studied the Li-H system by means of Knudsen effusion mass spectrometry, and identified all predicted molecules and ions as cited above(5), and reported the thermochemical properties of these gaseous species (, 2, ) ... 

Most of the gaseous species have been identified by the use of mass spectrometry. A variant of this technique, frequently used in inorganic gas phase chemistry, is Knudsen effusion mass spectrometry. Vaporization studies up to temperatures above 2500 K were carried out by this method and thermodynamic properties determined. The dissociation energies of numerous gaseous species were obtained in this way. 

The emphasis of this article is on the gaseous species observed in equilibrium high-temperature vapors and their dissociation energies. The fundamentals of Knudsen effusion mass spectrometry and recent methodic developments are described in Sect. 2. The results obtained by this method for metals and alloys, oxides, metal halides, and technical materials are reviewed in the following four sections. The results are discussed considering, in addition, relevant studies by... 

The maximum ionization cross sections of the atoms were computed by Otvos and Stevenson [69] as well as Mann [70, 71]. The cross sections determined by Mann are available for different ionization energies between 0 and 220 V with steps of 1 eV [72]. Mann s data are generally used in Knudsen effusion mass spectrometry. According to Drowart [11], Mann s maximum values agree within some 20 to 50% with the experimental ones the agreement is generally improved if the experimental and Mann s cross sections are com-... 

The polyatomic species Li4, Naj, Cuj, Agj, and Auj were detected for the first time by Knudsen effusion mass spectrometry, and their enthalpies of dissociation were determined [80, 127, 180]. The study of the molecule Li by Wu [180] complements his earlier investigation of Lij and Lij [223]. The binding energies of alkali metal dimers and polymers obtained by different... 

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. 

In the field of metals or alloys gaseous complex Zintl compounds were identified for the first time. The combination of the results of all electron ab initio computations and Knudsen effusion mass spectrometry have made the determination of improved values for bond energies possible. Moreover, bond energies for transition metal trimers were obtained for the first time. 

The vapour species above barium oxide has been investigated by means of effusion mass spectrometry from 1643 to 1803 K. Reaction enthalpies yield second- and third-law standard heats of formation of —26.5 and —28.3 kcal mol, respectively, for BaO (g). A third-law dissociation energy of 129.7 kcal mol was derived for BaO. Another determination gives a value of DS(BaO) =... 

An ionization threshold (appearance potential of PtJ measured by Knudsen-effusion mass spectrometry) was 7.3 + 2.OeV [35]. One-electron energies of the highest occupied MO were 3.74 and 12.22 eV from pseudopotential and EH calculations, respectively [31]. 

Gingerich 1989) and La2C n = 1-6, 8) (Pelino et al. 1984, Pelino and Gingerich 1989), as well as ScC (n = 2-6) (Haque and Gingerich 1981), have been observed in the equilibrium vapor above the metal graphite systems at high temperatures. Their thermodynamic characterizations, such as atomization energies etc., have been determined by the Knudsen effusion technique combined with mass spectrometry. The structure of the molecules have been inferred from the assumed models and consistency of the thermodynamic results based on various methods of evaluation. 

The dissociation energies of some monosulfides were determined by effusion methods and by mass spectrometry of their vapors (Cater et al., 1965 Coppens et al., 1967 Smoes et al., 1969 Stretz et al., 1975). The thermodynamic properties of the gaseous monosulfides were deduced. 

The dissociation energies of several diatomic MSe molecules, namely with M = Sc, Y, La to Gd, Ho, and Lu, have been determined, as for the rare earth sulfides, by the Knudsen effusion method combined with mass spectrometry. Also, as for the rare earth sulfides, the high temperature equilibria were of two main types, either sublimation equilibria where two vaporization processes occur simultaneously ... 

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