Dimers alkali metal vapors

The simplest kind of system where chemical reaction is present is an alkali metal vapor, where dimerization of atoms takes place, with a temperature- and pressure-dependent composition. This fact, allied to the high-temperature domain of the application of these vapors, makes the determination of the transport properties of these systems very challenging from a theoretical point of view. 

The calculation of the transport properties of alkali metal vapors involves the use of the kinetic theory of binary reacting mixtures and its application to a mixture of monomers and dimers. The use of the theory requires the evaluation of the monomer properties, the establishment of some rqrproximations to calculate the viscosity and the... 

Effect of molecular diffusion and vapor-phase chemical reactions Liquid metal vapors consist of molecules and gaseous atoms. Working with alkali metals, Ewing et al. (1967) found that the molecules are principally dimers and tetramers. The... 

Metal atoms with a simple electronic structure show very different effects when we compare vapor-phase dimers and crystals. The differences are large and of opposite sign in the case of alkali metals and alkaline earth metals. The dimers of alkaline metal atoms in the gas phase, Li2, Naj etc., are characterized by a weak covalent... 

This is a severe drawback in the case of equilibrium studies of metal molecules since, as a rule, such molecules are minor vapor components and maximum sensitivity is required for their thermodynamic evaluation. However, very precise ionization potentials can be measured using photoionization spectroscopy (5,28). Berkowltz (28) reviewed early work concerning alkali metal dimers. Herrmann et al. ( ) have measured the ionization potentials of numerous sodium, potassium and mixed sodium-potassium clusters. For most of these clusters the atomization energies of the neutral molecules are not known. Therefore, the dissociation energies of the corresponding positive ions cannot be calculated. 

The most abundant species in the equilibrium vapor over condensed metals are generally atoms as shown by Knudsen effusion mass spectrometry. The relative abundance of the homonuclear diatomic molecules was determined to be between 10 and 10 percent if they are detectable. Exceptions are bismuth and antimony. The dimer partial pressure over liquid bismuth at temperatures below 1000 K exceeds that of the monomer [83, 164, 165]. The tetramer is the most abundant species over liquid antimony [85]. Other polyatomic homonuclear species have so far been observed under equilibrium conditions for some of the alkali metals, as well as the group Ib, IVb, and Vb metals. Particularly large polymers up to 667 and Sn7 were detected for germanium [84,166,167] and tin [168],... 

Simple alkali metal halide molecules, for example NaCl and KCl, are found in the vapors above heated samples of the solid compounds. These vapor species have been studied by IR spectroscopy of gases and matrices, and by gas electron diffraction. The studies show that planar rhombic dimers are also present. 

Alkali halide vapor consists mainly of monomers and dimers. As stated in Sec. 1-22, Linevsky developed a technique to isolate these monomers and dimers, produced at high temperature, in inert gas matrices. This technique has been used extensively to study the infrared spectra and structures of a number of inorganic salts. Some references on metal halide dimers are LiF)2. (LiCOz, (LiBr)2, and (NaX)2 (X = F, Cl, Br, and 1). These dimers are known to be cydic-planar (02 ). On the other hand, (T1X)2(X — F and Cl) are linear and symmetrical (Doo X-Tl-Tl-X). Such structures have been well known for (HgX)2 (X = C1, Br, and 1). The dimer LiO)2 is also cyclic-planar. However, (Nbl)2 is frans-planar (C2J,) in a Nj matrix but takes the cis structure upon complex formation with the Cr(CO)s group. ... 

The alkali metal halides, MX, fonn solids at room temperature. The best known member of the family, NaCl, melts at 801°, the vapor pressure reaches 1 torr at 865°, and the melt boils at 1413°. The dominant species in gaseous alkali metal halides is the monomeric formula unit, MX(g), but smaller amounts of dimers (M2X2), trimers (M3X3) and tetramers (M4X4) have also been detected by mass spectroscopy. Information about the monomeric molecules obtained by spectroscopic studies at high temperatures is collected in Table 5.1 [1]. 

To a first approximation, the vapor phase of an alkali metal may be considered as a binary mixture of monomer and dimer species, whose molar fractions are dependent on both temperature and pressure. The viscosity of such a binary mixture with mole fractions yw and yu (subscripts M and D refer to monomer and dimer respectively) is well described by the first-order Chapman-Enskog theory (Hirschfelder et al. 1954 ... 

The vapor of the alkali metal is considered to be a chemical reacting mixture of monomers and dimers, in equilibrium... 

In a seeded beam a condensable material is mixed with a large excess of a rare gas (usually helium) which does not condense under the experimental expansion conditions. This technique is used with metals and other materials which have too low a vapor pressure to be expanded directly. Low boiling metals, such as the alkalis and alkaline earths, can be heated in a simple Knudsen cell. Stronger heating has been used for the more refractory transition metals. Most recently laser evaporation has been used to produce beams of many metals. This technique, which uses a pulsed, focused laser to evaporate metal from a rod, is well suited to the use of a time-of-flight mass spectrometer for cluster analysis. Analysis by laser-induced fluo-resence or laser ionization also allows many spectroscopic studies to be conducted in small clusters. In this way bond lengths and other parameters have been determined for dimers of Cr, Cu, and Sn to name just a few. 

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