Knudsen effusion mass spectrometry

Peck D-H, Miller M, and Hilpert K. Vaporization and thermodynamics of La j Ca,Cr03 investigated by Knudsen effusion mass spectrometry. Solid State Ionics 2001 143 391 400. 

Knudsen effusion mass spectrometry is used to measure the composition of the effusing beam (including the composition of the sample investigated, fragments or newly formed species) and in particular to determine the thermochemical data. The instrumentation is briefly described in Section 5.2.10.137... 

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, ) ... 

We have developed several new measurement techniques ideally suited to such conditions. The first of these techniques is a High Pressure Sampling Mass Spectrometric method for the spatial and temporal analysis of flames containing inorganic additives (6, 7). The second method, known as Transpiration Mass Spectrometry (TMS) (8), allows for the analysis of bulk heterogeneous systems over a wide range of temperature, pressure and controlled gas composition. In addition, the now classical technique of Knudsen Effusion Mass Spectrometry (KMS) has been modified to allow external control of ambient gases in the reaction cell (9). Supplementary to these methods are the application, in our laboratory, of classical and novel optical spectroscopic methods for in situ measurement of temperature, flow and certain simple species concentration profiles (7). In combination, these measurement tools allow for a detailed fundamental examination of the vaporization and transport mechanisms of coal mineral components in a coal conversion or combustion environment. 

Kerns = Knudsen effusion-mass spectrometry /Kete = Knudsen effusion-torsion effusion /tran = transpiration /Lang = Langmuir free evaporation /Lams = Langmuir-mass spectrometry... 

K-pressure data were obtained by Knudsen effusion mass spectrometry as discussed in detail elsewhere (16). 

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... 

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. 

The potential of Knudsen effusion mass spectrometry can be summarized as follows ... 

Mass spectrometry for the determination of physico-chemical data was first used by Chupka and Inghram [6] as well as Honig [7] to study the free evaporation of carbon. These authors identified different polymeric carbon species at high temperatures and they concluded that the trimer C3 is the most abundant vapor species at temperatures of 2450 and 2600 K. The numerous results obtained thereafter refer essentially to equilibrium vaporization investigated by Knudsen effusion mass spectrometry and are summarized in the first comprehensive review by Inghram and Drowart [8] in 1960. The systems studied and the results obtained up to 1964 are tabulated by Drowart [9]. An extensive compilation of literature is also given by Grimley [10] and in the recent review by Drowart [11]. 

Hastie [37] describes recent developments in Knudsen effusion mass spectrometry and in the application of mass spectrometry to high-temperature materials chemistry. 

The use of Knudsen effusion mass spectrometry for the study of non-equilibrium processes, such as catalytic decomposition or recrystallization of amorphous films, is described in Refs. 38-42. 

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-... 

Typical reactions studied by Knudsen effusion mass spectrometry are for example the dissociation, the isomolecular exchange, and sublimation reactions ... 

Hastie [131] coupled for the first time a quadrupole mass spectrometer with a Knudsen cell. One of the quadrupole mass spectrometer - Knudsen cell systems used at our laboratory is shown in Fig. 4. The system has been developed to study small alkali metal clusters under equilibrium conditions (see Sect. 3.2). Broad-band photoionization by a 1 kW Hg/Xe lamp is used for the first time in Knudsen effusion mass spectrometry to reduce fragmentation. Other quadrupole mass spectrometer - Knudsen cell systems have for example been developed by Hilpert [132, 133], Fraser and Rammensee [134], Plante [135], Ono et al. [136], Kematick et al. [137], and Edwards et al. [138]. Cryogenic pumping is used in the device by Hilpert to reduce mercury background ion intensities for the study of amalgams [132], The instruments described in Refs. 134,135 use a chopper to modulate the molecular beam from the Knudsen cell. Interfering background ion intensities can, thereby, be subtracted. The apparatus developed by the authors of Refs. 137, 138 renders possible the simultaneous application of Knudsen effusion mass spectrometry and the mass-... 

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],... 

Thermodynamic data of alloys, such as mixing properties, can additionally be evaluated from the data obtained for the gaseous species over these alloys. A comprehensive bibhography of the studies by Knudsen effusion mass spectrometry up to the year 1974 is given by Chatillon et al. [12]. 

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... 

A comparison between the two studies on Ct2 carried out so far by the use of Knudsen effusion mass spectrometry, which demonstrates also the difference in sensitivity, is rendered possible by Fig. 6 showing the different temperature ranges of investigation [126]. 

The interplay between theory and experiment is shown by the study of the molecules PdSi and PdGe (see Table 3). Shim et al. [207,208] investigated these molecules by Knudsen effusion mass spectrometry and additionally determined... 

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