Cesium ionization

When Pq and the interelectrode gap (d) have values greater than a specified value (Pq, d>20 mil Torr), the cesium ionizes and forms a plasma, referred to as the ignited mode. In this ignited mode, an output current density more than ten times that for the unignited mode is obtained. The output current-voltage characteristics obtained at Tg = 1800 K, 7 = 1000 K and Tg = 497 561 K are shown in Figure 7. 

We may suppose that it is essential to use a proper model for the metal. The adsorption of cesium on tungsten, a particularly simple case since the cesium ionizes completely on the metal surface, has been treated in this way most successfully by Langmuir (40) and de Boer (41). Catalysis requires a similar treatment for electronegative gases on metals, where the bond formed is largely homopolar, and not ionic as in the case of cesium. 

FIGURE 10-33. Effect of cesium ionization on the atomic absorption calibration curve of sodium. 

Indeed, the probability of Cesium to form Cs secondary ions is, in the majority of cases, independent of the position of the Fermi edge, which itself is dne to the very low ionization potential of Cesium, i.e. this places the Cesium ionization level well above the Fermi edge of most surfaces. This insensitivity to the position of the Fermi edge can thus explain the trends in Cs secondary ions from most Cs sputtered metal and semiconductor surfaces and why these are seen to increase over the transient region, with the increase simply reflecting the increased Cesium content implanted into the respective snbstrate. 

The performance of many metal-ion catalysts can be enhanced by doping with cesium compounds. This is a result both of the low ionization potential of cesium and its abiUty to stabilize high oxidation states of transition-metal oxo anions (50). Catalyst doping is one of the principal commercial uses of cesium. Cesium is a more powerflil oxidant than potassium, which it can replace. The amount of replacement is often a matter of economic benefit. Cesium-doped catalysts are used for the production of styrene monomer from ethyl benzene at metal oxide contacts or from toluene and methanol as Cs-exchanged zeofltes ethylene oxide ammonoxidation, acrolein (methacrolein) acryflc acid (methacrylic acid) methyl methacrylate monomer methanol phthahc anhydride anthraquinone various olefins chlorinations in low pressure ammonia synthesis and in the conversion of SO2 to SO in sulfuric acid production. 

Ion engines are used in sateUites for orientation control. Cesium is vaporized in a vacuum and ionized as it passes through a heated porous tungsten disk, the ions are accelerated by an electric field to about 135 km/s and are neutralized by the injection of electrons and exhausted from the thmster. However, mercury, xenon, and argon-based ion engines are preferred. 

Cesium ions are also sometimes used to enhance the secondary-ion yield of negative elemental ions and that of some polymer fragments [3.6]. They are produced by surface ionization with an extraction technique similar to that of FI sources. 

What trend is observed in the first ionization energy as you move from lithium down the column I metals On this basis, can you suggest a reason why potassium or cesium might be used in preference to sodium or lithium in photoelectric cells ... 

The first ionization energy is highest for elements close to helium and is lowest for elements close to cesium. Second ionization energies are higher than first ionization energies (of the same element) and very much higher if the electron is to be removed from a closed shell. Metals are found toward the lower left of the periodic table because these elements have low ionization energies and can readily lose their electrons. 

On the basis of your knowledge of periodicity, place each of the following sets of elements in order of decreasing ionization energy. Explain your choices, (a) Selenium, oxygen, tellurium (b) gold, tantalum, osmium (c) lead, barium, cesium. 

Mass Spectrometry. Mass spectrometry holds great promise for low-level toxin detection. Previous studies employed electron impact (El), desorption chemical ionization (DCI), fast atom bombardment (FAB), and cesium ion liquid secondary ion mass spectrometry (LSIMS) to generate positive or negative ion mass spectra (15-17, 21-23). Firm detection limits have yet to be reported for the brevetoxins. Preliminary results from our laboratory demonstrated that levels as low as 500 ng PbTx-2 or PbTx-3 were detected by using ammonia DCI and scans of 500-1000 amu (unpublished data). We expect significant improvement by manipulation of the DCI conditions and selected monitoring of the molecular ion or the ammonia adduction. 

Work with the Hhpp ligand was pioneered by Cotton and co-workers who showed that the di-metal complexes with Cr(II), Mo(II), or W(II), ionize readily, the latter more readily than cesium [41]. Recent work with the anionic hpp ligand has produced the compound [Au2(hpp)2Cl2] with a short Au-Au (2.47 A) distance [17, 18],... 

C. A., Van Dorn, L.O. and Wilkinson, C.C. (2002) Most easily-ionized, closed-shell molecules known easier than the cesium atom. Science, 298, 1971-1974 ... 

The first ionization potential is the energy required to pull the first electron from the outer orbital into space, and is given in table 4.2 and figure 4.2. It is seen that the required energy is lower for the metallic elements, and reaches a minimum at 3.9 eV for cesium it is higher for the nonmetallic elements, and reaches a maximum of 13.6 eV for hydrogen and 24.6 eV for helium. 

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