Alkali ionisation potentials

Because of the velocity bunching effect due to initial acceleration the ion beam is nearly monokinetic, and the neutralisation does not effect the velocity distribution The details of the method can be found in [ KAUF 78 ], [ NUE 78] By neutralisation in an alkali vapour, the atomic metastable states are preferentially populated since their energies match the ionisation potential of the corresponding alkali atom Therefore this technic is ideally suited for laser spectroscopy of rare gas, and is recently successfully used to study the heaviest one, radon Fig. 

Surface ionisation on a hot wire or ribbon is restricted to substances with low ionisation potentials and has been mainly used for detecting alkali atoms, dimers and halides and a few other similar species [30]. There is evidence that the efficiency of surface ionisation is dependent on the degree of internal excitation [104]. 

CH3C1 (90, 34 and 8 A2, respectively). These values suggest a similarity between these reactions and the corresponding ground state alkali atom reactions. The ionisation potential of Hg (3P2) is 4.974eV which is similar to those for the alkali atoms and so an electron jump mechanism is proposed for these chemiluminescent reactions of Hg (3P2 ) In contrast, the reaction of another spin-orbit state of metastable mercury with bromine, Hg (3P0) + Br2, has a much smaller chemiluminescent cross section [3 A2 compared with 150 A2 for Hg (3P2) + Br2] [406], which cannot be reconciled with an electron jump, suggesting the existence of a barrier to reaction of Hg (3P0) which is not present in the case of Hg(3P2). 

This effect will obviously be greatest with elements having low ionisation potentials such as the alkali and alkaline earth metals, e.g. barium is approximately 80% ionised in the nitrous oxide flame. Since the ground state therefore becomes depopulated, the sensitivity will decrease. 

This effect mostly occurs with alkali and alkaline earth metals. The low ionisation potentials of these elements cause them to be readily ionised in the flame with a resultant lowering of the population of ground state atoms and a suppression of sensitivity. The technique used to overcome this is to add an easily ionised salt such as potassium chloride to samples and standards. This ionises in preference to the analyte in the flame and enhances sensitivity. As an example, strontium, barium and aluminium are subject to ionisation in the flame. In water analyses, this is suppressed by adding potassium to the samples and standards so that the solution contains 2 000 mg l-1 potassium. 

Ions with the inert gas configuration are easily formed from the alkali metals, and those of the halogens are also stable. The ionisation potential for... 

In 1991, Li and Chan reported the use of indium to mediate Barbier-Grignard-type reactions in water.12 The work was designed on the basis of the first ionisation potentials of different elements, in which indium has the lowest first ionisation potential relative to the other metallic elements near it in the periodic table. On the other hand, indium metal is not sensitive to boiling water or alkali and does not form oxides readily in air. Such special properties of indium indicate that it is perhaps a promising metal for aqueous Barbier-Grignard-type reactions. Indeed, it appears that indium is the most reactive and... 

Ionisation potentials are displayed as a function of atomic number in fig. 1.1 The plot of ionisation potentials immediately brings out some interesting features one sees that the most stable (i.e. the most compact) atoms are the rare gases. In fact, the smallest atom is He. For the element following a rare gas (an alkali atom), the ionisation potential is particularly low. This can be understood as a consequence of the excellent screening of the nuclear charge by the compact closed shell of electrons. The same, however, is not true of closed subshells, or at least the effect is then much less pronounced. 

Fig. 1.2. Plot of the drop in ionisation potential between a rare gas and the following alkali against the ionisation potential of the rare gas X-experimental data O - Hartree-Fock calculations. Even though experiment differs considerably from theory, all points lie close to the same straight line.

Between these two extremes lie clusters of atoms more stable than the alkalis, but less so than rare gases, and which may actually effect a transition from van der Waals to metallic behaviour as a function of the cluster size N. A good example is provided by Hg clusters Hg atoms have a closed 6s2 subshell, and a resonably high ionisation potential. The small clusters (up to about 10 or 15 atoms) exhbit a van der Waals behaviour with quasiatomic 5d —> 6p transitions, while a conduction band appears for larger N [662] as the aggregates become metallic. 

Fig. 10. Electron affinities (EA) and ionisation potentials (IP) for alkali and alkaline-earth elements. The data for Na through Fr and Mg through Ra are experimental. The value for element 119 is DF CCSD [84,137] and DF for elements 120,165 and 166 [20].

In these equations, AG is the energy of formation of the indicated species, AGfat is the crystal lattice energy, / is the ionisation potential of the metal and A is the electron affinity of the halide. In sect. 2.11.2 the calculation of AG at and Ai/i t (at 298.15 K) is discussed and in Appendix 2.11.1 values are given for the alkali metal halides and a few other selected salts. 

Jortner has made use of this relation and the observed heats of solution of alkali metal in dilute metal-ammonia solutions, the tabulated heats of solution for the ions and tables of experimental data on sublimation energies and ionisation potentials to compile a table of for the alkali and alkaline earth metals (Table III). This table shows that does not show any systematic variation with size or charge of ion and is reasonably constant about a mean value of 1.7 0.2 ev or in other units, 39.2 4.6 kcal per mole. This mean value is in good agreement with earlier values... 

The alkali metals are easily determined by flame photometric methods which are always less time-consuming than chemical methods and offer the same order of accuracy. The precise procedure used will depend on the type of equipment which is available but certain generalisations can be made which are applicable to all instruments. These elements are easily excited and have relatively low ionisation potentials so a low temperature flame is always indicated and air-coal gas, air-propane and air-hydrogen are most convenient. As the resonance lines appear in the visible region of the spectrum filter instruments can be used so long as spectral interferences are negligible or can be corrected for. If a monochromator instrument is used such corrections are more easily and accurately made. 

Table Properties of alkali metals melting and boiling points, atomisation and ionisation enthalpies, ionic and standard electrode potentials...

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