Ions/ionization

A tabulation of the ECPSSR cross sections for proton and helium-ion ionization of Kand L levels in atoms can be used for calculations related to PIXE measurements. Some representative X-ray production cross sections, which are the product of the ionization cross sections and the fluorescence yields, are displayed in Figure 1. Although these A shell cross sections have been found to agree with available experimental values within 10%, which is adequate for standardless PKE, the accuracy of the i-shell cross sections is limited mainly by the uncertainties in the various Zrshell fluorescence yields. Knowledge of these yields is necessary to conven X-ray ionization cross sections to production cross sections. Of course, these same uncertainties apply to the EMPA, EDS, and XRF techniques. The Af-shell situation is even more complicated. 

Ion-molecule reactions can be investigated in a double mass spectrometer in two ways (a) In the collision between the incident ion and the gas molecule, transfer of part of one of these structures can take place. The pressure in the collision chamber must be low (b) The pressure in the collision chamber is increased. The slow incident ions ionize the gas molecules by charge exchange. Then ion-molecule reactions take place between the ionized gas molecules or their fragment ions and other gas molecules. 

FIG. 13. Cross sections for electron collisions for SiH4-H2 (a) SiH4. (b) Si2H (dotted lines) and H2 (solid lines). Abbreviations are ion, ionization dis. dissociation vib. vibrational excitation att, attachment. See Table II for details and references. (Adapted from G. J. Nienhuis. Ph.D. Thesis. Uni-versiteit Utrecht. Utrecht, the Netherlands. 1998. with permission.)... 

Discharge Ion Ionization Trap Source Detector Reflectron... 

To conclude, we shall estimate the characteristic distance r2, at which the electron tunnels during the ionization of the atoms in the external field. This estimation can be conducted with the help of the relation r2 cs I/F. Substituting into this formula, for instance, the energy of He ion ionization, 1 0.076 eV and F = 200kV cm-1, we find r2 = 40 A. According to eqn. (8), the probability of ionization by means of electron tunneling at a distance r2 at such values of I and F is equal to 5 x 109s. ... 

Fig. 21.1 Two atomic channels associated with two states of the ion (ionization limits), shown by bold lines. Above the first limit and below the second the nominally bound states of channel 2 autoionize into the continuum of channel 1.

Ionization isomers differ in the anion that is bonded to the metal ion. An example is the pair [Co(NH3)5Br]S04, a violet compound that has a Co-Br bond and a free sulfate anion, and [Co(NH3)5S04]Br, a red compound that has a Co-sulfate bond and a free bromide ion. Ionization isomers get their name because they yield different ions in solution. 

A potential curve of an endothermically chemisorbed atom or molecule represents an excited state with respect to the normal state of the physically adsorbed atom or molecule. When cesium atoms are adsorbed on salt layers or on cesium oxide, they are adsorbed as atoms and not, as they would be on metal surfaces, as ions. Ionization can be brought about by absorption of light 172) or by thermal excitation (173). 

The appearance potentials for molecular ions (ionization potentials) and for fragment ions formed in the mass spectra of metallocenes and related compounds are listed in Table XIII. These appearance potentials have been used to calculate bond dissociation energies and heats of formation of organometallic compounds, but the results obtained must be treated cautiously because the appearance potentials of fragment ions include excess energy due to excited species. The values obtained for the heats of formation are best considered as upper limits, rather than precise determinations. The extent to which energy due to excited states can contribute... 

Neutral gaseous molecules entering the ionization area are bombarded with electrons to smash the compound of interest to yield positively charged ions. Ionization is often followed by a series of spontaneous competitive decomposition reactions (fragmentation) which produce additional ions. The instrument operates under a high vacuum to prevent absorption of the charged particles by air molecules. 

Because of the different valence of the two cations, electron neutrality is maintained by hydrogen ions. Ionizing radiation releases an electron from the [A104] " group, which is then trapped by the hydrogen ion ... 

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