Secondary neutral energy distribution

The interaction of keV particles with solids has been characterized by the measurement of the angle and energy distribution of sputtered secondary ions and neutrals. The results are compared to classical dynamics calculations of the ion impact event. Examples using secondary ions are given for clean Ni 001), Cu 001) reacted with 0>, Ni 001 and Ni 7 9 11 reacted with CO, and Agllll) reacted with benzene. The neutral Rh atoms desorbed from Rh 001 are characterized by multiphoton resonance ionizaton of these atoms after they have left the surface. 

Angular distribution of neutral atoms angular distributions, 94,95f energy distributions, 93-9 1 schematic of detector, 93,94f Angular distribution of secondary Ions adsorbate-covered... 

It is important to point out that thermodynamic equilibria of hydrocarbons and those of derived carbocations are substantially different. Under appropriate conditions (traditional acid catalysts, longer contact time), the thermodynamic equilibrium mixture of hydrocarbons can be reached. In contrast, when a reaction mixture in contact with excess of strong (super) acid is quenched, a product distribution approaching the thermodynamic equilibrium of the corresponding carbocations may be obtained. The two equilibria can be very different. Since a large energy difference in the stability of primary < secondary < tertiary carbocations exists, in excess of superacid solution, generally the most stable tertiary cations predominate. This allows, for example, isomerization of n-butane to isobutane to proceed past the equilibrium concentrations of the neutral hydrocarbons, as the er -butyl cation is by far the most stable butyl cation. 

The outcome is referred to as the corrected intensity If actual yields are known, this corrected intensity can then be scaled to provide absolute ion yields as a function of emission velocity. Note However, similar to the sputtered neutral distribution, absolute secondary ion yields are difficult to derive. An example of the procedure described earlier is illustrated in Figure 3.32 for Cu secondary ions resulting from 7.5 keV 0 impact. This combination is shown as the relatively low energy and mass of 0 should result in a full isotropic linear cascade as assumed by the Sigmund-Thompson relation. The derived velocity distribution is shown in the inset of Figure 3.32. 

Figure 3.33 Overlay of the apparent velocity distributions derived on the assumption that the sputtered neutral population follows the Sigmund-Thompson distributions (Relation 3.4) for Cu" and Cn secondary ions emanating from a polycrystalline surface under 17.5 KeV Ar" " impact, 14.0 KeV Cs" " impact, 17.5 KeV02 impact, and7.5 KeV0 impact. The same calculations were applied to all data sets and with all plots arbitrarily normalized to unity at zero 1 jv. The lines represent the trends relayed by Relation 3.10(a) or (b) fitted to the lower 1 (higher emission energy) populations. Reproduced with permission from van der Heide and Karpusov (2000) Cop)night 2000 Elsevier.

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