Surface atom ionization

Surface atom ionization of covalent semiconductor electrodes... 

As more surface atoms ionize, more extra electrons remain inside the solid metal. The electrostatic attraction between these electrons and metal ions adsorbs the ions onto the metal surface. 

Small metal clusters are also of interest because of their importance in catalysis. Despite the fact that small clusters should consist of mostly surface atoms, measurement of the photon ionization threshold for Hg clusters suggest that a transition from van der Waals to metallic properties occurs in the range of 20-70 atoms per cluster [88] and near-bulk magnetic properties are expected for Ni, Pd, and Pt clusters of only 13 atoms [89] Theoretical calculations on Sin and other semiconductors predict that the stmcture reflects the bulk lattice for 1000 atoms but the bulk electronic wave functions are not obtained [90]. Bartell and co-workers [91] study beams of molecular clusters with electron dirfraction and molecular dynamics simulations and find new phases not observed in the bulk. Bulk models appear to be valid for their clusters of several thousand atoms (see Section IX-3). 

SIMS Secondary-ion mass spectroscopy [106, 166-168] (L-SIMS liquids) [169, 170] Ionized surface atoms are ejected by impact of -1 keV ions and analyzed by mass spectroscopy Surface composition... 

Plasma Carburizing. Plasma carburizing generates carbon atoms at the surface by ionization of a carbon-containing gas, eg, methane. The process is similar to that described for ion nitriding. Because the process is carried out in partial vacuum, there is less chance of oxidation. 

Figure 1 Schematic of DC glow-discharge atomization and ionization processes. The sample is the cathode for a DC discharge in 1 Torr Ar. Ions accelerated across the cathode dark space onto the sample sputter surface atoms into the plasma (a). Atoms are ionized in collisions with metastable plasma atoms and with energetic plasma electrons. Atoms sputtered from the sample (cathode) diffuse through the plasma (b). Atoms ionized in the region of the cell exit aperture and passing through are taken into the mass spectrometer for analysis. The largest fraction condenses on the discharge cell (anode) wall.

Appearance potential methods all depend on detecting the threshold of ionization of a shallow core level and the fine structure near the threshold they differ only in the way in which detection is performed. In all of these methods the primary electron energy is ramped upward from near zero to whatever is appropriate for the sample material, while the primary current to the sample is kept constant. As the incident energy is increased, it passes through successive thresholds for ionization of core levels of atoms in the surface. An ionized core level, as discussed earlier, can recombine by emission either of a characteristic X-ray photon or of an Auger electron. 

Liquid Metal Sources. The source feed is a metal of low melting point - Ga and In are commonly employed. It is introduced as a liquid film flowing over a needle towards the tip whose radius is relatively blunt (10 pm). The electrostatic and surface tension forces form the liquid into a sharp point known as the Taylor cone. Here the high electric field is sufficient to allow an electron to tunnel from the atom to the surface, leaving the atom ionized. 

In our third example (52), dissociative chemisorption of Li2, B2, C2, 02, N2, F2, CO, NO and ethylene on (100)W and Ni surfaces was examined. The metal surfaces are represented by means of nine-atom clusters, arranged as in Fig. 35. Experimental geometry was used for the adsorbates. The standard EHT method was used, i.e. with charge-independent atomic ionization potentials. Charge transfer between adsorbate and surface was explored... 

The terms surface ionization mass spectrometry (SIMS) and ion sputtering are often used when accelerated atoms such as Xe or ions such as Ar+ strike a surface causing ionization of the material on the surface. The surface can be solid or liquid in the form of a solution or a suspension in the solvent. In this section, the terms fast atom bombardment (FAB) and fast ion bombardment (FIB) will be used. 

In covalent semiconductors of single element S such as silicon, the covalent bonding electron is in the valence band and the valence band hole participates in the ionization of surface atoms as shown in Eqn. 3-13 and in Fig. 3-7 ... 

Fig. 3-8. Energy for formation of the standard gaseous ions, S(Vnj), from the surface atoms of a semiconductor of single element S dGnbi = standard free enthalpy of the surface atom sublimation h = ionization energy of gaseous atoms aj. = unitary level of the surface ion = - (dGsM + /s) = unitary level of the surface atom referred to the standard gaseous ions and elections.

Fig. 9-7. Ionization of surface at oms followed by ion tnnsfer across an electrode interface in anodic dissolution of covalent semiconductor S = covalently bonded atom in semiconductor S. = surface atom of semiconductor s = surface radical = surfisce ion 825 = hydrated ion OHP = outer Helmholtz plane.

Fig. 9-8. Potential energy profile for ionization of surface atoms in two steps on a covalent semiconductor electrode c, = band giq> energy tfi s electron level in an intermediate radical S " Ag = activation energy for the first step of radical formation in the conduction band mechanism df = activation energy for the first step of radical formation in the valence band mechanism = activation energy for the second step of radical ionization in the conduction band mechanism Ag = activation energy for the second step of radical ionization in the valence band mechanism beR = CR-Ev. [From Gerischer, 1970.]...

In the anodic dissolution of covalent semiconductors, the transfer of surface ions across the compact layer (Helmholtz la r) occurs following the ionization of surface atoms S, illustrated in Eqn. 9-33, as described in Sec. 9.2.1 ... 

When electronic equilibrium is established in the space charge layer, the concentration of interfacial electrons is given by n, = n exp (- e A /k T) and the concentration of interfacial holes is given by Pt = p exp(e A lk T) n and p are the concentrations of electrons and holes, respectively, in the semiconductor interior. In general, the ionization of surface atoms (Eqn. 9-24) is in quasiequilibrium so that the concentration of surface ions depends on the overvoltage... 

Fig. 1.33. Image mechanism of field-ion microscopy. A sharp tip, usually made of refractory metal, is placed in a chamber filled with He at about 10 torr. With an electric field of a few V/A, each of those atoms in the more protruding positions will have a He atom adsorbed on it. The He atom may be ionized to form a He ion, then be accelerated by the electric field to form an image of this surface atom. (Reproduced from Tsong, 1990, with permission.)...

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