Surface ionization energy

Murray JS, Brinck T, Politzer P (1992) Applications of calculated local surface ionization energies to chemical reactivity. J. Mol. Struct. (Theochem) 255 271-281... 

The energy required to remove an electron from the surface of a phthalo-cyanine crystal to infinity (surface ionization energy) has been measured by Pope (286) by an electrostatic method, and by Kearns and Calvin (176) by the photoelectric threshold method. The surface ionization energy is 5 eV and is independent of the presence or absence of the central metal ion. 

In 1991 Brincketal. [231] demonstrated a relationship between the pK s of azines and azoles and their molecular surface ionization energies, in the form of the descriptor Is jn. For a set of a dozen compounds they found a correlation coefficient of 0.99 between the aqueous pK values and values of to for the ring nitrogens. ... 

M. Haeberlein, J.S. Murray, T. Brinck, and P. Politzer, Calculated electrostatic potentials and local surface ionization energies of para-substitituted anilines as measures of substituent effects. Can. J. Chem. 70 (1992), pp. 2209-2214. 

T. Brinck, J.S. Murray, P. Politzer, and R.E. Carter, A relationship between experimentally determined pK s and molecular surface ionization energies for some azines and azoles, J. Org. Chem. 56 (1991), pp. 2934-2936. 

Similarly, adsorption of ions (n+) onto a metal surface leads to a heat of adsorption of Q,. Generally, Q is about 2-3 eV and is greater than Q, which itself is about 1 eV. The difference between Q, and is the energy required to ionize neutrals (n ) on a metal surface so as to give ions (n+) or vice versa. This difference, Q - Q, can be equal to, greater than, or less than the difference, I - ( ), between the ionization energy (1) of the neutral and the ease with which a metal can donate or accept an electron (the work function, ( )). Where Q, - Q, > I - ( ), the adsorbed... 

Therefore, the ratio of the number of ions to the number of neutrals desorbing from a heated filament depends not only on the absolute temperature but also on the actual surface coverage of ions and neutrals on the filament (C, C ) and crucially on the difference between the ionization energy and work function terms, I and (j). This effect is explored in greater detail in the following illustrations. 

If there is a different material (M) on the heated metal surface, M will be evaporated as the temperature increases. The ionization energy (ionization potential, I) of M is also measured in electron volts. 

Surface ionization. Takes place when an atom or molecule is ionized when it interacts with a solid surface. Ionization occurs only when the work function of the surface, the temperature of the surface, and the ionization energy of the atom or molecule have an appropriate relationship. 

Phofoelectron spectroscopy is a simple extension of the photoelectric effect involving the use of higher-energy incident photons and applied to the study not only of solid surfaces but also of samples in the gas phase. Equations (8.1) and (8.2) still apply buf, for gas-phase measuremenfs in particular, fhe work function is usually replaced by fhe ionization energy l so fhaf Equation (8.2) becomes... 

The main feature of the new wave function is its polarization by the field of the ion. That is, the presence of the ion changes the effective ionization energies of HA and HB and tends to pull electrons toward HB. The effect of the external ion on the potential surface for bond breaking is shown in Fig. 1.4. 

Enhancement of surface diffusion of the growth precursors is considered as one of the beneficial effects of ion bombardment [246,428]. The potential energy of ions, which is released when the ion is neutralized, is typically 10 eV. This energy can be a substantial fraction of the total energy transferred. The release of this ionization energy is sufficient to excite atoms into excited electronic states, thereby weakening their bonds and enhancing their mobilities [429]. 

Surface Ionization Sources. In this system, a low ionization potential atom (e.g. caesium) is adsorbed on a high work function metal (e.g. iridium). The temperature is raised so that the rate of desorption exceeds the rate of arrival of the atoms at the surface, and the Cs is then desorped as ions with very small energy spread (< 1 eY). The spot size - current characteristics of these sources lie between liquid metal and plasma discharge sources. 

Specific heat (Cp) Ionization energy ( , ,) -Surface tension (y)... 

Figure 4.9 SIMS spectra of a Rh(l 11) surface after adsorption of 0.12 ML of NO at 100 K, where NO adsorbs molecu-larly, and after heating to 400 K, illustrating dissociation of NO in to N and O atoms. Note that the yields of O and N differ in SIMS, because of different ionization energies of the respective clusters (adapted from van Hardeveld et al. [271).

There are several methods of producing gas-phase inorganic ions, the starting materials in mass spectrometric studies. The properties of the source of the ions required for study are important in the choice of ionization method. The production of bare metal ions from an involatile nonmolecular source requires a large amount of energy deposited on the surface of the material. The processes that occur after the initial ionization process may also affect the ions finally observed (e.g., clustering). At the other end of the ionization energy spectrum, gas-phase ions of a complexity similar to those observed in the condensed phases require a soft ionization process. A brief description of some of the ionization methods follows. 

Politzer and coworkers170 have recently presented a linear relationship (r = 0.99) between the measured aqueous pK values171 of a series of azines and azoles (9 pKa units) and the magnitude of the lowest value (/s,min>eV) of the average local ionization energy, 7(r), on the molecular surface 7(r) is defined within the framework of SCF-MO theory as... 

The unitary level of the surface ion referred to the standard gaseous ion S sTD) at the outer potential of the semiconductor is represented by the unitary real potential, Ug. (= - 7s). This unitary real potential is equivalent to the sum of the standard free enthalpy AG of sublimation of the semiconductor, the ionization energy Is of the gaseous atom S, and the electron energy sy at the upper edge level of the valence band as shown in Eqn. 3-14 ... 

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