Positive electron affinity

TABLE 4.4 Electron Affinities of Atoms, Molecules, and Radicals Electron affinity of an atom (molecule or radical) is defined as the energy difference between the lowest (ground) state of the neutral and the lowest state of the corresponding negative ion in the gas phase. A(g) + e = A-(g) Data are limited to those negative ions which, by virtue of their positive electron affinity, are stable. Uncertainty in the final data figures is given in parentheses. Calculated values are enclosed in brackets. ... 

A positive electron affinity signifies a release of energy when an electron attaches to a gas-phase atom or ion (Section 1.18). 

The electron affinity values for many of the elements shown in Figure 8-17 appear to lie on the x axis. Actually, these elements have positive electron affinities, meaning the resulting anion is less stable than the neutral atom. Moreover, the second electron affinity of every element is large and positive. Positive electron affinities cannot be measured directly. Instead, these values are estimated by other methods, as we show in Section 8-1. 

Although electron affinity values show only one clear trend, there is a recognizable pattern in the values that are positive. When the electron that is added must occupy a new orbital, the resulting anion is unstable. Thus, all the elements of Group 2 have positive electron affinities, because their valence S orbitals are filled. Similarly, all the noble gases have positive electron affinities, because their valence a p orbitals are filled. Elements with half-filled orbitals also have lower electron affinities than their neighbors. As examples, N (half-filled 2 p orbital set) has a positive electron affinity, and so does Mn (half-filled 3 d orbital set). 

C08-0066. According to Appendix C, each of the following elements has a positive electron affinity. For each one, constmct its valence orbital energy level diagram and use it to explain why the anion is unstable N, Mg, and Zn. 

The high potentials required for electrospray show that air at atmospheric pressure is not only a convenient, but also a very suitable ambient gas for ES, particularly when solvents with high surface tension, like water, are to be subjected to electrospray. The oxygen in the air, which has a positive electron affinity, captures free electrons and acts as a discharge suppressor. 

Specific Electron Capture. It is now customary to use solvents such as methanol (usually CD3OD) or methyl tetrahydrofuran (MTHF) as solvents if electron-capture by AB is required. These solvents form good glasses at 77 K, and for sufficiently dilute solutions of the substrate, AB, electron ejection occurs overwhelmingly from solvent molecules, so that AB+ centres are not formed. Electrons are fairly mobile, and hence AB radicals are formed provided AB has, effectively, a positive electron affinity. The hole centres, such as CD30D+, are not mobile because proton transfer to surrounding solvent molecules occurs rapidly at all temperatures. 

Data are limited to those negative ions which, by virtue of their positive electron affinity, are stable. Uncertainty in the final data figures is given in parentheses. Calculated values are enclosed in brackets. 

The relativistic coupled cluster method starts from the four-component solutions of the Drrac-Fock or Dirac-Fock-Breit equations, and correlates them by the coupled-cluster approach. The Fock-space coupled-cluster method yields atomic transition energies in good agreement (usually better than 0.1 eV) with known experimental values. This is demonstrated here by the electron affinities of group-13 atoms. Properties of superheavy atoms which are not known experimentally can be predicted. Here we show that the rare gas eka-radon (element 118) will have a positive electron affinity. One-, two-, and four-components methods are described and applied to several states of CdH and its ions. Methods for calculating properties other than energy are discussed, and the electric field gradients of Cl, Br, and I, required to extract nuclear quadrupoles from experimental data, are calculated. 

Dissociative electron attachment (DEA) occurs when the molecular transient anion state is dissociative in the Franck-Condon (FC) region, the localization time is of the order of or larger than the time required for dissociation along a particular nuclear coordinate, and one of the resulting fragments has positive electron affinity. In this case, a stable atomic or molecular anion is formed along with one or more neutral species. Dissociative electron attachment usually occurs via the formation of core-excited resonances since these possess sufficiently long lifetimes to allow for dissociation of the anion before autoionization. 

Desorption can proceed via several mechanisms. For solids with a negative electron alSnity such as Ar [49,149-151] and N2 [153], the extended electron cloud around a metastable center will interact repulsively with the surrounding medium and metastables formed at the film-vacuum interface will be expelled into vacuum (the so-called cavity expulsion mechanism [161]). Also permitted in solids with positive electron affinities (e.g., CO) is the transfer of energy intramolecular vibration to the molecule-surface bond with the resulting desorption of a molecule in lower vibrational level [153,155,158-160]. Desorption of metastables via the excitation of dissociative molecular (or excimer) electronic states is also possible [49,149-151,154,156,157]. A concise review of the topic can be found in Ref. 162. 

Note that the definition of electron affinity does not follow the usual thermodynamic convention in ihat a positive electron affinity is exothermic. See Chapter 2. 

If the reaction of interest is exothermic, the reaction can proceed without loading rare metals, which are usually quite expensive. In this case, surface heterogeneity of potentials may be caused by a surface defect or nonstoichiometry. Adsorption of reactant molecules possessing a positive electron affinity can act as a source of the potential drop near the surface of the semiconductor. 

A positive electron affinity indicates an affinity for electrons, in the sense that energy is released when an anion is formed. 

The other group consists of the so-called Feshbach resonances that are formed when the incident electron first excites the molecule directly and then is captured by the molecule which is now in an excited state, provided that by doing so the molecule benefits in energy. Very often this excited state of the molecule is one of the Rydberg states, since in that case a positive electron affinity is preferable. 

The electron affinity of an atom or ion is the counterpart of the ionisation potential. It is an intensive property, defined as the energy released when the atom in its ground state accepts an electron, i.e. the difference in energy between the ground state of E and that of E- with the sign convention that exothermic electron affinities are positive. Electron affinities, like ionisation potentials, are expressed in eV. 

The primary advantage of laser desorption is that abundant molecular or pseudomolecular ions are produced for many different classes of compounds. Positive pseudomolecular ions are most often formed by attachment of a cation, typically a proton, potassium, or sodium ion, to the parent molecule. Negative pseudomolecular ions can also be formed by laser desorption, usually by loss of a proton, or by electron attachment to molecules with a positive electron affinity. Often little fragmentation occurs (making this... 

Although they are not carbonylic compounds, carboxylate ions should be discussed here in conjunction with the carboxylic acids. Owing to its resonance stabilization, the —COjf group has no low-lying vacant orbital or any positive electron affinity thus it is non-reactive toward e q. Carboxylate ions with aliphatic chains, which may also carry OH or NH2 groups, are evidently non-reactive. This has been shown in the cases of formate, acetate, citrate, lactate, oxalate, glycinate and ethyl-enediaminetetra-acetate ions, all of which react with e q at rates lower than 1O0 m-1 sec-1 (Anbar and Neta, 1967a). 

The electron affinity of a semiconductor relates the vacuum level to the conduction band minimum at the surface. An informative way to view the electron affinity is as the conduction band offset between the semiconductor and vacuum. The band structure of the vacuum is simply the parabolic free electron bands, and the minimum energy (or vacuum level) refers to an electron at rest. For most materials, an electron at the bottom of the conduction band is bound to the material by a potential barrier of several volts. This barrier is the electron affinity and is defined as a positive electron affinity. In some instances, the vacuum level can actually align below the conduction band minimum. This means that an electron at the minimum of the conduction band would not see a surface barrier and could be freely emitted into vacuum. This situation is termed a negative electron affinity. 

While EQN (1) can be used to verify an NEA for wide bandgap semiconductors, another aspect that signals the presence of an NEA is the appearance of a sharp peak at the low kinetic energy end of the spectrum. This feature is attributed to electrons thermalised to the conduction band minimum. For a positive electron affinity, these electrons would be bound in the sample and not observed in the spectrum. 

---