The amount of energy release when an electron e at rest is captured by a species M, producing the negative ion M−. The electron affinity of a species M can also be thought of as the ionization potential of the negative ion M−. In these definitions, it is assumed that both M and M− are in the lowest electronic, vibrational, and rotational state.
If the electron affinity of M is negative, the M− ion is unstable with respect to decomposition into M + e. Most atoms have positive electron affinities, even though there is no net Coulomb attraction between the electron and the atom until the electron is close enough to be “a part of the atom.” The simple rules of chemical valence provide a qualitative guide to the magnitude of electron affinities. Thus the noble gases, which have a filled outer electronic shell and are chemically inert, are not capable of binding an additional electron to form a negative ion. The largest electron affinities are possessed by the halogens, atoms which require only one additional electron to fill the valence shell. See also: Halogen elements
The major exception to this concept is that multiply charged negative ions—for example, O2−, one of many such ions which are stable in solution—have not been observed in the gas phase. The ability to place more than one additional electron in the valence shell of a neutral atom or molecule arises from many particle electrostatic interactions with the ion's environment: either the solvent shell surrounding the ion in liquid solutions or the amorphous or crystalline region surrounding the ion in solids. See also: Valence
Accurate ionization potentials of the elements were known for a number of years before comparable data for electron affinities of the elements became available. In order to determine the ionization potential of an element, the element is vaporized, placed in an optical spectrometer, and observed for the onset of photoabsorption corresponding to the photoionization process, hν, + M → M+ + e. The photon energy corresponding to the threshold wavelength for this process is the ionization energy of the species M. The analogous method for determination of an electron affinity is through observation of the threshold of the very similar photodetachment reaction, hν + M− → M + e. Again, the threshold wavelength for this process corresponds to the electron affinity of the species M. Unfortunately, it has not proved possible to produce the sufficiently large densities of negative ions required to utilize photoabsorption spectrometers to observe directly the threshold for the photodetachment process. Consequently, determination of accurate electron affinities lagged far behind the determination of accurate ionization potentials. However, an ion velocity modulation scheme has been devised that permits the direct observation of vibrational photoabsorption in ions; it is not generally applicable to the electron detachment process required to determine electron affinities.
The two major experimental advances that have enabled accurate electron affinity determinations have been the development of ion-beam techniques affording the detection of photoabsorption by the monitoring of photodetachment products and the availability of intense light sources in the form of lasers.
In experiments to determine electron affinities, negative ions are formed in an electrical discharge, extracted through an aperture into a high-vacuum region, formed into a negative-ion beam, mass-analyzed, and intersected by an intense laser beam. The laser-beam–negative-ion-beam intersection takes place in a high-vacuum region where no collisions are likely. The occurrence of a photodetachment event is determined by detection of the photodetached electron.
Two experimental methods evolved which produce accurate electron affinities. In the first method the laser is tunable, and a search is made for the wavelength corresponding to the threshold for the photodetachment process. In this case the electron affinity is given directly by the threshold wavelength, and experimental methods have been developed that provide accuracies of 10−8 eV. In the second type of experiment, called photoelectron spectroscopy, a fixed-frequency laser (of known photon energy) is employed, and the electrostatic fields are used to determine the kinetic energy of the ejected electron. From simple energy conservation arguments, the electron affinity is then given by the photon energy less the kinetic energy of the ejected electron. This latter technique is quite general, but is limited in accuracy by the resolution of the electron energy analyzer (approximately 3 × 10−3 eV for state-of-the-art analyzers). See also: Laser; Laser spectroscopy; Mass spectrometry
These laser photodetachment studies dramatically improved knowledge of the electron affinities of the elements. The illustration is a periodic table showing values of the electron affinities of the elements. Most of the data shown here were obtained by using laser photodetachment methods. The periodic trends in electron affinities and the qualitative effects described earlier are immediately apparent. In addition, a number of more subtle trends are observable. For example, while it might be expected that filled-shell species such as the rare gases would not be capable of binding an additional electron, the illustration shows that half-filled shells (for example, N and P) also exhibit small or negative electron affinities. Again, this effect is the result of the fact that a half-filled valence shell is spherically symmetric and behaves somewhat as though it were a filled shell. A similar situation is also seen for half-filled d shells, as is the case for Mn, Tc, and Re in the transition metals.
These same techniques have provided a number of accurate electron affinity determinations for molecules and free radicals, and new insight into the structural and chemical properties of ions in the gas phase. See also: Ionization potential; Periodic table