Electron affinity, measurement

A neutral atom can add an electron to form an anion. The energy change when an electron is added to an atom is called the electron affinity (EA). Both ionization energy (IE) and electron affinity measure the stability of a bound electron, but for different species. Here, for example, are the values for fluorine ... 

The electron affinity measures the attractive force between the incoming electron and the nucleus the stronger the attraction, the more energy there is released. The factors that affect this attraction are exactly the same as those relating to ionization... 

Fc/Fc+) have been obtained, irrespective of the number of bridging thiophene rings [56]. Naito et al. [239] compared a variety of other electron-transport materials by their ionization potential and electron affinity measured by different methods. For example, some bis(styrylanthracenes) similar to 21 but with electron-withdrawing groups exhibit higher electron affinities than Alq3. The per-fluorinated compounds 19 and 34 showed irreversible electroreductions [62]. 

Only in the lithium catalysed copolymerization of styrene with the dienes in hydrocarbons do the monomers show an unexpected order of reactivity, an anomaly which disappears for polar solvents. The preference for the diene in the copolymer vanishes even in hydrocarbon solvents if sodium is used as initiator [231]. With the dienes, however, an added complication exists which makes simple experiments on electron affinity measured in solvents such as dioxane—water a poor guide to reactivity. They can react in more than one way to give a 3,4 (or 1,2) structure or alternatively a 1,4 structure. There appears to be good correlation between the amount of styrene in the copolymer and the percentage of... 

Much of our knowledge about the electronic structure of gas-phase clusters comes from ionization potential measurements as a function of cluster size. This is analogous to the measurement of the work function of metals. Kappes et al. recently reviewed ionization potential data for a large number of systems. Recently results were reported for Nb and V clusters as well. Some electron affinity measurements have also been reported. Most recently... 

FIGURE 5.25 Electron affinities (measured in kJ mol of gaseous atoms of the elements. An asterisk means that the element does not have a stable anion in the gas phase. 

A major objective of this book is to evaluate the reported values of molecular electron affinities and their errors and to assign them to specific states. Prior to 1970 the magnetron and ECD methods were used to measure the majority of gas phase molecular electron affinities. An extensive compilation of unevaluated experimental, empirical, and theoretical electron affinities of atoms, molecules, and radicals was published before 1990 [9]. The electron affinities measured in the gas phase are now available on the Internet but have not been evaluated [26]. The molecular Ea in this list is defined and evaluated in Appendix IV. Values that are significantly lower than the selected values will be assigned to excited states. Semi-empirical calculations and the CURES-EC technique support these assignments. Unpublished electron affinities and updated electron affinities from charge transfer complex data and half-wave reduction potentials are given in Appendix IV. 

Figure 6.5 ECD data plotted as In KT312 versus 1,000/7 illustrating the range of electron affinities measured in the ECD. This is 0.05 eV to 1.5 eV. The data have been published, but...

Figure 6.7 High-resolution photoelectron spectra of 02(—). The large peaks form a progression from one negative-ion state. The small peaks were unexplained in the original article, but coincide with electron affinities measured by other techniques. The PES spectrum was taken from [33],...

The Ea from half-wave reduction potential measurements and energies of charge transfer complex absorption in solution support the gas phase measurements from all the cited techniques. The major difference between the gas phase measurements and the solution or solid phase measurements is the interaction between the solvents or solid phase and the anions. These measurements provide a transition between the low values of valence-state electron affinities measured in the gas phase and the negative valence-state electron affinities. Thus, the valence electron affinities for naphthalene and pyridine are 0.17 eV and 0.0 0.2 eV by solution phase techniques [39, 77]. 

For the present tabulation the 2002 CODATA value elhc = 8065.54445 0.00069 cm eV (http //physics.nist.gov) has been used to convert electron affinities from the units used in spectroscopic work, cm, into eV for these tables. The 86 ppb uncertainty in e/hc is insignificant compared to uncertainties in the electron affinity measurements. 

Figure 16.4. Relationship of polymer n-eleetron band structure to vacuum and various energetic parameters. g is the optical band gap, BW is the band width of the fully occupied valence band, EA is the electron affinity (measured from the bottom of the conduction band to the vacuum) and IP is the ionization potential (measured from the top of the valence band to the vacuum).

Electrons can be removed from atoms successively because the cations formed are stable. (The remaining electrons are held more tightly by the nucleus.) On the other hand, adding electrons to an atom results in an increasing electrostatic repulsion in the anions, leading to instability. For this reason, it is difficult and often impossible to carry electron affinity measurements beyond the second step in most cases. 

Since the IP of hydrogen is a constant equal to 313.6 kcal/mol [16], the enthalpy change A// , depends basically on the difference between the bond dissociation energy D iA—H), and the electron affinity EA(A). For many years the experimental way of obtaining AH for a compound was from dissociation energies and electron affinity measurements. For an interesting explanation of details of such procedures, see Bartmess and Mclver [17] and Lias and Bartmess [18]. A more recent and comprehensive discussion of the various experimental techniques available for measuring these properties can be found in the NIST webbook [18]. 

It is important to understand the difference between ionization energy and electron affinity Ionization energy measures the ease with which an atom loses an electron, whereas electron affinity measures the ease with which an atom gains an electron. 

Fig. 1.13. A schematic illustration of the spectroscopic techniques and the portion of the band structure that they probe. The techniques illustrated are ionization potential (IP) measmements, electron affinity measurements (Sa), Bremsstrahlrmg isochromat spectroscopy (BIS), Ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), Scanning tunneling spectroscopy (STS)...

---