Affinity level

Conversely, an atom in Fig. 6.23 with an affinity level that initially is empty becomes partly occupied upon adsorption. Hence, charge is transferred from the metal to the atom. This sets up a dipole that increases the surface contribution to the work function. This is the case for adsorbed halides, which will be negatively charged at the surface. We will later see that such dipole fields can explain promotion and inhibition effects caused by various adsorbates in catalysis. 

The thicket of models is complicated, and with misunderstood notation (including homo/lumo), the careful user or reader of models has to be aware of exactly what is being done in any given analysis. While it is possible to decry the use of (in particular) the homo/lumo language, that language is universal. This can be avoided simply by thinking of them as affinity levels and detachment levels, as they really are. 

Elastic tunneling spectroscopy is discussed in the context of processes involving molecular ionization and electron affinity states, a technique we call orbital mediated tunneling spectroscopy, or OMTS. OMTS can be applied readily to M-I-A-M and M-I-A-I -M systems, but application to M-A-M junctions is problematic. Spectra can be obtained from single molecules. Ionization state results correlate well with UPS spectra obtained from the same systems in the same environment. Both ionization and affinity levels measured by OMTS can usually be correlated with one electron oxidation and reduction potentials for the molecular species in solution. OMTS can be identified by peaks in dl/dV vs bias voltage plots that do not occur at the same position in either bias polarity. Because of the intrinsic... 

Keywords Inelastic Elastic Tunneling Spectroscopy Orbital mediated Vibrational Electronic Ionization levels Affinity levels... 

When the sample is biased positively (Ub > 0) with respect to the tip, as in Fig. 9c, and assuming that the molecular potential is essentially that of the substrate [85], only the normal elastic current flows at low bias (<1.5 V). As the bias increases, electrons at the Fermi surface of the tip approach, and eventually surpass, the absolute energy of an unoccupied molecular orbital (the LUMO at +1.78 V in Fig. 9c). OMT through the LUMO at — 1.78 V below the vacuum level produces a peak in dl/dV, seen in the actual STM based OMTS data for nickel(II) octaethyl-porphyrin (NiOEP). If the bias is increased further, higher unoccupied orbitals produce additional peaks in the OMTS. Thus, the positive sample bias portion of the OMTS is associated with electron affinity levels (transient reductions). In reverse (opposite) bias, as in Fig. 9b, the LUMO never comes into resonance with the Fermi energy, and no peak due to unoccupied orbitals is seen. However, occupied orbitals are probed in reverse bias. In the NiOEP case, the HOMO at... 

A different view of the OMT process is that the molecule, M, is fully reduced, M , or oxidized, M+, during the tunneling process [25, 26, 92-95]. In this picture a fully relaxed ion is formed in the junction. The absorption of a phonon (the creation of a vibrational excitation) then induces the ion to decay back to the neutral molecule with emission (or absorption) of an electron - which then completes tunneling through the barrier. For simplicity, the reduction case will be discussed in detail however, the oxidation arguments are similar. A transition of the type M + e —> M is conventionally described as formation of an electron affinity level. The most commonly used measure of condensed-phase electron affinity is the halfwave reduction potential measured in non-aqueous solvents, Ey2. Often these values are tabulated relative to the saturated calomel electrode (SCE). In order to correlate OMTS data with electrochemical potentials, we need them referenced to an electron in the vacuum state. That is, we need the potential for the half reaction ... 

While the first electrochemical reduction potential provides an estimate for Ac (assuming it is a reversible process), the second and higher reduction potentials do not provide the spectrum of single electron affinity levels. Rather, they provide information about two-electron, three-electron, and higher electron reduction processes, and, therefore, depend on electron pairing energy. Thus, the utility of solution-phase reduction potentials for estimating solid-state affinity levels is... 

At this point, we have reached the stage where we can describe the adatom-substrate system in terms of the ANG Hamiltonian (Muscat and Newns 1978, Grimley 1983). We consider the case of anionic chemisorption ( 1.2.2), where a j-spin electron in the substrate level e, below the Fermi level (FL) eF, hops over into the affinity level (A) of the adatom, whose j-spin electron resides in the lower ionization level (I), as in Fig. 4.1. Thus, the intra-atomic electron Coulomb repulsion energy on the adatom (a) is... 

Fig. 4.1. Anionic chemisorption energy-level diagram showing transfer of j-spin electron from substrate level ek to affinity level A on adatom, while experiencing Coulomb repulsion U from j-spin electron in ionization level I.

As a cation approaches a metal or semiconducting surface, an electron from the solid may resonantly tunnel through the vacuum to fill a vacant level on the ion. This process of resonant neutralization becomes facile, when (1) the affinity level of the ion is resonant with the occupied levels of the solid, and (2) the ion is sufficiently proximate to the surface... 

Fig. 16. One-electron energy level diagram for neutralization of a positive ion near a metal surface. The metal conduction band is shown on the left. The dashed and solid curves represent the potential energy for an electron when the ion is at infinite and close distances to the surface, respectively. The affinity level for the ion shifts upward and broadens as the ion approaches the surface.

Similar arguments can be applied to resonant charge exchange processes between H+ and H or H and H, In the last case, we define the H-affinity level. A, by means of the equation... 

As emphasized by Norskov and co-workers, for reactive systems another effect occurs. As Z decreases, the affinity level (LUMO) of the molecule begins to feel an image-charge type attraction, causing a shifting of its energy as... 

Compounds from some of the above series have been optimized to low nanomolar affinity levels. The Johnson and Johnson group disclosed Nutlin-3 with a K, of 36 nM [153], and Nutlins have shown anticancer effects in vitro and in vivo [168]. Further development of spirooxindoles produced MI-219 [169], which has a 5 nM K,. and also shows effects in vitro and in vivo. Many compounds exhibit potent cellular activity and good pharmacokinetics. Several other series have appeared in recent years [170], and a few compounds have reached clinical trials [171]. 

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