Alkali atoms

A more dramatic type of restmctiiring occurs with the adsorption of alkali metals onto certain fee metal surfaces [39]. In this case, multilayer composite surfaces are fomied in which the alkali and metal atoms are intemiixed in an ordered stmcture. These stmctiires involve the substitution of alkali atoms into substrate sites, and the details of the stmctiires are found to be coverage-dependent. The stmctiires are influenced by the repulsion between the dipoles fomied by neighbouring alkali adsorbates and by the interactions of the alkalis with the substrate itself [40]. 

Todd C J and Rhodin T N 1974 Adsorption of single alkali atoms on tungsten using field emission and field desorption Surf. Sc/. 42 109-21... 

If the molecules could be detected with 100% efficiency, the fluxes quoted above would lead to impressive detected signal levels. The first generation of reactive scattering experiments concentrated on reactions of alkali atoms, since surface ionization on a hot-wire detector is extremely efficient. Such detectors have been superseded by the universal mass spectrometer detector. For electron-bombardment ionization, the rate of fonnation of the molecular ions can be written as... 

The occupation of each tetraliedral and octaliedral site in tliese regularly oriented arrays of cavities by, for example, alkali atoms results in tire transfer of a single electron to tire fullerene s conduction band (ti ) [58]. Consequently,... 

Plllet P, Crubelller A, Bleton A, Dulleu O, Nosbaum P, Mourachko I and Masnou-Seeuws F 1997 Photoassociation In a gas of cold alkali atoms I. Perturbative quantum approach J.Phys.B At.MoLOpt.Phys. 30 2801-20... 

For an introduction to current research in alkali-atom BEC see tire special issue on BEC in the Journal of Research of the National Institute of Standards and Technology ... 

The atomic volumes of the alkali metals increase with atomic number, as do those of the inert gases. Notice, however, that the volume occupied by an alkali atom is somewhat larger than that of the adjacent inert gas (with the exception of the lithium and helium—helium is the cause of this anomaly). The sodium atom in sodium metal occupies 30% more volume than does neon. Cesium occupies close to twice the volume of xenon. 

Figure 6-10. Models (to scale) oj halogen atoms, inert gas atoms, and alkali atoms.

Regardless of the exact extent (shorter or longer range) of the interaction of each alkali adatom on a metal surface, there is one important feature of Fig 2.6 which has not attracted attention in the past. This feature is depicted in Fig. 2.6c, obtained by crossploting the data in ref. 26 which shows that the activation energy of desorption, Ed, of the alkali atoms decreases linearly with decreasing work function . For non-activated adsorption this implies a linear decrease in the heat of chemisorption of the alkali atoms AHad (=Ed) with decreasing > ... 

At high alkali coverages (near monolayer coverage), when the adsorbed alkali overlayer shows a metal-like character, alkali-methoxy species are formed. As shown by TPD experiments in the system K/Ru(001) these alkali-methoxy species are more stable than the methoxy species on clean Ru(001) and adsorbed methanol on 0.1K/Ru(001). On metal surfaces inactive for methanol decomposition, e.g. Cu(lll), these alkali-methoxy species are formed even at low alkali coverages, due to the weaker interaction of the alkali atoms with the metal surface. The formation of these species stabilizes the methoxy species on the metal surface and enhances the activity of the metal surface for methanol decomposition. 

It is worth noting that each Na atom appears to perturb the electron density of the Pt(lll) surface over large ( 12) atomic distances. This can explain nicely the observed long-range promotional effect of Na on Pt surfaces. It is strongly reminiscent of the IR spectroscopic work of Yates and coworkers who showed that a single adsorbed alkali atom can affect the IR spectra of up to 27 coadsorbed CO molecules.80... 

Fig. 14.3 Polyhedral packing plots for the two-dimensional layers of [RE(P2S6),/2(PS4)P in the series of solids A2RE(P2S6)i/2(PS4), where A=K, Cs RE = Y, La. Rare-earth polyhedra are striped PS4 polyhedra are black phosphorous atoms in P2S6 are shown as black circles. Alkali atoms are not shown for clarity. Although these phases have distinctly different structures based on space group symmetry and atomic positions, the compounds are clearly related upon close inspection of the building blocks.

Consider an atom approaching the surface in Fig. 6.23. If the upper level of the atom originally contained an electron, then upon adsorption it will transfer part of this electron density to the metal and become positively charged. This is the case with alkali atoms. The atom forms a dipole with the positive end towards the outside, which counteracts the double layer that constitutes the surface contribution to the work function of the metal (Fig. 6.13). Thus alkali atoms reduce the work function of a metal surface simply because they all have a high-lying s electron state that tends to donate charge to the metal surface. 

This approach was used by Elliott and co-workers to control the ionization of alkali atoms by one- and two-photon excitation. Wang and Elliott [72] measured the interference between outgoing electrons in different angular momentum states. They showed, for example, that the angular flux of the p2P and the d2D continua of Rb is determined by the phase difference... 

Bradley, R. Sputtering of alkali atoms by inert gas ions of low energy. 

Hooymayers HP, Alkemade CTJ (1966) Quenching of excited alkali atoms and related effects in flames Part II. Measurements and discussion. J Quant Spectrosc Radiat Transfer 6 847-874... 

Systematic TRSHG studies on alkali-atom adsorbed metal surfaces by Matsumoto and coworkers provided a deep insight on how coherent motions are created under very different electronic configurations [15, 77, 78]. The results showed that the coherent phonon generation critically depends on the surface and bulk electronic structure of the substrate. 

The relative position of the electronic level eo to the Fermi level depends on the electrode potential. We perform estimates for the case where there is no drop in the outer potential between the adsorbate and the metal - usually this situation is not far from the pzc. In this case we obtain for an alkali ion eo — Ep — where is the work function of the metal, and I the ionization energy of the alkali atom. For a halide ion eo — Ep = electron affinity of the atom. 

Table 18.2 Occupation probability of the valence orbital of a few alkali and halide ions adsorbed on mercury ( = 4.5 eV). For alkali atoms eo denotes the ionization energy for halide atoms, the electron affinity.

Figure 9.10 Up work function of alkali-promoted metals as a function of alkali coverage (see also Table 9.2). Down electrostatic potential around a single alkali atom adsorbed on jellium. The effective local work function at each position is the sum of the substrate work function and the value of the electrostatic potential in the figure (from Lang el at. [39]).

In the situation as sketched in Figs. A.9 and A. 10a, level 1 remains occupied and level 2 empty, implying that the adsorbate atom retains the same charge as in the free atom. However, other situations can arise also. Suppose that the atom has a low ionization potential, smaller than the work function of the metal. Then the broadened level 1 falls largely above the Fermi level of the metal, with the result that most of the electron density of level 1 ends up on the metal. Hence, the adatom is positively charged (Fig. A. 10b). This happens with alkali atoms on many metal surfaces see for example the discussion of potassium on rhodium in Chapter 9. 

The resonant level model readily explains the change in work function associated with chemisorption. It is well known that alkali atoms such as potassium lower the work function of the substrate, whereas electronegative atoms such as chlorine increase the work function [2,8,19]. Figure A. 10 indicates that potassium charges positively and chlorine negatively when adsorbed on jellium. Remember that the surface contribution to the work function is caused by... 

Alkali and alkaline earth metals. Results obtained for the group I and group II atoms are encouraging. As Table 5.6 shows, calculations for the alkali atoms are slightly more reliable than those for the alkaline earths. The largest error obtains for the quasidegenerate Be atom. ECP bases provide a convenient alternative to all-electron treatments. 

Figure 3. Isomer shifts, O (13), and number of iodine p holes, hp, obtained from NMR chemical shift (4), X, and from dynamic quadrupole coupling (25), A, vs. alkali atomic number for alkali iodides. Data points for each alkali spread horizontally for clarity...

The author also studied the possibility of enhancing the electron density and temperature of an alkali-seeded plasma by pumping the resonance lines of the alkali atoms with a suitably tuned laser 283)... 

The metal salts of peroxynitrite have also been studied using ab initio methods. We use the same nomenclature as McGrath and Powland s early paper on ONO—OH , which distinguishes different conformers by their dihedral angles. For example, the structure with a cis ONO—O arrangement and the alkali atom bonded to the terminal oxygen with a perpendicular orientation is called cis,perp Figure 1 describes the conformational aspects of the conformers studied. 

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