Simple Electron Counting Research Articles

Scanning tunneling microscopy of the GaAs (311)A surface reconstruction

Reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM) are used to study the reconstruction of the GaAs (311)A surface. During and after molecular-beam epitaxy, in-situ RHEED of the surface shows a lateral periodicity of 3.2 nm perpendicular to the [ 2 33] direction. This periodicity is confirmed employing in-situ STM. The height of this periodic surface structure is two monolayers, i.e., 0.34 nm. We show how this reconstruction, characterized by a dimerization of the surface As atoms, can be formed using a simple electron counting model. The excellent agreement with the experimental results further support this model, that was already found to explain the reconstructions of the (100) and (111) surfaces.

Chemisorption of carbon monoxide on platinum cluster anions

Reactions of platinum cluster anions with carbon monoxide are investigated using a flow tube reactor (flowing afterglow) instrument. The platinum cluster anions are prepared with a cathodic discharge source. The cluster anions, Pt n−(n≤7), react with carbon monoxide in a flowing helium/argon buffer gas at 0.5 Torr total pressure and at 300 K. Clusters with four or more platinum metal atoms react by addition of CO with an effective bimolecular reaction rate of greater than 70% of the calculated collision rate, while for n=3 the reaction rate is 16% of the collision rate. At high CO concentrations, additional CO molecules attach to the clusters to form platinum carbonyl anion intermediates, with little loss of metal atoms. Saturation limits were found, above which no further CO addition occurred. The saturated cluster species are Pt3(CO)6−, Pt4(CO)8−, Pt5(CO)9−, and Pt6(CO)10−. The application of simple geometry arguments and electron counting rules to infer possible skeletal structures of the metal clusters is discussed.

First-principles calculations of molecular- and atomic-hydrogen reactions on As-terminated GaAs(100) surfaces.

We have investigated the reaction of molecular and atomic hydrogen on an As-terminated GaAs(100) surface, on which As dimers exist, by performing a first-principles total-energy calculation. According to the present calculations, the ${\mathrm{H}}_{2}$ molecule neither dissociates nor sits on the surface while the H atoms sit on several sites accompanied by As-dimer distortion. The As-dimer distortion is well described by the simple electron counting model, taking orbital energies into consideration. We have also found that the total energy of the H-atom-adsorbed surface is comparable to that of the GaAs(100) 2\ifmmode\times\else\texttimes\fi{}4 surface exposed to arsine and hydrogen gases.

Work function, electron affinity, and band bending at decapped GaAs(100) surfaces

Work function and band bending measurements are performed on n- and p-type GaAs(100) surfaces prepared by molecular-beam epitaxy and As decapping. The band bending is almost independent of the decapping temperature and surface reconstruction [(1×1)→C(4×4)→C(2×8)→(4×2)]. The work function and electron affinity undergo large and systematic variations of up to 400 meV which consistently establish maxima and minima of these quantities for the (2×4)-C(2×8) and (4×2)-C(8×2) structures, respectively. These variations are due to surface dipoles induced by charge exchanges between the cation and anion dangling bonds in the top two layers of the crystals. These charge exchanges are fully explained by applying a simple electron counting model to each reconstruction.

The choice of optimum exposure time for a simple electron counting system

The question of an optimum exposure time for an event counting charge coupled device electron detection system is discussed [B. D. Hall, M. Flüeli, J. -P. Borel, and R. Monot, Rev. Sci. Instrum. (in press)]. The system is a null-event detector, providing a single piece of information per exposure: whether an event has not been detected. It is shown that when the number of exposure periods is fixed, optimum operation conditions require high exposures, in spite of the loss of information due to multiple events being detected as single ones.

Recognition of toroidal and spheroidal geometry in metal clusters

For some categories of Pt and Au cluster compounds, a simple electron count theory is successful in predicting its structure. Six or eight cluster valence electrons correspond to toroidal or spherical structures, respectively. As the characterization of more complex clusters as toroidal and spheroidal may seem to be somewhat arbitrary, an objective method for classifying the geometry is developed. From atom position coordinates, a numerical parameter can be calculated which gives unambiguous indication of the geometry. It is shown that there is a perfect correlation between this parameter and the electron count.

Counting electrons and rationalizing structures of clusters and clusters of clusters

The investigation of the electronic structures of high nuclearity metal clusters constitutes an important prerequisite for the basic understanding of the relationship between discrete metal clusters and the extended bulk metal. Simple electron counting principles are particularly useful in this regard. It not only allows a systematization or rationalization of the observed structures of these clusters, but also can lead to prediction of clusters of yet unknown molecular architectures and/or formulations. In this review, electron counting principles for the metal clusters of varying nuclearity (magic numbers), size, shape and packing arrangements will be discussed. Emphasis will be focused upon: (1) the topological electron counting ( TEC) rule; (2) the C 2 model for clusters of clusters; (3) the Shell model for v n polytopal close-packed metal clusters; and (4) the Extended Hume-Rothery rule for metal clusters of close-packed arrangements.

Electron counting model and its application to island structures on molecular-beam epitaxy grown GaAs(001) and ZnSe(001).

The principal reconstructions found on the low-index planes of GaAs and ZnSe can be explained in terms of a simple electron counting model. A surface structure satisfies this model if it is possible to have all the dangling bonds on the electropositive element (Ga or Zn) empty and the dangling bonds on the electronegative element (As or Se) full, given the number of available electrons. This condition will necessarily result in there being no net surface charge. The justification for this model is discussed. The GaAs(001)-(2\ifmmode\times\else\texttimes\fi{}4) reconstruction is known to involve surface dimers. It is shown that a (2\ifmmode\times\else\texttimes\fi{}4) unit cell with three dimers and one dimer vacancy is the smallest unit cell that satisfies the electron counting model for this surface. The electron counting model is used to explain the structure of islands imaged by scanning tunneling microscopy on the GaAs(001)-(2\ifmmode\times\else\texttimes\fi{}4) surface. The model shows that island structures built up from complete (2\ifmmode\times\else\texttimes\fi{}4) unit cells can be stable if they extend in the 2\ifmmode\times\else\texttimes\fi{} direction, but not if they extend in the 4\ifmmode\times\else\texttimes\fi{} direction. These island structures can also provide an explanation for the different step structures seen on GaAs(001) vicinal surfaces. Much less is known experimentally about step and island structures on ZnSe(001). Structures on this surface predicted by the electron counting model differ significantly from those found on GaAs(001).

Neutral and ionic clusters of antimony and bismuth: A comparison of magic numbers

Neutral and singly charged anion or cation clusters of antimony and bismuth are produced by laser vaporization in a pulsed nozzle cluster source. Neutral clusters are photoionized near threshold with a fluorine excimer laser, while ionized clusters are sampled directly from the laser vaporization plasma. All species are mass analyzed with a time-of-flight spectrometer. Abundance patterns in these systems exhibit ‘‘magic numbers’’ that are strikingly charge dependent. Compared at the same charge, corresponding neutral and cation spectra of these metals are quite similar, but significant differences arise in the two anion cluster distributions. Neutral cluster growth in these systems ends abruptly at five atoms while the ionized distributions fall off gradually out to beyond 12–15 atoms. These unique patterns are examined in the context of cluster growth dynamics and suspected cluster stability based on the predictions of simple electron counting models.

Growth patterns in binary clusters of Group IV and V metals

Binary clusters composed of Group IV and Group V metals (e.g., Sn/Bi) are prepared by laser vaporization of corresponding alloys in a pulsed nozzle molecular beam source. Relative abundances of these clusters are studied with laser photoionization mass spectroscopy at 193 and 157 nm. Mass spectral abundance patterns for all systems studied (Sn/Bi, Pb/Sb, Sn/As) exhibit highly nonstatistical combinations of component elements. Preferential stoichiometries follow common patterns throughout these cluster systems, which can be related to the bonding and stability of condensed phase cluster ions of these same metals. These patterns are predicted by a simple valence electron counting model.

ChemInform Abstract: ELECTRONIC STRUCTURE AND FORCE CONSTANTS OF THE DITHIONITRONIUM CATION NS2‐

Abstract The standard GF method was employed to determine the valence force constants of the dithionitronium cation (NS 2 + ), using the experimental IR and Raman frequencies. The calculation force constants ( f NS = 8.256, f NS,NS = 0.663 and f SNS = 0.236 mdyn A −1 ) reproduce well the 32 SN 34 S + frequencies and suggest a bond order greater than 1, as well as a delocalization of the π electrons in the trinuclear system. CNDO/2 calculations reveal positive charges on sulfur atoms, contrary to simple valence electron count rules. The CNDO/2 preferred geometry is D ∞h rather than a C 2V cyclic structure, in agreement with the X-ray diffraction data. A very important S⋯S interaction has been found.

Electronic structure and force constants of the dithionitronium cation NS 2+

The standard GF method was employed to determine the valence force constants of the dithionitronium cation (NS 2 +), using the experimental IR and Raman frequencies. The calculation force constants ( f NS = 8.256, f NS,NS = 0.663 and f SNS = 0.236 mdyn Å −1) reproduce well the 32SN 34S + frequencies and suggest a bond order greater than 1, as well as a delocalization of the π electrons in the trinuclear system. CNDO/2 calculations reveal positive charges on sulfur atoms, contrary to simple valence electron count rules. The CNDO/2 preferred geometry is D ∞h rather than a C 2V cyclic structure, in agreement with the X-ray diffraction data. A very important S⋯S interaction has been found.

A simple electron counting rule for close-packed high nuclearity metal clusters

A simple electron counting rule based on an extended Hume–Rothery rule has been developed for close-packed high nuclearity metal clusters.

ChemInform Abstract: THEORY OF PHOTOCHEMICAL REACTIONS

AbstractVortrag, gehalten auf dem 8th International Congress on Photochemistry, Edmonton, Canada,v0m 7 ‐ 13.August 1975.

Theory of photochemical reactions.

Although the great number of electronic states available to an excited molecule might seem to preclude a coherent picture of photochemical reaction mechanisms, it is possible to bring out some basic features common to a great many reactions. The electronic states of the primary diradical intermediates, surface crossings, topicity, and avoided surface crossings have been shown to be essential components of the electronic theory of photochemical reactions. Diradicals have four important electronic states. Knowing these states, and making a simple electron count, it is possible to draw state correlation diagrams. Some diagrams show a typical surface crossing of the ground singlet state with the lowest (singlet, triplet) pair of excited states, with clear-cut consequences of quantum yields under various conditions. In other reactions the surfaces stay apart. The critical discriminating feature that determines the type of correlation diagram is the topicity. Photochemical reactions can be classified according to topicity, which is useful in interpreting their mechanisms (53). Avoided surface crossings can also be classified into different types. Figure 7, which illustrates the interplay of a covalent and an ionic surface responsible for photochemical electron transfer, is a typical multidimensional representation of a photochemical reaction. The chemical behavior of the excited zwitterionic states of common intermediates, such as twisted ethylene or diallyl, reflects the quantum mechanical nature of photochemical processes. In these states, for perfectly symmetric systems, charge oscillates back and forth between two symmetry-equivalent sites. Slight geometric perturbations can create a sudden polarization of the excited molecule, with localization of almost a full charge at one end of the molecule. A photon is transformed into an electrical signal thanks to an appropriate molecular distortion. Nature may have used this simple process in the N-retinylidene visual chromophore to trigger an electrical response to vision.