The magic gold cluster AU(20)

in International Journal of Quantum Chemistry (2007), 107(14), 2922-2934

The 20-nanogold cluster Au-20 exhibits a large variety of two- and three-dimensional isomeric forms. Among them is the ground-state isomer Au-20(T-d) representing the stable cluster with a unique ... [more ▼]

The 20-nanogold cluster Au-20 exhibits a large variety of two- and three-dimensional isomeric forms. Among them is the ground-state isomer Au-20(T-d) representing the stable cluster with a unique tetrahedral shape, with all atoms on the surface, and large HOMO-LUMO gap which even slightly exceeds that of the buckyball fullerene C-60. The anionic cluster Au-20(-) (T-d) that holds its parent tetrahedral symmetry features a high catalytic activity. The list of the properties of the 20-nanogold clusters surveyed in the present work ranges from the energetic order of stability of its isomers to the optical absorption and excitation spectra of the Au-20(T-d) cluster. We also report the structures and properties of its doubly charged clusters Au-20(2+) and Au-20(2-) and computationally confirm that Au-20 is indeed stable. The zero-point-energy-corrected adiabatic second electron affinity of Au-20(T-d) amounts to 0.43-0.53 eV that is consistent with the experimental data. In addition, we provide computational evidence of the existence of the novel, hollow cage isomer of Au-20 and analyze its key properties. (C) 2007 Wiley Periodicals, Inc. [less ▲]

Towards parallel computing: representation of a linear finite state digital logic machine by a molecular relaxation process

in European Physical Journal D -- Atoms, Molecules, Clusters & Optical Physics (2007), 42(1), 49-59

A chemical system displaced not far from equilibrium is shown to offer a physical realization of a linear sequential digital logic machine. The requirement from the system is that its state is described ... [more ▼]

A chemical system displaced not far from equilibrium is shown to offer a physical realization of a linear sequential digital logic machine. The requirement from the system is that its state is described by giving the current values of the concentration of different chemical species. The time evolution is therefore described by a classical master equation. The Landau-Teller process of vibrational relaxation of diatomic molecules in a buffer gas is used as a concrete example where each vibrational level is taken to be a distinct species. The probabilities (= fractional concentrations) of the species of the physicochemical system are transcribed as words composed of letters from a finite alphabet. The essential difference between the finite precision of the logic machine and the seemingly unbounded number of significant figures that could be used to specify a concentration is emphasized. The transcription between the two is made by using modular arithmetic that is, is the arithmetic of congruence. A digital machine corresponding to the vibrational relaxation process is constructed explicitly for the simple case of three vibrational levels. In this exploratory effort we use words of only one letter. Even this is sufficient to achieve an exponentially large number of memory states. [less ▲]

The gold-ammonia bonding patterns of neutral and charged complexes Au m 0+/-1-(NH3)n. I. Bonding and charge alternation.

in Journal of Chemical Physics (2007), 127(19), 194305

The gold-ammonia bonding patterns of the complexes which are formed between the ammonia clusters (NH/sub 3/)/sub 1<or=n<or=3/ and gold clusters of different sizes that range from one gold atom to the tri ... [more ▼]

The gold-ammonia bonding patterns of the complexes which are formed between the ammonia clusters (NH/sub 3/)/sub 1<or=n<or=3/ and gold clusters of different sizes that range from one gold atom to the tri-, tetra-, and 20-nanogold clusters are governed by two basic and fundamentally different ingredients: the anchoring Au-N bond and the nonconventional N-H ... Au hydrogen bond. The latter resembles, by all features, a conventional hydrogen bond and is formed between a typical conventional proton donor N-H group and the gold cluster that behaves as a nonconventional proton acceptor. We provide strong computational evidence that the gold-ammonia bonding patterns exhibit distinct characteristics as the Z charge state of the gold cluster varies within Z=0,+/-1. The analysis of these bonding patterns and their effects on the N-H ... N H-bonded ammonia clusters are the subject of this paper. [less ▲]

The emergence of a coupled quantum dot array in a doped silicon nanowire gated by ultrahigh density top gate electrodes

in Journal Of Physical Chemistry c (2007), 111(48), 17852-17860

The electrical characteristics of Si nanowire gated by an array of very closely spaced nanowire gate electrodes are experimentally determined and theoretically modeled. Qualitative and quantitative ... [more ▼]

The electrical characteristics of Si nanowire gated by an array of very closely spaced nanowire gate electrodes are experimentally determined and theoretically modeled. Qualitative and quantitative changes in the transport characteristics of these devices, as a function of gate-array voltage, are described. Experiments are reported for two widths of Si nanowires, 40 and 17 nm, and for a varying number of gate electrodes, all spaced at a pitch of 33 tim. We find that these top nanowire gate electrodes can be utilized to locally deplete the carriers in the underlying Si nanowire and thus define an array of coupled quantum dots along the nanowire. Reproducible Coulomb blockade is observed, and clear diamond features are obtained when the conductance is plotted in the plane of the source-drain and gate voltages. The regularity of the diamond diagrams is imposed by the regularity of the SNAP top gate electrodes. Model computations of the electronic structure starting from a tight-biding Hamiltonian in the atomic basis suggest that the control made possible by the top gate voltage induces the emergence (and reversible submergence) of a coupled quantum dot structure in an otherwise homogenously doped Si nanowire. [less ▲]

Probing ultrafast purely electronic charge migration in small peptides

in Zeitschrift Fur Physikalische Chemie-International Journal Of Research In Physical Chemistry & Chemical Physics (2007), 221(5), 647-661

A pump-probe experiment that can examine a pure charge migration on a time scale short compared to the onset of nuclear motion is discussed. The mass spectrometric studies of Schlag et al. suggest that ... [more ▼]

A pump-probe experiment that can examine a pure charge migration on a time scale short compared to the onset of nuclear motion is discussed. The mass spectrometric studies of Schlag et al. suggest that short peptide terminated by an aromatic amino acid are particularly suitable test compounds. The pump pulse needs to ionize the molecule on a time scale short compared to the period of the electronic motion, typically sub-fs. However, ionization occurs preferentially when the electrical field of the light is maximal so that the duration of the pulse envelope can be somewhat longer. Detection by photoelectron spectrometry of the peptide cation, to produce a dication, is shown to be able to probe the electronic rearrangement. [less ▲]

Electrical transport in saturated and conjugated molecular wires

in Faraday Discussions (2006), 131

The mechanism for charge transport in dithio molecular wires tethered between two gold electrodes is investigated, using both a steady state and a time-dependent quantum mechanical approach. The interface ... [more ▼]

The mechanism for charge transport in dithio molecular wires tethered between two gold electrodes is investigated, using both a steady state and a time-dependent quantum mechanical approach. The interface with the electrodes is modeled by two gold clusters and the electronic structure of the entire Au-n-S-bridge-S-Au-n system is computed ab initio at the DFT level and semi-empirically, with the extended Huckel theory. Current vs. applied bias, I-V, curves are computed using a scattering Landauer-type formalism in a steady state picture. The applied source-drain and gate voltages are included at the ab initio level in the electronic Hamiltonian and found to influence strongly the I-V characteristics. The time evolution of a non stationary electronic wave packet initially localized on a gold atom at one end of the extended system shows that charge transfer proceeds sequentially, by a hopping mechanism, to the opposite end. Analysis of the effective one electron Hamiltonian matrix shows that the sulfur atom endows a resistive character to the Au-C-S junctions. The S atoms are however rather well coupled to both the gold and carbon atoms so that typically the super exchange limit for electron transfer is not reached unless the molecular bridge is saturated and the Fermi window function is narrow. [less ▲]

All-optical digital logic: Full addition or subtraction on a three-state system

in Physical Review. A (2006), 73(3),

Stimulated Raman adiabatic passage (STIRAP) is a well-studied pump-probe control scheme for manipulating the population of quantum states of atoms or molecules. By encoding the digits to be operated on as ... [more ▼]

Stimulated Raman adiabatic passage (STIRAP) is a well-studied pump-probe control scheme for manipulating the population of quantum states of atoms or molecules. By encoding the digits to be operated on as "on" or "off" laser input signals we show how STIRAP can be used to implement a finite-state logic machine. The physical conditions required for an effective STIRAP operation are related to the physical conditions expected for a logic machine. In particular, a condition is derived on the mean number of photons that represent an on pulse. A finite-state machine computes Boolean expressions that depend both on the input and on the present state of the machine. With two input signals we show how to implement a full adder where the carry-in digit is stored in the state of the machine. Furthermore, we show that it is possible to store the carry-out digit as the next state and thereby return the machine to a state ready for the next full addition. Such a machine operates as a cyclical full adder. We further show how this full adder can equally well be operated as a full subtractor. To the best of our knowledge this is the first example of a nanosized system that implements a full subtraction. [less ▲]

Thermometer ions for matrix-enhanced laser desorption/ionization internal energy calibration

in Rapid Communications in Mass Spectrometry : RCM (2003), 17(16), 1847-1854

This work describes a method to use relative fragmentation yields to characterize the internal energy distribution of ions produced by matrix-enhanced laser desorption/ionization mass spectrometry (MELDI ... [more ▼]

This work describes a method to use relative fragmentation yields to characterize the internal energy distribution of ions produced by matrix-enhanced laser desorption/ionization mass spectrometry (MELDI-MS, see: Wright LG, Cooks RG, Wood KL. Biomed. Mass Spectrom. 1985; 12:153162). Assuming that the fragmentation proceeds statistically and that the collisions in the source lead to a Boltzmann-like distribution of the internal energy, a characteristic parameter, the effective temperature, is introduced to describe the internal energy distribution of the ions observed. The hypotheses, advantages and drawbacks of the implementation of the method that uses substituted benzylpyridinium salts as thermometer ions are discussed. Use is made of two matrices that produce no matrix cations in MELDI and are suitable for small cationic salts. The actual value of this effective temperature significantly depends on an accurate determination of the threshold dissociation energies and on the time spent in the source, in addition to the statistical hypothesis itself. The method could be applied to normalize spectra in order to compare results issued from different instruments. [less ▲]

Unimolecular Reaction Dynamics from Kinetic Energy Release Distributions. 2. A Study of the Reaction C6H5Br+->C6H5++Br by the Maximum Entropy Method.

in Journal of Physical Chemistry (1996), 100(19), 8003-8007

The kinetic translational energy released in the unimolecular fragmentation reaction C6H5Br+->C6H5++Br has been experimentally studied in the microsecond time scale and theoretically analyzed by the ... [more ▼]

The kinetic translational energy released in the unimolecular fragmentation reaction C6H5Br+->C6H5++Br has been experimentally studied in the microsecond time scale and theoretically analyzed by the maximum entropy formalism. The appropriate functional form relating the actual distribution to its prior distribution (eq.2.3) involves the square root of the kinetic energy (i.e. the momentum associated with the relative translational energy). A value of 0.26+/-0.02 eV is obtained by the entropy deficiency distribution at an internal energy of 0.85 eV above the reaction threshold. From this value, it can be concluded that 77% of the transition state phase space is efficiently sampled. [less ▲]

Quantum effects in competitive decay

in Chemical Physics (1991), 153