A single molecule logic gate using electronically excited states and ionization/fragmentation can take advantage of the differences in cross-sections for one and two photon absorption. Fault tolerant optically pumped half adder and full adder are discussed as applications. A full adder requires two separate additions, and the logic concatenation that is required to implement this is physically achieved by an intramolecular transfer along the side chain of 2-phenylethyl-N,N-dimethylamine (PENNA). Solutions of the kinetic equations for the temporal evolution of the concentration of different states in the presence of time-varying laser fields are used to illustrate the high contrast ratios that are potentially possible for such devices.

During a chemical reaction the electronic structure of the reactants reforms to the structure of the products. In the simplest case, an old bond is broken and simultaneously a new bond is formed. Even when this change occurs adiabatically, meaning that the electronic charge distribution tracks the motion of the nuclei without change in its quantum state, there is a displacement of charge, particularly so if there is a switch from a covalent to an ionic bonding. This reorganization can be probed by light emitted during the very collision. The F + H(2) reaction is used as a computational example.

When the valence molecular orbital is localized sudden ionization can cause the nascent hole to move rapidly even before any relaxation of the geometry occurs. Hydrogen bonded clusters offer suitable test systems where the hole is initially localized on one moiety. Computational studies are reported for the water dimer and water-methanol bimer. The local ionization potential of water is different in the methanol-water and water-methanol conformers and this difference is very clearly reflected in the dynamics of charge migration. For the NO dimer the results are that its structure is symmetric so that the two NO molecules are equivalent and do not exhibit the required localization. The role of symmetry is also evident in the charge propagation for holes created in different orbitals. Localization of the initial hole distribution even if absent in the bare molecule can still be induced by the intense electric field of a sudden photoionization. This effect is computationally studied for the NO dimer in the presence of a static electric field. (c) 2006 American Institute of Physics.

Conduction spectroscopy measures the current I through a nanosystem as a function of the voltage V between two electrodes. The differential conductance, dl/dV, has peaks that can be assigned to resonance conditions with different electronic levels of the system. Between these increments, the current has roughly constant plateaus. We discuss how measurements of the current vs. voltage can be used to perform Boolean operations and hence construct finite state logic machines and combinational circuits. The inputs to the device are the source-drain voltage, including its sign, and a gate voltage applied in a manner analogous to optical Stark spectroscopy. As simple examples, we describe a two-state set-reset machine (a machine whose output depends on the input and also on its present state) and a full adder circuit (a circuit that requires three inputs and provides two outputs).

Matter under extreme conditions can be generated by a collision of a hypersonic cluster with a surface. The ultra-high-pressure interlude lasts only briefly from the impact until the cluster shatters. We discuss the theoretical characterization of the pressure using the virial theorem and develop a constrained molecular-dynamics procedure to compute it. The simulations show that for rare-gas clusters the pressures reach the megabar range. The contribution to the pressure from momentum transfer is comparable in magnitude and is of the same sign as that (''the internal pressure'') due to repulsive interatomic forces. The scaling of the pressure with the reduced mechanical variables is derived and validated with reference to the simulations.

The temperature and solvent composition dependence of the electrochemically stimulated rate of shuttling of the redox-active cyclophane, cyclobis(paroquat-p-phenylene), on a molecular string has been studied. The molecular string includes a pi-donor diiminebenzene-site that is associated on one side with on electrode, and stoppered on the other side with on adamantane unit. The cyclophone rests on the pi-donor site, owing to stabilizing pi-donor-acceptor interactions. Electrochemical reduction of the cyclophone units, to the bis-radical cation cyclophone, results in the shuttling of the reduced cyclophane towards the electrode, a process that is driven by the removal of the stabilizing donor-acceptor interactions, and the electrostatic attraction of the reduced product by the electrode. The latter process is energetically downhill, and is temperature-independent. Upon oxidation of the reduced cyclophane that is associated with the electrode, the energetically uphill shuttling of the oxidized cyclophane to the pi-donor site proceeds. The rote of this translocation process has been found to be temperature-dependent, and controlled by the solvent composition. The experimental results have been theoretically analyzed in terms of Kramers' molecular friction model. The theoretical fitting of the experimental results, using solutions of variable composition, reveals that the rate-constants for the uphill reaction in a pure aqueous solution follow the temperature-dependence of the viscosity of water. The results demonstrate the significance of friction phenomena in shuttling processes within molecular machines.

Transmission of electrons through orbitals of molecules is discussed using high-level ab initio methods that allow an orbital interpretation. The transmission is computed for a molecule tethered between gold atoms. Strong coupling to the gold is achieved using a sulfur atom at each end of the molecule. sigma-type orbitals can conduct as well as conjugated bridges provided that the orbitals have weights on the sulfur atoms. The current depends exponentially on the voltage applied to the molecule and this voltage can also shift the electronic density and further alter the transmission. The conductivity can also be modified by a gate voltage. (C) 2003 Elsevier B.V. All rights reserved.

We study ordered arrays of Quantum Dots (QDs) as model systems for the electronic structure and response of solids and devices built from nanoscale components. QDs self-assemble as two-dimensional solids, with novel optical and electric properties, which can be experimentally tuned. The properties are controlled chemically via the selection of the composition and size of the individual QDs and physically through such external controls as the packing, temperature, and electrical and magnetic fields. The freedom of the architectural design is constrained because even the best synthesis does not yield dots of exactly the same size. We discuss the effects of disorder on the electronic structure of arrays of metallic dots and on their transport properties. (C) 2004 Wiley Periodicals, Inc.

A possible mechanism for negative differential resistance is discussed. The level crossing is induced by the source-drain voltage applied across the bridge. The effect is most dramatic when the zero field levels that are resonant with the electrodes are almost degenerate. It is suggested that such degeneracies can arise often when the junctions on either side are weakly coupled by the bridge. Quantitative results for I-V curves are reported on the basis of high-level electronic structure computations for the junction-bridge-junction region and where the electric field is included in the Hamiltonian. (C) 2004 American Institute of Physics.

Microelectronic sensors require the nanowiring of a selectively active site to an electrode. Different molecules can be used as the bridge that establishes the electrical communication. We report computational results for the current carried by molecules tethered between two gold clusters as a function of the overvoltage. The computations include the effect of the voltage at the ab initio level. The trends are consistent with the currents as measured by electrochemical means and suggest that the rate of charge migration can reach far higher values than measured given somewhat higher applied bias or the application of a gate voltage. The role of polarization of the molecular charge density by the applied voltage can be quite significant with definite propensity for the orientation of the molecular charge density with respect to the field. Conduction spectroscopy is therefore analogous to optical spectroscopy in strong laser fields where the field is not a weak probe but C dresses the system and can be used to control it.

The chemical kinetic description of time evolution where the phase is random but the states are discrete is discussed as a basis for a computational approach. This proposed scheme uses numbers in the entire range of 0 to 1 to represent Boolean propositions. In the implementation by chemical kinetics these numbers are the mole fractions of different species. Vibrational relaxation in a mixture of HCI and DCI is the physical system that is used to illustrate the approach. Energy exchange in such a mixture corresponds to two strongly coupled two-level systems. A search problem, previously discussed in the quantum computing literature, is solved as an example. The solution requires the same number of function evaluations as in the quantal case. The action of the oracle is described in detail.

The hypersonic impact of a molecular cluster at a hard surface generates a hot and compressed globule that in a very short while expands and shatters. Even at impact velocities below the onset of ionization this hot matter has a time varying transient dipole that can emit light. We discuss the spectral range and the power (in absolute units) of the emission spectrum. The computational results for the emission spectrum from our molecular dynamics simulation are compared to extrapolations of experimental results for collision-induced absorption at lower energies. The very short time interval during which the cluster survives intact means that the emitted power is low so options for increasing the yield of photons are discussed.

Emission spectra of mixed rare gas clusters, heated by impact with a hard surface at hypersonic velocities, are shown to extend into the near-IR and visible regimes. The emission is due to the transient dipole that arises during the collision of dissimilar atoms. The simulations are for a cluster that remains in the electronic ground state throughout the collision and use classical dynamics to determine the positions of the atoms vs time. The spectrum is computed as the Fourier transform of the (quantum mechanical) time rate of change of the dipole of the cluster. The time dependence of the dipole velocity is obtained by replacing the positions of the atoms by the computed classical functions of time. Taking the Fourier transform of the dipole velocity rather than of the dipole itself introduces a quantal correction with the result that the computed spectrum satisfies the oscillator sum rule. Binary collisions make the major contribution to the spectrum and there are hardly any caging effects. The spectral density of emitted photons is found to be thermal with a temperature that scales linearly with the impact velocity. Using the oscillator sum rules, this temperature is related to the deformation energy of the electronic charge cloud of the cluster. The hot cluster shatters and the fragments are in translational thermal equilibrium with a mean energy that scales linearly with the energy of impact. The temperature of the emitted light is, therefore, significantly lower than the translational temperature.

Experimental and computational studies demonstrating that the conduction of compressed, two-dimensional arrays of hexagonally ordered Ag quantum dots (QDs) may be varied through the influence of applied electric fields are reported and discussed. Monolayers of Ag QDs are incorporated into three-terminal (gated) devices, in which temperature, source-drain voltage (V-sd), gating voltage (V-g), compression of the array, and QD size distribution may all be varied. Experimental and computational results are compared in an effort to construct a physical picture of the system. Current vs V-sd plots at low temperatures exhibit systematic nonlinearities that change over to an ohmic-like behavior at higher temperatures and/or higher V-sd. The voltage-induced transition is discussed as a transition of the conducting states from domain localized to delocalized. Such a transition was previously observed in the temperature dependence of the resistance. The computational model reveals that this transition is also highly sensitive to both the compression of the array and the size-distribution of the dots. We calculate the influence of V-g on the conductivity of the QD array, using the same computational model. In both the experiment and the model, we find a significant voltage gating effect and we observe hole-type conductivity of the array. Overall, the results demonstrate that low-temperature transport measurements provide a spectroscopic-like probe of the electronic states of the QD lattice. The theoretical approach further suggests that quite different gating behavior can be observed for electrodes with a different Fermi energy than the gold electrodes used in the experiment.

Collision induced spectra allow a characterization of the rate of change of the dipolar asymmetry of the electronic charge distribution. We compute such spectra using classical trajectories but include essential quantal corrections. These corrections are necessary to satisfy the sum rules to leading order in Planck's constant. A corrected computation using classical dynamics for the motion during the collision results when the spectrum is computed from the dipole velocity rather than from the dipole itself. The resulting spectrum is then an asymmetric function of frequency. The Laplace distribution is discussed as a convenient representation of the asymmetric spectrum over both the negative and positive frequency axis. For the emission spectrum the frequency distribution corresponds to the Planck equation with a radiation temperature that is equal to the mean deformation energy of the electronic charge distribution. Therefore, collision induced emission provides a thermometer for the electronic deformation during the collision. (C) 2003 American Institute of Physics.

Measuring the wavelength dependence of photodissociation transition amplitudes to crossing electronic states is discussed with reference to extracting time domain information. Outwards from the crossing the system evolves on one or the other state as ascertained from the electronic state of the products. One can therefore measure the phase difference for the motion on the two potentials accumulated between the Franck-Condon region and the crossing point. A computational example, the photodissociation of HI and DI, is provided. The interference varies rapidly with wavelength and provides considerable leverage for determination of ultra-short time differences. The interference exhibits a significant isotope effect. (C) 2003 Elsevier Science B.V. All rights reserved.

Computed dc transport in compressed arrays of metallic quantum dots exhibits a voltage-induced phase transition at low temperatures. The transition is seen in the temperature dependence of the conductance at different voltages: from a variable hopping dependence at low voltage to an ohmic, activated behavior at higher voltages. The computations also exhibit the transition as a break in the current versus voltage plots at low temperatures where, at higher voltages, the plot is linear. At higher temperatures, the conductance is ohmic. A many-electron basis is used. The same transition is seen in the surface potential contours. (C) 2003 American Institute of Physics.