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

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

Electrical transport in saturated and conjugated molecular wires

peer reviewedThe 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

The time scale for electronic reorganization upon sudden ionization of the water and water-methanol hydrogen bonded dimers and of the weakly bound NO dimer

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

Attosecond pumping of nonstationary electronic states of LiH: Charge shake-up and electron density distortion

peer reviewe

Time-resolved electrochemical spectroscopy of charge migration in molecular wires: Computational evidence for rich electronic dynamics

peer reviewedElectrical conduction through a molecule tethered by thiol bridges between two gold clusters is examined from a time-dependent point of view. The shortest electronic time scale for charge migration is a few femtoseconds transit, which is too swift for coupling to the nuclei, that proceeds by super exchange. An order of magnitude slower transfer occurs sequentially through the lower-in-energy sigma bonds. The electronic structure computations are performed at a high level ab initio density functional theory level where the external electric fields are included as part of the Hamiltonian. The structure computed includes the thiol bridge as well as the gold trimer at either end of the molecule. The results shown in detail are for the saturated dithiohexane -S-(CH2)(6)-S- bridge between the two gold trimers

Laser steered ultrafast quantum dynamics of electrons in LiH.

The response of the electronic system of LiH to a few-cycle strong field is computed by a time-dependent multiconfiguration method using a large, adaptive, basis set. The intensity, pulse duration, polarization, and phase of carrier frequency can all be tuned to steer the motion of the electrons. It is shown possible to, e.g., direct the electrons to move along the Li-H bond or normal to it. By shifting the phase, the electrons can be driven toward the Li nucleus or away from it. When the pulse is polarized not along the bond the result is a rotation of the charge density

Redox Executed Logic Operations through the Reversible Voltammetric Response Characteristics of Electroactive Self-Assembled Monolayers

peer reviewe

Pump and probe ultrafast electron dynamics in LiH: a computational study

A time-dependent multiconfiguration method with a large electronic basis set is used to compute the response of all the electrons of LiH to a few-cycle intense pump field followed by a probe pulse. The ultrashort pump pulse excites a coherent superposition of stationary electronic states and, by changing the pump parameters such as intensity, duration, polarization and phase of carrier frequency, one can steer the motion of the electrons. Particular attention is given to the control provided by the polarization and by the phase. For example, a change in polarization is used to select an electronic wave packet that is rotating in a plane perpendicular to the bond or rotation in a plane containing the bond. The electronic wave packet can be probed by a delayed second pulse. This delayed probe pulse is also included in the Hamiltonian with the result that the frequency dispersed probe spectrum can be computed and displayed as a two-dimensional plot

Quantum Nuclear Dynamics Pumped and Probed by Ultrafast Polarization Controlled Steering of a Coherent Electronic State in LiH

peer reviewedSteering chemical reactivity DCS001262