270 research outputs found
Rotation-Induced Breakdown of Torsional Quantum Control
Control of the torsional angles of nonrigid molecules is key for the development of emerging areas like molecular electronics and nanotechnology. Based on a rigorous calculation of the rotation-torsion-Stark energy levels of nonrigid biphenyl-like molecules, we show that, unlike previously believed, instantaneous rotation-torsion-Stark eigenstates of such molecules, interacting with a strong laser field, present a large degree of delocalization in the torsional coordinate even for the lowest energy states. This is due to a strong coupling between overall rotation and torsion leading to a breakdown of the torsional alignment. Thus, adiabatic control of changes on the planarity of this kind of molecule is essentially impossible unless the temperature is on the order of a few Kelvin
Intrinsically biased electrocapacitive catalysis
We propose the application of the contact potential from metal-metal junctions or the built-in potential of semiconductor p-np-n junctions to induce or catalyze chemical reactions. Free of external sources, this intrinsic potential across microscale and nanoscale vacuum gaps establishes electric fields in excess of 10^7V/m. The electrostatic potential energy of these fields can be converted into useful chemical energy. As an example, we focus on the production of superthermal gas ions to drive reactions. Analysis indicates that this intrinsically biased electrocapacitive catalysis can achieve locally directed ion energies up to a few electron volts and local gas temperatureboosts in excess of 10^4K. Practical considerations for implementation and experimental tests are considered
Enhanced Molecular Orientation Induced by Molecular Anti-Alignment
We explore the role of laser induced anti-alignment in enhancing molecular
orientation. A field-free enhanced orientation via anti-alignment scheme is
presented, which combines a linearly polarized femtosecond laser pulse with a
half-cycle pulse. The laser pulse induces transient anti-alignment in the plane
orthogonal to the field polarization, while the half-cycle pulse leads to the
orientation. We identify two qualitatively different enhancement mechanisms
depending on the pulse order, and optimize their effects using classical and
quantum models both at zero and non-zero temperature
Nuclear spin selective laser control of rotational and torsional dynamics
We explore the possibility of controlling rotational-torsional dynamics of
non-rigid molecules with strong, non-resonant laser pulses and demonstrate
that transient, laser-induced torsional alignment depends on the nuclear spin
of the molecule. Consequently, nuclear spin isomers can be manipulated
selectively by a sequence of time-delayed laser pulses. We show that two
pulses with different polarization directions can induce either overall
rotation or internal torsion, depending on the nuclear spin.Nuclear spin
selective control of the angular momentum distribution may open new ways to
separate and explore nuclear spin isomers of polyatomic molecules
Perfect coupling of light to surface plasmons with ultra-narrow linewidths
We examine the coupling of electromagnetic waves incident normal to a thin
silver film that forms an oscillatory grating embedded between two otherwise
uniform, semi-infinite half spaces. Two grating structures are considered, in
one of which the mid point of the Ag film remains fixed whereas the thickness
varies sinusoidally, while in the other the mid point oscillates sinusoidally
whereas the film thicknesses remains fixed. On reducing the light wavelength
from the long wavelength limit, we encounter signatures in the transmission, T,
and reflection, R, coefficients associated with: i) the short-range surface
plasmon mode, ii) the long-range surface plasmon mode, and iii) electromagnetic
diffraction tangent to the grating. The first two features can be regarded as
generalized (plasmon) Wood's anomalies whereas the third is the first-order
conventional (electromagnetic) Wood's anomaly. The energy density at the film
surface is enhanced for wavelengths corresponding to these three anomalies,
particularly for the long range plasmon mode in thin films. When exciting the
silver film with a pair of waves incident from opposite directions, we find
that by adjusting the grating oscillation amplitude and fixing the relative
phase of the incoming waves to be even or odd, T+R can be made to vanish for
one or the other of the plasmon modes; this corresponds to perfect coupling
(impedance matching in the language of electrical engineering) between the
incoming light and these modes.Comment: 13 pages, 5 figures. accepted J. Chem. Phy
Vibrational effects in laser driven molecular wires
The influence of an electron-vibrational coupling on the laser control of
electron transport through a molecular wire that is attached to several
electronic leads is investigated. These molecular vibrational modes induce an
effective electron-electron interaction. In the regime where the wire electrons
couple weakly to both the external leads and the vibrational modes, we derive
within a Hartree-Fock approximation a nonlinear set of quantum kinetic
equations. The quantum kinetic theory is then used to evaluate the laser
driven, time-averaged electron current through the wire-leads contacts. This
novel formalism is applied to two archetypical situations in the presence of
electron-vibrational effects, namely, (i) the generation of a ratchet or pump
current in a symmetrical molecule by a harmonic mixing field and (ii) the laser
switching of the current through the molecule.Comment: 12 pages, 7 figures, RevTeX4 require
Traversal time for electron tunneling in water
The traversal time for tunneling is a measure of the time during which the
transmitted particle can be affected by interactions localized in the barrier.
The Buttiker-Landauer approach, which estimates this time by imposing an
internal clock on the system, has been applied so far for relatively simple
1-dimensional models. Here we apply this approach to estimate the traversal
time for electron tunneling through a realistic 3-dimensional model of a water
layer. Observed structure in the energy dependence of times computed reflects
the existence of transient tunneling resonances associated with instantaneous
water structures.Comment: 9 pages, 3 figures. Submitted to the Journal of Chemical Physic
Monotonically convergent optimal control theory of quantum systems under a nonlinear interaction with the control field
We consider the optimal control of quantum systems interacting non-linearly
with an electromagnetic field. We propose new monotonically convergent
algorithms to solve the optimal equations. The monotonic behavior of the
algorithm is ensured by a non-standard choice of the cost which is not
quadratic in the field. These algorithms can be constructed for pure and
mixed-state quantum systems. The efficiency of the method is shown numerically
on molecular orientation with a non-linearity of order 3 in the field.
Discretizing the amplitude and the phase of the Fourier transform of the
optimal field, we show that the optimal solution can be well-approximated by
pulses that could be implemented experimentally.Comment: 24 pages, 11 figure
Squeezing of Atoms in a Pulsed Optical Lattice
We study the process of squeezing of an ensemble of cold atoms in a pulsed
optical lattice. The problem is treated both classically and
quantum-mechanically under various thermal conditions. We show that a dramatic
compression of the atomic density near the minima of the optical potential can
be achieved with a proper pulsing of the lattice. Several strategies leading to
the enhanced atomic squeezing are suggested, compared and optimized.Comment: Latex, 9 pages, 10 figures, submitted to PR
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