451 research outputs found
Mechanisms in Adaptive Feedback Control: Photoisomerization in a Liquid
The underlying mechanism for Adaptive Feedback Control in the experimental
photoisomerization of NK88 in methanol is exposed theoretically. With given
laboratory limitations on laser output, the complicated electric fields are
shown to achieve their targets in qualitatively simple ways. Further, control
over the cis population without laser limitations reveals an incoherent
pump-dump scenario as the optimal isomerization strategy. In neither case are
there substantial contributions from quantum multiple-path interference or from
nuclear wavepacket coherence. Environmentally induced decoherence is shown to
justify the use of a simplified theoretical model.Comment: 10 pages, 3 figures, to be published in Phys. Rev. Let
Robust ultrafast currents in molecular wires through Stark shifts
A novel way to induce ultrafast currents in molecular wires using two
incident laser frequencies, and , is demonstrated. The
mechanism relies on Stark shifts, instead of photon absorption, to transfer
population to the excited states and exploits the temporal profile of the field
to generate phase controllable transport. Calculations in a
\emph{trans}-polyacetylene oligomer coupled to metallic leads indicate that the
mechanism is highly efficient and robust to ultrafast electronic dephasing
processes induced by vibronic couplings.Comment: 4 pages, 2 figures, accepted to Physical Review Letter
Generic Quantum Ratchet Accelerator with Full Classical Chaos
A simple model of quantum ratchet transport that can generate unbounded
linear acceleration of the quantum ratchet current is proposed, with the
underlying classical dynamics fully chaotic. The results demonstrate that
generic acceleration of quantum ratchet transport can occur with any type of
classical phase space structure. The quantum ratchet transport with full
classical chaos is also shown to be very robust to noise due to the large
linear acceleration afforded by the quantum dynamics. One possible experiment
allowing observation of these predictions is suggested.Comment: 4 pages, 4 figure
Overlapping Resonances Interference-induced Transparency: The Photoexcitation Spectrum of Pyrazine
The phenomenon of "overlapping resonances interference-induced transparency"
(ORIT) is introduced and studied in detail for the
photoexcitation of cold pyrazine (CHN). In ORIT a molecule becomes
transparent at specific wavelengths due to interferences between envelopes of
spectral lines displaying overlapping resonances. An example is the
internal conversion in pyrazine where destructive
interference between overlapping resonances causes the light
absorption to disappear at certain wavelengths. ORIT may be of practical
importance in multi-component mixtures where it would allow for the selective
excitation of some molecules in preference to others. Interference induced
cross section enhancement is also shown.Comment: 13 pages, 7 figure
Classical-Wigner Phase Space Approximation to Cumulative Matrix Elements in Coherent Control
The classical limit of the Wigner-Weyl representation is used to approximate
products of bound-continuum matrix elements that are fundamental to many
coherent control computations. The range of utility of the method is quantified
through an examination of model problems, single-channel Na_2 dissociation and
multi-arrangement channel photodissociation of CH_2IBr. Very good agreement
with the exact quantum results is found for a wide range of system parameters.Comment: 17 pages, 12 figures, to appear in Journal of Chemical Physic
Laser-induced currents along molecular wire junctions
The treatment of the previous paper is extended to molecular wires.
Specifically, the effect of electron-vibrational interactions on the electronic
transport induced by femtosecond laser fields along unbiased
molecular nanojunctions is investigated. For this, the photoinduced vibronic
dynamics of trans-polyacetylene oligomers coupled to macroscopic metallic leads
is followed in a mean-field mixed quantum-classical approximation. A reduced
description of the dynamics is obtained by introducing projective lead-molecule
couplings and deriving an effective Schr\"odinger equation satisfied by the
orbitals in the molecular region. Two possible rectification mechanisms are
identified and investigated. The first one relies on near-resonance
photon-absorption and is shown to be fragile to the ultrafast electronic
decoherence processes introduced by the wire's vibrations. The second one
employs the dynamic Stark effect and is demonstrated to be highly efficient and
robust to electron-vibrational interactions.Comment: 14 pages, 10 figures. Accepted in J. Chem. Phy
Symmetric Rotating Wave Approximation for the Generalized Single-Mode Spin-Boson System
The single-mode spin-boson model exhibits behavior not included in the
rotating wave approximation (RWA) in the ultra and deep-strong coupling
regimes, where counter-rotating contributions become important. We introduce a
symmetric rotating wave approximation that treats rotating and counter-rotating
terms equally, preserves the invariances of the Hamiltonian with respect to its
parameters, and reproduces several qualitative features of the spin-boson
spectrum not present in the original rotating wave approximation both
off-resonance and at deep strong coupling. The symmetric rotating wave
approximation allows for the treatment of certain ultra and deep-strong
coupling regimes with similar accuracy and mathematical simplicity as does the
RWA in the weak coupling regime. Additionally, we symmetrize the generalized
form of the rotating wave approximation to obtain the same qualitative
correspondence with the addition of improved quantitative agreement with the
exact numerical results. The method is readily extended to higher accuracy if
needed. Finally, we introduce the two-photon parity operator for the two-photon
Rabi Hamiltonian and obtain its generalized symmetric rotating wave
approximation. The existence of this operator reveals a parity symmetry similar
to that in the Rabi Hamiltonian as well as another symmetry that is unique to
the two-photon case, providing insight into the mathematical structure of the
two-photon spectrum, significantly simplifying the numerics, and revealing some
interesting dynamical properties.Comment: 11 pages, 5 figure
Cold Atomic Collisions: Coherent Control of Penning and Associative Ionization
Coherent Control techniques are computationally applied to cold (1mK < T < 1
K) and ultracold (T < 1 microK) Ne*(3s,3P2) + Ar(1S0) collisions. We show that
by using various initial superpositions of the Ne*(3s,3P2) M = {-2,-1,0,1,2}
Zeeman sub-levels it is possible to reduce the Penning Ionization (PI) and
Associative Ionization (AI) cross sections by as much as four orders of
magnitude. It is also possible to drastically change the ratio of these two
processes. The results are based on combining, within the "Rotating Atom
Approximation", empirical and ab-initio ionization-widths.Comment: 4 pages, 2 tables, 2 figure
A Selective Advantage for Conservative Viruses
In this letter we study the full semi-conservative treatment of a model for
the co-evolution of a virus and an adaptive immune system. Regions of viability
are calculated for both conservatively and semi-conservatively replicating
viruses interacting with a realistic semi-conservatively replicating immune
system. The conservative virus is found to have a selective advantage in the
form of an ability to survive in regions with a wider range of mutation rates
than its semi-conservative counterpart. This may help explain the existence of
a rich range of viruses with conservatively replicating genomes, a trait which
is found nowhere else in nature.Comment: 4 pages, 2 figure
Overlapping resonances in the control of intramolecular vibrational redistribution
Coherent control of bound state processes via the interfering overlapping
resonances scenario [Christopher et al., J. Chem. Phys. 123, 064313 (2006)] is
developed to control intramolecular vibrational redistribution (IVR). The
approach is applied to the flow of population between bonds in a model of
chaotic OCS vibrational dynamics, showing the ability to significantly alter
the extent and rate of IVR by varying quantum interference contributions.Comment: 10 pages, 7 figure
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