297 research outputs found
Anisotropy Control in Photoelectron Spectra: A Coherent Two-Pulse Interference Strategy
Coherence among rotational ion channels during photoionization is exploited
to control the anisotropy of the resulting photoelectron angular distributions
at specific photoelectron energies. The strategy refers to a robust and single
parameter control using two ultra-short light pulses delayed in time. The first
pulse prepares a superposition of a few ion rotational states, whereas the
second pulse serves as a probe that gives access to a control of the molecular
asymmetry parameter for individual rotational channels. This is
achieved by tuning the time delay between the pulses leading to channel
interferences that can be turned from constructive to destructive. The
illustrative example is the ionization of the state of
Li. Quantum wave packet evolutions are conducted including both
electronic and nuclear degrees of freedom to reach angle-resolved photoelectron
spectra. A simple interference model based on coherent phase accumulation
during the field-free dynamics between the two pulses is precisely exploited to
control the photoelectron angular distributions from almost isotropic, to
marked anisotropic
Optimally Controlled Field-Free Orientation of the Kicked Molecule
Efficient and long-lived field-free molecular orientation is achieved using
only two kicks appropriately delayed in time. The understanding of the
mechanism rests upon a molecular target state providing the best efficiency
versus persistence compromise. An optimal control scheme is referred to for
fixing the free parameters (amplitudes and the time delay between them). The
limited number of kicks, the robustness and the transposability to different
molecular systems advocate in favor of the process, when considering its
experimental feasibility.Comment: 5 pages, 2 figures (version 2 contains some minor additions and
corrects many misprints
Kaariye Camii mozaikleri
Taha Toros Arşivi, Dosya No: 102-Camilerİstanbul Kalkınma Ajansı (TR10/14/YEN/0033) İstanbul Development Agency (TR10/14/YEN/0033
Molecular Alignment and Orientation: From Laser-Induced Mechanisms to Optimal Control
Genetic algorithms, as implemented in optimal control strategies, are currently successfully exploited in a wide range of problems in molecular physics. In this context, laser control of molecular alignment and orientation remains a very promising issue with challenging applications extending from chemical reactivity to nanoscale design. We emphasize the complementarity between basic quantum mechanisms monitoring alignment/orientation processes and optimal control scenarios. More explicitly, if on one hand we can help the optimal control scheme to take advantage of such mechanisms by appropriately building the targets and delineating the parameter sampling space, on the other hand we expect to learn, from optimal control results, some robust and physically sound dynamical mechanisms. We present basic mechanisms for alignment and orientation, such as pendular states accommodated by the molecule-plus-field effective potential and the "kick" mechanism obtained by a sudden excitation. Very interestingly, an optimal control scheme for orientation, based on genetic algorithms, also leads to a sudden pulsed field bearing the characteristic features of the kick mechanism. Optimal pulse shaping for very efficient and long-lasting orientation, together with robustness with respect to temperature effects, are among our future prospects
Towards Laser Control of Open Quantum Systems: Memory Effects
Laser control of Open Quantum Systems (OQS) is a challenging issue as
compared to its counterpart in isolated small size molecules, basically due to
very large numbers of degrees of freedom to be accounted for. Such a control
aims at appropriately optimizing decoherence processes of a central two-level
system (a given vibrational mode, for instance) towards its environmental bath
(including, for instance, all other normal modes). A variety of applications
could potentially be envisioned, either to preserve the central system from
decaying (long duration molecular alignment or orientation, qubit decoherence
protection) or, to speed up the information flow towards the bath (efficient
charge or proton transfers in long chain organic compounds). Achieving such
controls require some quantitative measures of decoherence in relation with
memory effects in the bath response, actually given by the degree of
non-Markovianity. Characteristic decoherence rates of a Spin-Boson model are
calculated using a Nakajima-Zwanzig type master equation with converged HEOM
expansion for the memory kernel. It is shown that, by adequately tuning the
two-level transition frequency through a controlled Stark shift produced by an
external laser field, non-Markovianity can be enhanced in a continuous way
leading to a first attempt towards the control of OQS
Time-dependent unitary perturbation theory for intense laser driven molecular orientation
We apply a time-dependent perturbation theory based on unitary
transformations combined with averaging techniques, on molecular orientation
dynamics by ultrashort pulses. We test the validity and the accuracy of this
approach on LiCl described within a rigid-rotor model and find that it is more
accurate than other approximations. Furthermore, it is shown that a noticeable
orientation can be achieved for experimentally standard short laser pulses of
zero time average. In this case, we determine the dynamically relevant
parameters by using the perturbative propagator, that is derived from this
scheme, and we investigate the temperature effects on the molecular orientation
dynamics.Comment: 16 pages, 6 figure
Ultrafast Molecular Imaging by Laser Induced Electron Diffraction
We address the feasibility of imaging geometric and orbital structure of a
polyatomic molecule on an attosecond time-scale using the laser induced
electron diffraction (LIED) technique. We present numerical results for the
highest molecular orbitals of the CO2 molecule excited by a near infrared
few-cycle laser pulse. The molecular geometry (bond-lengths) is determined
within 3% of accuracy from a diffraction pattern which also reflects the nodal
properties of the initial molecular orbital. Robustness of the structure
determination is discussed with respect to vibrational and rotational motions
with a complete interpretation of the laser-induced mechanisms
- …