55 research outputs found
Resonantly enhanced pair production in a simple diatomic model
A new mechanism for the production of electron-positron pairs from the
interaction of a laser field and a fully stripped diatomic molecule in the
tunneling regime is presented. When the laser field is turned off, the Dirac
operator has resonances in both the positive and the negative energy continua
while bound states are in the mass gap. When this system is immersed in a
strong laser field, the resonances move in the complex energy plane: the
negative energy resonances are pushed to higher energies while the bound states
are Stark shifted. It is argued here that there is a pair production
enhancement at the crossing of resonances by looking at a simple 1-D model: the
nuclei are modeled simply by Dirac delta potential wells while the laser field
is assumed to be static and of finite spatial extent. The average rate for the
number of electron-positron pairs produced is evaluated and the results are
compared to the single nucleus and to the free cases. It is shown that
positrons are produced by the Resonantly Enhanced Pair Production (REPP)
mechanism, which is analogous to the resonantly enhanced ionization of
molecular physics. This phenomenon could be used to increase the number of
pairs produced at low field strength, allowing the study of the Dirac vacuum.Comment: 11 pages, 4 figure
Long-range Energy Transfer and Ionization in Extended Quantum Systems Driven by Ultrashort Spatially Shaped Laser Pulses
The processes of ionization and energy transfer in a quantum system composed
of two distant H atoms with an initial internuclear separation of 100 atomic
units (5.29 nm) have been studied by the numerical solution of the
time-dependent Schr\"odinger equation beyond the Born-Oppenheimer
approximation. Thereby it has been assumed that only one of the two H atoms was
excited by temporally and spatially shaped laser pulses at various laser
carrier frequencies. The quantum dynamics of the extended H-H system, which was
taken to be initially either in an unentangled or an entangled ground state,
has been explored within a linear three-dimensional model, including two z
coordinates of the electrons and the internuclear distance R. An efficient
energy transfer from the laser-excited H atom (atom A) to the other H atom
(atom B) and the ionization of the latter have been found. It has been shown
that the physical mechanisms of the energy transfer as well as of the
ionization of atom B are the Coulomb attraction of the laser driven electron of
atom A by the proton of atom B and a short-range Coulomb repulsion of the two
electrons when their wave functions strongly overlap in the domain of atom B.Comment: 11 pages, 7 figure
Visualizing quantum entanglement and the EPR paradox during the photodissociation of a diatomic molecule using two ultrashort laser pulses
We investigate theoretically the dissociative ionization of a H2+ molecule
using two ultrashort laser (pump-probe) pulses. The pump pulse prepares a
dissociating nuclear wave packet on an ungerade surface of H2+. Next, an UV (or
XUV) probe pulse ionizes this dissociating state at large (R = 20 - 100 bohr)
internuclear distance. We calculate the momenta distributions of protons and
photoelectrons which show a (two-slit-like) interference structure. A general,
simple interference formula is obtained which depends on the electron and
protons momenta, as well as on the pump-probe delay on the pulses durations and
polarizations. This interference can be interpreted as visualization of an
electron state delocalized over the two-centres. This state is an entangled
state of a hydrogen atom with a momentum p and a proton with an opposite
momentum. -p dissociating on the ungerade surface of H2+. This pump-probe
scheme can be used to reveal the nonlocality of the electron which intuitively
should be localized on just one of the protons separated by the distance R much
larger than the atomic Bohr orbit
Transition State Theory For Laser-Driven Reactions
Recent developments in transition state theory brought about by dynamical systems theory are extended to time-dependent systems such as laser-driven reactions. Using time-dependent normal form theory, the authors construct a reaction coordinate with regular dynamics inside the transition region. The conservation of the associated action enables one to extract time-dependent invariant manifolds that act as separatrices between reactive and nonreactive trajectories and thus make it possible to predict the ultimate fate of a trajectory. They illustrate the power of our approach on a driven Henon-Heiles system, which serves as a simple example of a reactive system with several open channels. The present generalization of transition state theory to driven systems will allow one to study processes such as the control of chemical reactions through laser pulses
Landau-Zener-St\"uckelberg interferometry in pair production from counterpropagating lasers
The rate of electron-positron pair production in linearly polarized
counter-propagating lasers is evaluated from a recently discovered solution of
the time-dependent Dirac equation. The latter is solved in momentum space where
it is formally equivalent to the Schr\"odinger equation describing a strongly
driven two-level system. The solution is found from a simple transformation of
the Dirac equation and is given in compact form in terms of the
doubly-confluent Heun's function. By using the analogy with the two-level
system, it is shown that for high-intensity lasers, pair production occurs
through periodic non-adiabatic transitions when the adiabatic energy gap is
minimal. These transitions give rise to an intricate interference pattern in
the pair spectrum, reminiscent of the Landau-Zener-St\"uckelberg phenomenon in
molecular physics: the accumulated phase result in constructive or destructive
interference. The adiabatic-impulse model is used to study this phenomenon and
shows an excellent agreement with the exact result.Comment: 22 pages, 7 figure
Transition state theory for laser-driven reactions
This is a pre-print.Recent developments in Transition State Theory brought about by dynamical systems theory
are extended to time-dependent systems such as laser-driven reactions. Using time-dependent
normal form theory, we construct a reaction coordinate with regular dynamics inside the transition
region. The conservation of the associated action enables one to extract time-dependent invariant
manifolds that act as separatrices between reactive and non-reactive trajectories and thus make it
possible to predict the ultimate fate of a trajectory. We illustrate the power of our approach on
a driven H´enon-Heiles system, which serves as a simple example of a reactive system with several
open channels. The present generalization of Transition State Theory to driven systems will allow
one to study processes such as the control of chemical reactions through laser pulses
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