506 research outputs found
Qualitatively Different Theoretical Predictions for Strong-Field Photoionization Rates
We give examples showing that two well-known versions of the S-matrix theory,
which describes a nonresonant multiphoton ionization of atoms and ions in
intense laser fields, lead to qualitatively different results. The latter refer
not only to total ionization rates, but also to energy distributions of
photoelectrons, for instance in a polarization plane of the laser field. It
should be possible to make an experiment testing predictions of both theories
in the near future.Comment: 12 pages, 5 figures; submitted to Physical Revie
Coulomb focusing at above-threshold ionization in elliptically polarized mid-infrared strong laser fields
The role of Coulomb focusing in above-threshold ionization in an elliptically
polarized mid-infrared strong laser field is investigated within a
semiclassical model incorporating tunneling and Coulomb field effects. It is
shown that Coulomb focusing up to moderate ellipticity values is dominated by
multiple forward scattering of the ionized electron by the atomic core that
creates a characteristic low-energy structure in the photoelectron spectrum and
is responsible for the peculiar energy scaling of the ionization normalized
yield along the major polarization axis. At higher ellipticities, the electron
continuum dynamics is disturbed by the Coulomb field effect mostly at the exit
of the ionization tunnel. Due to the latter, the normalized yield is found to
be enhanced, with the enhancement factor being sharply pronounced at
intermediate ellipticities
Production of high energy particles in laser and Coulomb fields and e^+e^- antenna
A strong laser field and the Coulomb field of a nucleus can produce
e^{+}e^{-} pairs. It is shown for the first time that there is a large
probability that electrons and positrons created in this process collide after
one or several oscillations of the laser field. These collisions can take place
at high energy resulting in several phenomena. The quasielastic collision
e^{+}e^{-} -> e^{+}e^{-} allows acceleration of leptons in the laser field to
higher energies. The inelastic collisions allow production of high energy
photons e^{+}e^{-}-> 2 gamma and muons, e^{+}e^{-} -> mu^{+}mu^{-}. The yield
of high-energy photons and muons produced via this mechanism exceeds
exponentially their production through conventional direct creation in laser
and Coulomb fields. A relation of the phenomena considered with the
antenna-mechanism of multiphoton absorption in atoms is discussed.Comment: 4 page
Strong-field approximation for intense-laser atom processes: the choice of gauge
The strong-field approximation can be and has been applied in both length
gauge and velocity gauge with quantitatively conflicting answers. For
ionization of negative ions with a ground state of odd parity, the predictions
of the two gauges differ qualitatively: in the envelope of the angular-resolved
energy spectrum, dips in one gauge correspond to humps in the other. We show
that the length-gauge SFA matches the exact numerical solution of the
time-dependent Schr\"odinger equation.Comment: 5 pages, 3 figures, revtex
Classical Effects of Laser Pulse Duration on Strong-field Double Ionization
We use classical electron ensembles and the aligned-electron approximation to
examine the effect of laser pulse duration on the dynamics of strong-field
double ionization. We cover the range of intensities
for the laser wavelength 780 nm. The classical scenario suggests that the
highest rate of recollision occurs early in the pulse and promotes double
ionization production in few-cycle pulses. In addition, the purely classical
ensemble calculation predicts an exponentially decreasing recollision rate with
each subsequent half cycle. We confirm the exponential behavior by trajectory
back-analysis
Origin of Unexpected Low Energy Structure in Photoelectron Spectra Induced by Mid-Infrared Strong Laser Fields
Using a semiclassical model which incorporates tunneling and Coulomb field
effects, the origin of the unexpected low-energy structure (LES) in
above-threshold ionization spectrum observed in recent experiments [C. I. Blaga
et al., Nature Phys. {\bf 5}, 335 (2009) and W. Quanet al., Phys. Rev. Lett.
{\bf 103}, 093001 (2009)] is identified. We show that the LES arises due to an
interplay between multiple forward scattering of an ionized electron and the
electron momentum disturbance by the Coulomb field immediately after the
ionization. The multiple forward scattering is mainly responsible for the
appearance of LES, while the initial disturbance mainly determines the position
of the LES peaks. The scaling laws for the LES parameters, such as the contrast
ratio and the maximal energy, versus the laser intensity and wavelength are
deduced
Lorentz force on an electron in a strong plane-wave laser field and the low-frequency limit for ionization
A motion of a classical free charge in an electromagnetic plane wave can be
found exactly in a fully relativistic case. We have found an approximate
non-parameter form of the suitable equations of motion. In a linearly polarized
wave, in the simplest frame of reference, the charge moves along the well-known
"figure-8" path. We have numerically calculated the Lorentz force acting on the
charge as a function of time. In virtue of this, for the low frequency
ionization (or detachment) rate, we discuss a manifestation of nondipole and
relativistic effects.Comment: 14 pages, 7 figures; submitted to Physical Review
Instantaneous Multiphoton Ionization Rate and Initial Distribution of Electron Momenta
The Yudin-Ivanov formula [Phys. Rev. A 64, 013409 (2001)] is generalized such
that the most general analytical expression for single-electron spectra, which
includes the dependence on the instantaneous laser phase, is obtained within
the strong field approximation. No assumptions on the momentum of the electron
are made. Previously known formulas for single-electron spectra can be obtained
as approximations to the general formula.Comment: 9 pages, 3 figure
Influence of molecular symmetry on strong-field ionization: Studies on ethylene, benzene, fluorobenzene, and chlorofluorobenzene
Using the molecular strong-field approximation we consider the effects of
molecular symmetry on the ionization of molecules by a strong, linearly
polarized laser pulse. Electron angular distributions and total ionization
yields are calculated as a function of the relative orientation between the
molecule and the laser polarization. Our studies focus on ethylene
(CH), benzene (CH), fluorobenzene (CHF), and ortho
chlorofluorobenzene (1,2 CHClF), the molecules representing four
different point groups. The results are compared with experiments, when
available, and with the molecular tunneling theory appropriately extended to
non-linear polyatomic molecules. Our investigations show that the orientational
dependence of ionization yields is primarily determined by the nodal surface
structure of the molecular orbitals.Comment: 13 pages, 10 figures. Submitted to Physical Review
Wavelength and intensity dependence of multiple forward scattering at above-threshold ionization in mid-infrared strong laser fields
The nonperturbative role of multiple forward scattering for Coulomb focusing
in mid-infrared laser fields and its dependence on a laser intensity and
wavelength are investigated for low-energy photoelectrons at above-threshold
ionization. We show that high-order rescattering events can have comparable
contributions to the Coulomb focusing and the effective number of rescattering
depends weakly on laser parameters in the classical regime. However, the
relative contribution of the forward scattering to the Coulomb focusing and the
Coulomb focusing in total decrease with the rise of the laser intensity and
wavelength
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