20 research outputs found
Alignment-Dependent Ionization of Molecular Hydrogen in Intense Laser Fields
The alignment dependence of the ionization behavior of H exposed to
intense ultrashort laser pulses is investigated on the basis of solutions of
the full time-dependent Schr\"odinger equation within the fixed-nuclei and
dipole approximation. The total ionization yields as well as the
energy-resolved electron spectra have been calculated for a parallel and a
perpendicular orientation of the molecular axis with respect to the
polarization axis of linear polarized laser pulses. For most, but not all
considered laser peak intensities the parallel aligned molecules are easier to
ionize. Furthermore, it is shown that the velocity formulation of the
strong-field approximation predicts a simple interference pattern for the ratio
of the energy-resolved electron spectra obtained for the two orientations, but
this is not confirmed by the full ab initio results.Comment: 4 figure
Explicit summation of the constituent WKB series and new approximate wave functions
The independent solutions of the one-dimensional Schr\"odinger equation are
approximated by means of the explicit summation of the leading constituent WKB
series. The continuous matching of the particular solutions gives the uniformly
valid analytical approximation to the wave functions. A detailed numerical
verification of the proposed approximation is performed for some exactly
solvable problems arising from different kinds of potentials.Comment: 8 pages, LaTeX, minor changes and stylistic improvements, final
versio
Ionization of molecular hydrogen and deuterium by a frequency-doubled Ti:sapphire laser pulses
A theoretical study of the intense-field single ionization of molecular
hydrogen or deuterium oriented either parallel or perpendicular to a linear
polarized laser pulse (400 nm) is performed for different internuclear
separations and pulse lengths in an intensity range of W
cm. The investigation is based on a non-perturbative treatment that
solves the full time-dependent Schr\"odinger equation of both correlated
electrons within the fixed-nuclei and the dipole approximation. The results for
various internuclear separations are used to obtain the ionization yields of
molecular hydrogen and deuterium in their ground vibrational states. An atomic
model is used to identify the influence of the intrinsic diatomic two-center
character of the problem.Comment: 13 pages, 9 figure
Generalized gauge-invariant formulations of the strong-field approximation
The gauge problem in the so-called strong-field approximation (SFA)
describing atomic or molecular systems exposed to intense laser fields is
investigated. Introducing a generalized gauge and partitioning of the
Hamiltonian it is demonstrated that the S-matrix expansion obtained in the SFA
depends on both gauge and partitioning in such a way that two gauges always
yield the same S-matrix expansion, if the partitioning is properly chosen.Comment: 11 page
Exact Keldysh theory of strong-field ionization: residue method vs saddle-point approximation
In recent articles [Mishima et al., Phys. Rev. A, 66, 033401(2002); Chao,
Phys. Rev. A, 72, 053414 (2005)] it was proposed to use the residue theorem for
the exact calculation of the transition amplitude describing strong-field
ionization of atomic systems within Keldysh theory. This should avoid the
necessity to apply the method of steepest descent (saddle-point approximation).
Comparing the results of both approaches for atomic hydrogen a difference by a
factor of 2 was found for the 1s, and an even more drastic deviation for the 2s
state. Thus it was concluded that the use of the saddle-point approximation is
problematic. In this work the deviations are explained and it is shown that the
previous conclusion is based on an unjustified neglect of an important
contribution occurring in the application of the residue theorem. Furthermore,
the applicability of the method of steepest descent for the ionization of
Rydberg states is discussed and an improvement of the standard result is
suggested that successfully removes the otherwise drastic failure for large
principal quantum numbers.Comment: 7 pages, 3 figures, 1 tabl
Alignment-Dependent Ionization of N, O, and CO in Intense Laser Fields
The ionization probability of N, O, and CO in intense laser
fields is studied theoretically as a function of the alignment angle by solving
the time-dependent Schr\"odinger equation numerically assuming only the
single-active-electron approximation. The results are compared to recent
experimental data [D.~Pavi{\v{c}}i{\'c} et al., Phys.\,Rev.\,Lett.\ {\bf 98},
243001 (2007)] and good agreement is found for N and O. For CO a
possible explanation is provided for the failure of simplified
single-active-electron models to reproduce the experimentally observed narrow
ionization distribution. It is based on a field-induced coherent core-trapping
effect.Comment: 5 pages, 2 figure
Feshbach resonances of harmonically trapped atoms
Employing a short-range two-channel description we derive an analytic model
of atoms in isotropic and anisotropic harmonic traps at a Feshbach resonance.
On this basis we obtain a new parameterization of the energy-dependent
scattering length which differs from the one previously employed. We validate
the model by comparison to full numerical calculations for Li-Rb and explain
quantitatively the experimental observation of a resonance shift and
trap-induced molecules in exited bands. Finally, we analyze the bound state
admixture and Landau-Zener transition probabilities.Comment: 4 pages, 2 figures; revised version with extension to anisotropic
traps and new paragraph on trap-induced molecules in excited band