A theoretical study of the intense-field multiphoton ionization of hydrogenlike systems is performed by solving the time-dependent Dirac equation within the dipole approximation. It is shown that the velocity-gauge results agree to the ones in length
gauge only if the negative-energy states are included in the time propagation. On the other hand, for the considered laser parameters, no significant difference is found in length gauge if the negative-energy states are included or not. Within the adopted dipole approximation the main relativistic effect is the shift of the ionization potential. A simple scaling procedure is proposed to account for this effect.
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 d
iffers 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.
The ionization probability of N$_2$, O$_2$, and CO$_2$ in intense laser fields is studied theoretically as a function of the alignment angle by solving the time-dependent Schrodinger equation numerically assuming only the single-active-electron appro
ximation. 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$_2$ and O$_2$. For CO$_2$ 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.
The alignment dependence of the ionization behavior of H$_2$ exposed to intense ultrashort laser pulses is investigated on the basis of solutions of the full time-dependent Schrodinger equation within the fixed-nuclei and dipole approximation. The to
tal 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.
The collision of two atoms is an intrinsic multi-channel (MC) problem as becomes especially obvious in the presence of Feshbach resonances. Due to its complexity, however, single-channel (SC) approximations, which reproduce the long-range behavior of
the open channel, are often applied in calculations. In this work the complete MC problem is solved numerically for the magnetic Feshbach resonances (MFRs) in collisions between generic ultracold 6Li and 87Rb atoms in the ground state and in the presence of a static magnetic field B. The obtained MC solutions are used to test various existing as well as presently developed SC approaches. It was found that many aspects even at short internuclear distances are qualitatively well reflected. This can be used to investigate molecular processes in the presence of an external trap or in many-body systems that can be feasibly treated only within the framework of the SC approximation. The applicability of various SC approximations is tested for a transition to the absolute vibrational ground state around an MFR. The conformance of the SC approaches is explained by the two-channel approximation for the MFR.
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 i
n an intensity range of $(2-13)times10^{13} $W cm$^{-2}$. The investigation is based on a non-perturbative treatment that solves the full time-dependent Schrodinger 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.
For the description of an H2 molecule an effective one-electron model potential is proposed which is fully determined by the exact ionization potential of the H2 molecule. In order to test the model potential and examine its properties it is employed
to determine excitation energies, transition moments, and oscillator strengths in a range of the internuclear distances, 0.8 < R < 2.5 a.u. In addition, it is used as a description of an H2 target in calculations of the cross sections for photoionization and for partial excitation in collisions with singly-charged ions. The comparison of the results obtained with the model potential with literature data for H2 molecules yields a good agreement and encourages therefore an extended usage of the potential in various other applications or in order to consider the importance of two-electron and anisotropy effects.
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 th
e 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.
A theoretical comparison of the electronic excitation and ionisation behaviour of molecular hydrogen oriented either parallel or perpendicular to a linear polarised laser pulse is performed. The investigation is based on a non-perturbative treatment
that solves the full time-dependent Schrodinger equation of both correlated electrons within the fixed-nuclei approximation and the dipole. Results are shown for two different laser pulse lengths and intensities as well as for a large variety of photon frequencies starting in the 1- and reaching into the 6-photon regime. In order to investigate the influence of the intrinsic diatomic two-center problem even further, two values of the internuclear separation and a newly developed atomic model are considered.
