The ionization probability of N2, O2, and CO2 in intense laser fields is studied theoretically as a function of the alignment angle by solving the time-dependent Schrödinger equation numerically assuming only the single-active-electron approximation. The results are compared to recent experimental data [D. Pavicić, Phys. Rev. Lett. 98, 243001 (2007)] and good agreement is found for N2 and O2. For CO2 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.

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High harmonic spectra show that laser-induced strong field ionization of water has a significant contribution from an inner-valence orbital. Our experiment uses the ratio of H2O and D2O high harmonic yields to isolate the characteristic nuclear motion of the molecular ionic states. The nuclear motion initiated via ionization of the highest occupied molecular orbital (HOMO) is small and is expected to lead to similar harmonic yields for the two isotopes. In contrast, ionization of the second least bound orbital (HOMO-1) exhibits itself via a strong bending motion which creates a significant isotope effect. We elaborate on this interpretation by simulating strong field ionization and high harmonic generation from the water isotopes using the time-dependent Schrödinger equation. We expect that this isotope marking scheme for probing excited ionic states in strong field processes can be generalized to other molecules.

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A method for solving the time-dependent Schrödinger equation describing the electronic motion of molecular hydrogen exposed to very short intense laser pulses has been developed. The fully correlated three-dimensional time-dependent electronic wavefunction is expressed in terms of field-free wavefunctions. These are obtained from a configuration-interaction calculation where the one-electron basis functions are built from B splines. The reliability of the method is tested by comparing results in the low-intensity regime to the prediction of lowest order perturbation theory. The onset of non-perturbative effects is shown for higher intensities and the validity of the single-active electron approximation is briefly discussed. Finally, the ability of the method to calculate photoelectron spectra including above-threshold-ionization peaks is demonstrated.

A B-spline based configuration–interaction method for diatomic two-electron molecules has been developed and implemented. The molecular symmetry of the problem is fully accounted for by using the prolate-spheroidal coordinate system. The performance of the method is demonstrated in a number of test applications. This includes the calculation of the energies of the ground and excited states of H2 (even autoionizing doubly-excited states) as well as transition dipole moments. Furthermore, a partial photoionization cross-section for HeH+ was calculated. In all these cases a favourable comparison to the literature values is found. This indicates the broad applicability of the present approach.

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.

The alignment dependence of the ionization behavior of H2 exposed to intense ultrashort laser pulses is investigated on the basis of solutions of the full time-dependent Schrödinger 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.PACS numbers: 32.80. Rm, 33.80.Rv Time-resolved imaging of the dynamics of nuclei and electrons on a femtosecond or even sub-femtosecond time scale is a prerequisite for the real-time investigation of the formation and breaking of chemical bonds. Ultrashort laser pulses have recently been demonstrated to pave a possible path to the experimental realization of this longstanding dream. For example, ways have been proposed and experimentally verified that allow monitoring nuclear motion with sub-femtosecond and sub-Ångstrom resolution in real time [1,2,3]. It was also experimentally demonstrated that the high-harmonic radiation or the electrons emitted in an intense laser pulse may in principle reveal information on the electronic structure [4,5] and thus have the potential for time-resolved imaging of changes of the electronic structure in, e. g., a chemical reaction. To reach this goal it is, however, important to understand the relation between electronic structure and the strong-field response of molecules. This includes the fundamental question whether the rather clear correspondence between the symmetry of the highest-occupied molecular orbital (HOMO) and the strong-field signal as indicated for N 2 and O 2 in [4,5] is really a universal phenomenon.Already some time ago it was found that within the molecular strong-field approximation (MO-SFA) -formulated in velocity gauge (VG) and within the framework of a linear combination of atomic orbitals (LCAO) -the molecular response to intense laser fields should reflect the symmetry of the highest occupied molecular orbital (HOMO) [6,7]. In the case of diatomic molecules, a simple interference picture arises in the MO-SFA-VG that seems to plausibly explain the occurrence or absence of suppressed ionization [6]. The term suppressed ionization describes the effect that a molecule with the same ionization potential as the one of some so-called companion atom is harder to ionize in an intense laser pulse.For example, molecular nitrogen shows a similar ionization behavior as atomic Ar, while the ionization yield of oxygen is much smaller than the one of Xe atoms; despite the almost identical ionization potentials of either N...