[en] We studied the ionization probability of Na atoms in an intense Ti-sapphire laser at 800 nm versus the laser pulse duration. The ionization probabilities for the Na(3s) and Na(3p) states were found to vary non-monotonically with the pulse duration at a constant peak laser intensity. The abnormal (non-monotonic) pulse duration dependence was traced to the competition between resonant and non-resonant multiphoton ionization processes, which occurs when the frequency broadening due to the finite pulse duration is compatible with the energy difference between the excitation energy and multiphoton energy

[en] We have determined resonant strengths of the KLn (2≤n≤5) resonances for helium-like Ti ions and (3≤n≤5) resonances for helium-like Fe ions. The results were obtained using the Tokyo electron beam ion trap. Characteristic X-rays from both dielectronic recombination and radiative recombination were detected as the electron beam energy was scanned through the resonances

[en] We propose an empirical formula for the static field ionization rates of atoms and molecules by extending the well-known analytical tunnelling ionization rates to the barrier-suppression regime. The validity of this formula is checked against ionization rates calculated from solving the Schroedinger equation for a number of atoms and ions. The empirical formula retains the simplicity of the original tunnelling ionization rate expression but can be used to calculate the ionization rates of atoms and molecules by lasers at high intensities

[en] We have shown that the whole time-dependent vibrational wave packet of D₂⁺ ions can be reconstructed from the kinetic energy release of the D⁺ ion pairs when it is probed with an attosecond xuv pulse. Such a full interrogation of the wave packet will pave the way for controlling the generation of tailor-designed wave packets for favorable chemical reaction paths, as well as for probing the time evolution of their interaction with the medium

[en] We studied the angular distributions of the fragmented ions of diatomic molecules in an intense linearly polarized short laser pulse. In addition to the well-known dynamic alignment of the neutral molecules before ionization, we identified a more important post ionization alignment effect of the molecular ions. The latter is modelled quantum mechanically as resulting from the breakup of a rotating linear rotor. We showed that only for very short pulses are the two alignment mechanisms not important. In this case the angular distributions of the fragmented ions mimic the shape of the electronic density of the outermost molecular orbital

[en] We proposed recently a theoretical method to study the infrared (IR) laser assisted photoabsorption cross sections over a broad energy range by a single calculation. We apply this method to study the IR laser assisted photoabsorption of Ne atoms near the first ionization threshold. The simulation results show that the IR field modifies the photoabsorption cross sections and the energy structures. This photoabsorption cross section is an important quantity to analyze the IR-laser assisted dynamical processes by an attosecond pulse, a pulse train or a free electron laser.

[en] We investigated the Coulomb focusing effect on the space distribution of the rescattering electron wavepacket created in the laser-atom interaction solving the time integral equation. The Coulomb focusing is conspicuous in the even-number returns since the rescattering electron energies are lower there. The Coulomb force focuses the rescattering electron beam into a small space and the rescattering electron beam current intensity can reach the order of 10¹⁰ A cm^-2, much more intense than the conventional electron beam used in scattering experiments. The Coulomb focusing effect plays a less important role for the first return and this explains why the released kinetic energy spectra have a peak at the third return in the experiments of the double ionization of H₂ molecules.

[en] Complete text of publication follows. We have presented a detailed investigation of the asymptotic form of the stopping power for charged particles moving parallel to a metal surface at large distances [1,2]. A comparative study of the linear response employing the specular reflection model (SRM) and time-dependent density functional theory (TDDFT) shows dramatic differences. In the vicinity of the surface the SRM underestimates the stopping power since it underestimates the local electron density that provides the drag force. At large distances, the roles are reversed. The TDDFT calculation for a jellium surface neglecting lattice effects and coupling to phonons underestimates the stopping power. The latter originates from the vanishing (surface) plas- mon line width in the long-wavelength limit. Since the SRM contains the experimental line width on a phenomenological level, its asymptotic stopping power results in the correct power law ∼ (distance)^-3 and dominates over the TDDFT result ∼ (distance)^-4. We have proposed a simple extension of the TDDFT description that remedies this deficiency [1]. While in the vicinity of the surface this new stopping power calculation gives values significantly larger than obtained by SRM, it converges to the SRM in the asymptotic region above the surface. As an application of our stopping power calculations we present the energy loss for a protons passing through an Al microcapillary. The reason for this choice is that for small z differences between TDDFT and SRM are more pronounced. Note however, that integrated energy loss is much larger and more easily measurable for highly charged ions. For the study of the problem of energy loss we have performed a classical trajectory Monte Carlo simulation with an ensemble of 5 x 10⁶ primary trajectories. In our simulations a capillary with a radius of 2360 a.u. and length L=30000 a.u. was used. Fig.1 shows the energy loss probability for a 2.1 keV/amu H⁺ ions passing through an Al microcapillary. The energy loss exceeding about 0.3 eV correspond- ing to ΔE=E=0.014 % should be experimentally observable. We note that with increasing projectile charge the maximum reachable energy loss for TDDFT approach to the value obtain with SRM. The work was supported by the Hungarian Scientific Research Found: OTKA Nos. T038016, T046095, the grant 'Bolyai' from the Hungarian Academy of Sciences, TeT Grant No. A-15/04, the Austrian Fonds zur Foerderung der wissenschaftlichen Forschung, FWF (Austria), and EU under contract no. HPRI-CT-2001-50036. We gratefully acknowl edge invaluable inspiration and guidance over many years by our friend and pioneer of this field, Rufus Ritchie. (author)

[en] We have used momentum imaging techniques to measure in high resolution the kinetic energy release spectra and angular distributions of coincident O⁺ and N⁺ ion pairs produced by short laser pulses (8-35 fs) on targets of N₂ and O₂ at peak intensities between 1 and 12 x 10¹⁴ W cm^-2. We record the full momentum vectors of both members of each pair and achieve a kinetic energy release resolution of less than 0.3 eV. We find that the process proceeds through well-defined electronic states of the excited molecular dications. Using linear and circularly polarized light, we identify two mechanisms for the production of these states, rescattering and sequential ionization. By using 8 fs pulses, we observe that the internuclear distance can be frozen during the pulse. For low intensities and 8 fs pulses, emission from N₂ is strongly directed along the polarization vector, while that for O₂ is not, a result we interpret as being due to the different symmetries of the outer orbitals of these molecules. For high intensities and longer pulses, the distributions increasingly fold towards the polarization vector, ultimately peaking at zero degrees for both molecules. For oxygen, a local peaking for molecules aligned at right angles to the polarization vector is seen. A discussion and interpretation of the results are presented

[en] We examined nearest-neighbor spacing (NNS) statistics in doubly excited states of helium near the double ionization threshold. Using the Brody parameter q to measure the NNS distribution between the regular Poisson distribution (q=0) and the chaotic Wigner distribution (q=1), we showed that for levels near the N=20 threshold of He⁺, or at about 0.13 eV below the double ionization threshold, the NNS distribution has q=0.66. The result shows the slow approach of the NNS of helium energy levels towards the Wigner distribution vs the excitation energy. Using an s-wave model where the angular momentum of each electron is restricted to zero, we also examined the NNS for levels up to the N=30 threshold of He⁺. We showed the gradual increase in q as the excitation energy is increased. To generate the theoretical data needed for the NNS analysis, we have used the hyperspherical close-coupling method, with the recently proposed diabatization of potential curves and the truncation of channels, to greatly reduce the complexity in the calculation. We also investigated the dependence of q vs the nuclear charge in the same scaled energy region, and for different symmetries, to assess their relation with the rate of approaching the q=1 Wigner distribution