[en] We demonstrate that the electron emission from atoms can be temporally controlled on an attosecond time scale. Electron wave-packets are formed by ionizing an atomic target with an attosecond pulse train composed of both odd and even high-order harmonics in the presence of a relatively weak infrared field. We show that interference between one- and two-photon transitions produces a large asymmetry in the angular distribution of the photoelectrons. The direction of the emission can be controlled on an attosecond time scale by varying the time delay between the two pulses. In addition, we show that such asymmetric emission is also related to the properties of the attosecond pulse train. The temporal analysis of the modulated electron emission, based on an accurate description of the atomic physics of the photoionization process, then provides a way to measure the temporal profile of the attosecond pulse

[en] This paper presents an investigation of the charge-state and energy dependences of transfer-ionization (TI) and single-electron-capture (SC) processes for fluorine ions (q=4+ to 9+) incident on a supersonic He jet target. The measurements were made for beam energies between 0.5 and 2.5 MeV/u. A recoil ion momentum spectrometer was used to separate TI and SC based on the longitudinal momentum transfer and time of flight of the recoil ions. The cross-section ratios for TI to SC, R=σ_TI/σ_SC, were determined and observed to decrease monotonically with velocity. The values of R were combined with measured total transfer cross sections to deduce the cross sections for both SC and TI. Coupled-channel calculations of the energy dependence of TI and SC for F⁹⁺+He were compared to the experimental cross sections as well as the values of R. The calculated cross sections were found to be slightly lower and the R values slightly higher than the measured values, but with approximately the same energy dependences. A q² scaling of the He²⁺+He data was also compared to the present data and was found to give unexpected good agreement

[en] We investigate the dissociation dynamics of electronically and vibrationally excited O₂⁺ molecular ions in intense ultrashort infra-red (IR) probe pulses by analyzing fragment-kinetic-energy release (KER) spectra as a function of the pump-probe delay and quantum-beat frequency.

[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 have investigated the spectral splitting of high harmonics generated in a semi-infinite gas cell. By performing an EUV–IR cross-correlation experiment, we are able to use the phase behaviour of the different sub-peaks of each harmonic to identify them with different electronic trajectories. Both microscopic and macroscopic analyses of the spectra effects are made. The identification of a particular trajectory with a particular component of the splitting on the basis of a single-atom model is found to be incorrect, while the full macroscopic treatment is in agreement with the experiment. (paper)

[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] Laser-induced dissociation of O₂⁺ is studied in the strong-field limit using two independent methods, namely a crossed laser-ion-beam coincidence 3D momentum imaging method and a supersonic gas jet velocity map imaging technique (790 and 395 nm, 8-40 fs, ∼10¹⁵ W/cm²). The measured kinetic energy release spectra from dissociation of O₂⁺ and dissociative ionization of O₂ reveal vibrational structure which persists over a wide range of laser intensities. The vibrational structure is similar for O₂⁺ produced incoherently in an ion source and coherently by laser pulses. By evaluation of the potential energy curves, we assign the spectral energy peaks to dissociation of the v=10-15 vibrational states of the metastable a ⁴Π_u state via the dissociation pathway |a ⁴Π_u>→|f ⁴Π_g-1ω>--a mechanism equivalent to bond softening in H₂⁺.

[en] We demonstrate an experimental control of electron localization in the deuterium molecular ion created and dissociated by the combined action of an attosecond pulse train (APT) and a femtosecond IR laser pulse. A left-right asymmetric ejection of the deuterium ions, characterized by an asymmetry parameter A, exhibits oscillations with a full laser period when the time-delay between the APT and IR pulses is scanned with 300as resolution.

[en] We have used few-cycle IR and EUV laser pulses in pump-probe arrangement to trace out the dissociation pathways in CO when exploded by strong laser fields. We present two preliminary sets of data of different pump pulses. In these sets, different excited state of CO cations are populated using (< 10 fs) IR, and EUV pulses respectively. We followed the time evolution of these states using the time-resolved Coulomb explosion imaging technique. We compare the time evolution of IR- and EUV-induced excited states by measuring the KER of the fragment ions as a function of the time delay between the pump and the IR probe pulse.

[en] Two-color (800 nm and 400 nm) short (45 fs) linearly polarized pulses are used to ionize and dissociate H₂(D₂) into a neutral hydrogen (deuterium) atom and a proton(deuteron). The yields and energies of the ions are measured left and right along the polarization vector. As the relative phase of the two colors is varied, strong yield asymmetries are found in the ion energy regions corresponding to bond softening, above-threshold dissociation and rescattering. A model based on the dynamic coupling by the laser field of the gerade and ungerade states in the molecular ion accounts for many of the observed features.