We demonstrate that the structure of the outermost orbitals of oxygen and nitrogen can be observed in the angular distribution of coincident ion pairs generated by the double ionization of these molecules by 8 fs laser pulses. We do this by establishing that these ions emerge from well defined excited electronic states of ${\mathrm{O}}_{\mathrm{2}}^{\mathrm{2}\mathrm{+}}$ and ${\mathrm{N}}_{\mathrm{2}}^{\mathrm{2}\mathrm{+}}$ respectively and that they are produced dominantly through a process which involves electron rescattering. The angular distributions of the ions from the two targets are very different, reflecting the different structures of the outermost orbitals of the two molecules.

We exploit a method to precisely calibrate the peak intensity of intense laser pulses by measuring the momentum transferred to the ion in single ionization by circularly polarized light. Using this approach, we show how the experimental data can be used to get information about the absolute ionization rate at a specific local laser-peak intensity within the focal volume. The results of this method are compared to other results obtained with different methods of laser-peak-intensity calibration described in the literature.

We have measured coincident ion pairs produced in the Coulomb explosion of ${\mathrm{H}}_2$ by 8–30 fs laser pulses at different laser intensities. We show how the Coulomb explosion of ${\mathrm{H}}_2$ can be experimentally controlled by tuning the appropriate pulse duration and laser intensity. For laser pulses less than 15 fs, we found that the rescattering-induced Coulomb explosion is dominated by first-return recollisions, while for longer pulses and at the proper laser intensity, the third return can be made to be the major one. Additionally, by choosing suitable pulse duration and laser intensity, we show ${\mathrm{H}}_2$ Coulomb explosion proceeding through three distinct processes that are simultaneously observable, each exhibiting different characteristics and revealing distinctive time information about the ${\mathrm{H}}_2$ evolution in the laser pulse.

We have measured momentum spectra and branching ratios of charged ionic fragments emitted in the double ionization of ${\mathrm{D}}_2$ (and ${\mathrm{H}}_2$) molecules by short intense laser pulses. We find high-energy coincident ${\mathrm{D}}^{+}$ (and ${\mathrm{H}}^{+}$) ion pairs with kinetic energy releases between 8 and 19 eV which appear for linearly polarized light but are absent for circularly polarized light. The dependence on the polarization, the energy distributions of the ions, and the dependence on laser intensity of yield ratios lead us to interpret these ion pairs as due to a rescattering mechanism for the double ionization. A quantitative model is presented which accounts for the major features of the data.

We have measured angular distributions of ion fragments produced in dissociative double ionization of CO, $\mathrm{C}{\mathrm{O}}_2$, and ${\mathrm{C}}_2{\mathrm{H}}_2$ by intense ultrashort laser pulses. This report extends similar recent studies of ${\mathrm{O}}_2$ and ${\mathrm{N}}_2$ to a wider set of molecules. We found that for sub-$10\text{−}\mathrm{fs}$ pulses of sufficiently low intensity the fragments’ angular distributions for all studied molecules are determined by angular dependence of the first ionization step. Those experimental angular distributions were in good agreement with angular dependent ionization probabilities calculated with the molecular tunneling ionization theory. The measured angular distributions directly reflect the symmetry of the corresponding molecular orbitals. For higher laser intensities and longer pulse durations, dynamic alignment and postionization alignment start to affect the angular distributions and ion fragments are preferentially ejected along the laser-polarization direction.