We study the effects of two-photon absorption in the region of the double continuum of helium, with extreme ultraviolet (xuv) laser fields. The nonrelativistic time-dependent Schrödinger equation is solved for the two active electron systems, interacting with strong laser pulses. The dynamic calculations include electronic correlations. The final double continuum states are calculated by treating the electronic term ${1/r}_{12}$ within the zero- and first-order perturbation theory. We calculate the total single and double ionization probabilities and cross sections for photon energies of 45 and 57 eV, as well as the electron energy distributions in the double continuum. The effects of electronic correlations are discussed in the context of ultrashort pulses.

We present a theoretical study of ${\mathrm{H}}_2^{+}$ ionization under strong IR femtosecond pulses by using a method designed to extract correlated (2D) photoelectron and proton kinetic energy spectra. The results show two distinct ionization mechanisms—tunnel and multiphoton ionization—in which electrons and nuclei do not share the energy from the field in the same way. Electrons produced in multiphoton ionization share part of their energy with the nuclei, an effect that shows up in the 2D spectra in the form of energy-conservation fringes similar to those observed in weak-field ionization of diatomic molecules. In contrast, tunneling electrons lead to fringes whose position does not depend on the proton kinetic energy. At high intensity, the two processes coexist and the 2D plots show a very rich behavior, suggesting that the correlation between electron and nuclear dynamics in strong field ionization is more complex than one would have anticipated.

Energies and widths of autoionising states in the helium isoelectronic sequence (Z=5-10) converging to the n=3 and n=4 hydrogenic thresholds are computed. States of symmetry ¹S^e, ¹P^o, ¹D^e are considered. Using the Feshbach formalism the eigenvalues of QHQ are calculated by the truncated diagonalisation method of Altick and Moore (1965) or Lipsky et al. (1977) and the continuum functions by the static exchange approximation. The author computes both total widths and partial widths associated with the various open channels.