No abstract

Understanding and controlling the dynamic evolution of electrons in matter is among the most fundamental goals of attosecond science. While the most exotic behaviors can be found in complex systems, fast electron dynamics can be studied at the fundamental level in atomic systems, using moderately intense (≲103 W/cm2) lasers to control the electronic structure in proof-of-principle experiments. Here, we probe the transient changes in the absorption of an isolated attosecond extreme ultraviolet (XUV) pulse by helium atoms in the presence of a delayed, few-cycle near infrared (NIR) laser pulse, which uncovers absorption structures corresponding to laser-induced “virtual” intermediate states in the two-color two-photon (XUV+NIR) and three-photon (XUV+NIR+NIR) absorption process. These previously unobserved absorption structures are modulated on half-cycle (~1.3 fs) and quarter-cycle (~0.6 fs) timescales, resulting from quantum optical interference in the laser-driven atom.

We present an ab initio three-dimensional ͑3D͒ calculation of multiphoton ionization ͑MPI͒ and high-order harmonic generation ͑HHG͒ of the hydrogen molecular ions subject to intense linearly polarized laser pulses. The orientation of the molecular axis with respect to the polarization of the laser field can be arbitrary. The numerical procedure involves the extension of the generalized pseudospectral ͑GPS͒ method for nonuniform spatial discretization of the Hamiltonian and wave functions and time propagation using the split-operator technique in the energy representation. The calculations were performed for the ground and two first excited electronic states of H 2 + at the internuclear separation R = 2.0 a.u. The laser pulse has a sine-squared envelope and contains 20 optical cycles with the wavelength 800 nm. The dependence of MPI and HHG on the orientation angle is analyzed. We show that orientation effects are strongly affected by the symmetry of the wave function and the corresponding distribution of the electron density. While the anisotropy of MPI and HHG is rather weak for the 1 g state, both processes are suppressed at the orientation angle 90°for the 1 u state and at the angle 0°for the 1 u state. We discuss the multiphoton resonance and two-center interference effects in the HHG spectra which can lead both to enhancement and suppression of the harmonic generation.

No abstract