Ab initio study of the orientation effects in multiphoton ionization and high-order harmonic generation from the ground and excited electronic states of H2 +

We present an ab initio three-dimensional 3D calculation of multiphoton ionization MPI and high-order harmonic generation HHG 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 GPS 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 H2 + 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.