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We study enhanced ionization (EI) in asymmetric molecules by solving the 3D time-dependent Schrödinger equation for HeH2+ driven by a few-cycle laser pulse linearly polarized along the molecular axis. We find that EI is much stronger when the laser's carrier-envelope phase is such that the electric field at the peak of the pulse is antiparallel to the permanent dipole of the molecule (PDM). This phase dependence is explained by studying the molecule in the presence of a static electric field. When this field is antiparallel to the PDM, the energy of the dressed ground state moves up (with increasing internuclear distance R) to cross with excited states, leading to a stronger ionization via intermediate state resonances and via tunneling. We predict analytically the laser and molecular parameters at which these crossings are expected to occur in any asymmetric molecule.

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Exact (Born-Oppenheimer) 3-D numerical solutions of the time-dependent Schrödinger equation are obtained for the one electron linear H+-H2+ atom-molecule system at large internuclear distance R in interaction with two-cycles intense (I>10(14) W cm(-2)) 800 nm laser pulses. High-order harmonic generation (HHG) spectra are obtained with an energy cutoff larger than the atomic maximum of I(p)+3U(p), where I(p) is the ionization potential and U(p) is the ponderomotive energy. At large R, this extended cutoff is shown to be related to the nature of electron transfer, whose direction is shown to depend critically on the carrier-envelope phase (CEP) of the ultrashort pulse. Constructive and destructive interferences in the HHG spectrum resulting from coherent superpositions of electronic states in the H+-H2+ system are interpreted in terms of multiple electron trajectories extracted from a time profile analysis.

We report correlated two-electron ab initio calculations for the hydrogen molecule H 2 in interaction with intense ultrashort laser pulses, via a solution of the full three-dimensional time-dependent Schrödinger equation. Our results for ionization and excitation probabilities (at 800 and 400 nm) as a function of internuclear distance R show strong evidence of enhanced ionization, in both single and double ionization, as well as enhanced excitation, in single and double excitation, as the internuclear distance R increases from the equilibrium value R e . The enhancement of all these molecular processes exhibits a maximum at a critical distance R c , which can be predicted from simple electrostatic and recollision models.

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We report a calculation of the harmonic emission from a one-electron heteronuclear nonsymmetric molecule (HeH 2+ ) interacting with a few-cycle laser pulse linearly polarized along the molecular axis. We find that a 180 • rotation of the molecule (or equivalently a 180 • change in the carrier-envelope phase) leads to substantial changes in the harmonic emission of the molecule. Phase-dependent plateaux and cutoffs appear in the harmonic spectrum as a consequence of the phase-dependent electric field of few-cycle pulses. Asymmetries in the intensity of harmonics result from the phase dependence of ionization rates in nonsymmetric molecules, and from the fact that depending on the molecular orientation, the ionized electron wavepacket can be Coulomb focused as it visits the proton twice before the recollision leading to harmonic generation.