Three-dimensional time-profile analysis of high-order harmonic generation in molecules: Nuclear interferences in H₂⁺

We describe a numerical method used previously [ Phys. Rev. A 70 011404 (2004)] for solving the three-dimensional time-dependent Schrödinger equation for H₂⁺ (with fixed nuclei) in interaction with an intense, arbitrary oriented laser pulse. In this approach, we use the prolate spheroidal coordinate system, and expand the time-dependent wave function in a complex basis of Laguerre polynomials and Legendre functions. Our results indicate that ionization, excitation, and harmonic generation are strongly influenced by the orientation of the molecular axis with respect to the laser polarization axis. We evaluate the contribution of each nucleus to harmonic generation, as this permits a quantitative and nonambiguous assessment of interference effects as a function of molecular orientation. A time-profile analysis, using a Gabor transform of the harmonic spectrum around certain harmonics, shows that every half-cycle high-order harmonics are emitted by each nucleus when the electron wave packet returns for a recollision with the molecular core, thus confirming the strong field recollision model in molecules. In general, each nucleus emits both odd and even harmonics, but even harmonics are destroyed by interferences between contributions of each nucleus. These interferences are shown to be maximum at certain harmonic orders as a function of molecular orientation. A comparison of acceleration and dipole formulations of the harmonic emission process is made in order to assess the use of high-order harmonic generation for electron wave-function imaging.