Influence of energetically close orbitals on molecular high-order harmonic generation

We investigate the contributions from the $3\sigma_{g}$ and $1\pi_{u}$ and molecular orbitals in high-order harmonic generation in $N_{2}$, with particular emphasis on quantum-interference effects. We consider both the physical processes in which the electron is freed and returns to the same orbital, and those in which it is ionized off one orbital and recombines with the other. We show that the quantum-interference patterns observed in the high-order harmonic spectra are predominantly determined by the $3\sigma_{g}$ orbital. This holds both for the situation in which only the $1\pi_{ux}$ orbital is considered, and the dynamics of the electron is restricted to the plane $p_{x}p_{z},$ or in the full three-dimensional case, if the azimuthal angle is integrated over and the degeneracy of $1\pi_{u}$ is taken into account.