Molecular high-order harmonic generation with more than one active orbital: Quantum interference effects

We investigate quantum interference effects in high-order harmonic generation in N-2 and N-2(+) beyond the single-active-orbital approximation, with particular emphasis on the role of sigma and pi orbitals in the overall spectra. In the former case, we consider a simplified multielectron wave function which incorporates the 3 sigma(g) and 1 pi(u) orbitals, and, in the latter, we assume that the optically active electron is in a coherent superposition of the 3 sigma(g) and 1 pi(g)* one-electron states. If the orbitals are energetically close, such as the 3 sigma(g) and the 1 pi(u) orbitals of N-2, we show that the quantum interference patterns observed in the high-order harmonic spectra are predominantly determined by the 3 sigma(g) orbital. If, on the other hand, there is a significant difference in their binding energies, such as for the 3 sigma(g) and the 1 pi(g)* orbitals of N-2(+), the most loosely bound orbital will determine the shape of the spectra. Due, however, to the different cutoffs encountered, the more deeply bound orbital will leave an imprint on the high-energy harmonics. This holds both for the situation in which the dynamics of the electron is restricted to the plane pxp(z) and for the full three-dimensional case, if the azimuthal angle is integrated over and the degeneracy of the p orbitals is taken into account.