Minimum in the high-order harmonic generation spectrum from molecules: role of excited states

We model the process of high-order harmonic generation by solving the time-dependent Schrodinger equation for H-2(+) in the fixed nuclei approximation including full 3D electron motion for nonvanishing angles between the nuclear axis and the linear polarization of the driving pulse. We show that the coherent laser coupling induced between the (2)Sigma(+)(g) (1s sigma(g)) ground state and the first excited (2)Sigma(+)(u) (2p sigma(u)) state leads to two dominating amplitudes for the high-order harmonic generation that may interfere: amplitudes describing recombination back into the sigma(g) and sigma(u) states, respectively. These two amplitudes may interfere destructively or constructively. The effect of a destructive interference is very clear through the occurrence of a minimum in the high-order harmonic spectrum. We show cases where such a minimum in the spectrum is approximately at the position predicted by the simple two-centre interference formula.