High-order harmonic generation from coherently excited molecules

High-order harmonic generation (HHG) from electronically excited molecules exposed to single-cycle laser pulses is investigated. H(2)(+) ions are used as an example under the condition that they are prepared in a coherent superposition of the two lowest lying states. We show that the populated excited state can be used to gate the ionization and the HHG processes. The HHG yields are found dependent on the coherent phase and the carrier-envelope phase of the laser pulse, providing the possibility of monitoring electron localization dynamics. We further demonstrate that the presence of multi-path recombinations leads to interference in harmonic yields enabling the determination of harmonic chirp.