High-order harmonic phase in molecular nitrogen

Electronic structure in atoms and molecules modulates the amplitude and phase of high harmonic generation (HHG). We report measurements of the high harmonic spectral amplitude and phase in N₂. The phase is measured interferometrically by beating the N₂ harmonics with those of an Ar reference oscillator in a gas mixture. A rapid phase shift of 0.2π is observed in the vicinity of the HHG spectral minimum, where a shift of π had been presumed [J. Itatani et al., Nature (London) 432, 867 (2004)]. We compare the phase measurements to a simulation of the HHG recombination step in N₂ that is based on a simple interference model. The results of the simulation suggest that modifications beyond the simple interference model are needed to explain HHG spectra in molecules.