Attosecond measurement via high-order harmonic generation in low-frequency fields

The spectral phase of high-order harmonic and attosecond pulses is typically shaped by the interaction of the recollision electron with the strong field in the continuum. However, the phase of the transition-moment coupling bound and continuum states can be significant in shaping the emitted radiation. It has commonly been assumed that the propagation and recombination steps of the recollision process can be described independently. Here, we investigate the effect that the transition moment has recollision trajectories by incorporating the transition-moment phase into the Lewenstein model of recollision. We then use our model to investigate the all-optical measurement of the transition-moment phase around the Cooper minimum in argon and the spectral minimum due to two-center interference in a diatomic molecular system. Our results indicate that, while all-optical methods are generally sensitive to the transition-moment phase, they are insensitive to the phase shifts due to two-center interference and ionic structure. Thus, we have resolved the apparent discrepancy between studies with conflicting conclusions regarding the sensitivity of all-optical approaches to the transition-moment phase. Our work demonstrates that all-optical measurements focus on photorecombination time delays attributable to electronic structure and dynamics. Our method will allow any laboratory capable of generating attosecond pulses to perform these measurements, even at wavelengths at which the single-photoionization cross section becomes small.