The emission of above-ionization-threshold harmonics results from the recombination of two electron wavepackets moving along a short and a long trajectory in the atomic continuum. Attosecond pulse train generation has so far been attributed to the sh
ort trajectory, attempted to be isolated through targeted trajectory-selective phase matching conditions. Here, we provide experimental evidence for the contribution of both trajectories to the harmonic emission, even under phase matching conditions unfavorable for the long trajectory. This is finger printed in the interference modulation of the harmonic yield as a function of the driving laser intensity. The effect is also observable in the sidebands yield resulting from the frequency mixing of the harmonics and the driving laser field, an effect with consequences in cross-correlation pulse metrology approaches.
The two basic approaches underlying the metrology of attosecond pulse trains are compared, i.e. the 2nd order Intensity Volume Autocorrelation and the Resolution of Attosecond Beating by Interference of Two photon Transitions (RABITT). They give rath
er dissimilar results with respect to the measured pulse durations. It is concluded that RABITT may underestimate the duration due to variations of the driving intensity, but in conjunction with theory, allows an estimation of the relative contributions of two different electron trajectories to the extreme-ultraviolet emission.
