The absolute timing of the high-harmonic attosecond pulse train with respect to the generating IR pump cycle has been measured for the first time. The attosecond pulses occur 190±20 as after each pump field maxima (twice per optical cycle), in agreement with the "short" quantum path of the quasiclassical model of harmonic generation.

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Journal Phys. Rev. Lett.
Ciubotariu-Dinu, L, Muller, H. G, Kazamias, S, Mullot, G, Augé, F, Balcou, P, … Agostini, Pierre. (2003). Measurement of the subcycle timing of attosecond xuv bursts in high-harmonic generation. Phys.Rev.Lett., 91(Article number: 63901), 1–4.