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.

Phys. Rev. Lett.

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