Direct optical observation of attosecond electron wavepacket interference

Attosecond transient absorption spectroscopy is used to investigate electron dynamics in the vicinity of the first ionization threshold of helium. Rapid oscillations of the absorption of individual harmonics are observed when delaying an attosecond pulse train (APT) with respect to a superimposed, moderately strong infrared laser field. The phase relation between the absorption modulation of individual harmonics gives direct evidence for the interference of transiently bound electronic wavepackets as the underlying mechanism. Furthermore, the understanding of these dynamics opens a route towards attosecond XUV pulse shaping.