Role of electron wavepacket interference in the optical response of helium atoms

Attosecond control of the optical response of helium atoms to extreme ultraviolet radiation in the presence of moderately strong infrared laser light has been recently demonstrated both by employing attosecond pulse trains (APTs) and single attosecond pulses. In the case of APTs the interference between different transiently bound electron wavepackets excited by consecutive attosecond light bursts in the train was indicated as the predominant mechanism leading to the control. We studied the same physical system with transient absorption spectroscopy using elliptically polarized infrared pulses or APTs with a varying number of pulses down to a single pulse. Our new results are not consistent with this kind of wavepacket interference being the dominant mechanism and show that its role in the control over the photoabsorption probability has to be rediscussed.