Attosecond time delays near the photoionisation threshold of neon

Spectroscopy with attosecond pulse trains is routinely applied for the investigation of electronic dynamics in atoms and molecules [1]. Specifically, the process of photoionization has been investigated using a two-color photoionization scheme, demonstrating the existence of tiny, but measurable, delays in photoionization [2]. In the vast majority of these investigations, extreme ultraviolet harmonics of a fundamental frequency with photon energies above the ionization threshold are used. The time delays can be decomposed in a term inherent to the group delay of the different harmonics (τ_GD) and an atomic time delay (τ_a).

We present a combined experimental and theoretical study of two-color photoionization of neon, showing that the delay τ_a determined from one harmonic below and one above the ionization threshold largely deviates from the value expected for sidebands generated only above threshold. The variation is attributed to excited states of neon that can be effectively populated by the 13^th harmonic of the XUV frequency comb.

[1] F. Krausz and M. Ivanov, Rev. Mod. Phys. 81, 163 (2009).

[2] J. M. Dahlström, A. L’Huillier, and A. Maquet, J. Phys. B 45, 183001 (2012).