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Abstract

In this work, the strong-field-modified dipole response at the
ionization threshold of helium is studied. The dipole response is induced by an
attosecond pulse in the extreme ultraviolet spectral range and is manipulated by an
ultrashort and strong femtosecond pulse in the near-infrared. To probe the response,
the transient absorption spectrum of helium is recorded for different time delays
between both pulses and different intensities of the femtosecond pulse. From the
spectra, the dipole response of the ionization threshold is reconstructed, which is
linked to the dynamics of excited electrons with energies in the transition region from
bound to free. To identify the underlying processes of light-matter interaction leading
to the observed structures in the time and spectral domain, different quantummechanical model simulations are conducted. As a result, the measured dipole
response reveals light-induced energy shifts of the photoelectron’s kinetic energy
close to the parent ion, signatures for field-driven recollisions of a photoelectron into
the parent ion, and a temporal amplitude and phase gating mechanism. With the latter,
the build-up dynamics of complex spectral structures are temporally resolved, which
are the time-dependent separation and line-shape modification of the doubly excited
Rydberg series as well as the temporal build-up of the ionization threshold.