  We present the first fully ab initio calculations for attosecond transient
absorption spectroscopy of atomic krypton with overlapping pump and probe
pulses. Within the time-dependent configuration interaction singles (TDCIS)
approach, we describe the pump step (strong-field ionization using a
near-infrared pulse) as well as the probe step (resonant electron excitation
using an extreme- ultraviolet pulse) from first principles. We extent our TDCIS
model and account for the spin-orbit splitting of the occupied orbitals. We
discuss the spectral features seen in a recent attosecond transient absorption
experiment [A. Wirth et al., Science 334, 195 (2011)]. Our results support the
concept that the transient absorption signal can be directly related to the
instantaneous hole population even during the ionizing pump pulse. Furthermore,
we find strong deformations in the absorption lines when the overlap of pump
and probe pulses is maximum. These deformations can be described by relative
phase shifts in the oscillating ionic dipole. We discuss possible mechanisms
contributing to these phase shifts. Our finding suggests that the
non-perturbative laser dressing of the entire N -electron wave function is the
main contributor.
