Attosecond transient absorption spectrum of argon at the L2,3 edge

Progress in high-harmonic generation has led to high-energy attosecond pulses with cutoff above the carbon $1s$ edge (283.8 eV). These pulses are essential to extend time-resolved spectroscopies to the water window in order to control electron dynamics in solvated organic species. Here we report a step towards this goal: the measurement, with subcycle time resolution, of the attosecond transient absorption spectrum of argon at the $2{p}^{\ensuremath{-}1} {L}_{2,3}$ edge ($\ensuremath{\sim}250$ eV) in the presence of a short-wave infrared control pulse. The measurements, supported by theoretical simulations, demonstrate the concurrent role of Auger decay and tunnel ionization in the driven evolution of inner-valence holes of polyelectronic atoms.