  Few-femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy,
performed with optical 500-1000 nm supercontinuum and broadband XUV pulses
(30-50 eV), simultaneously probes dynamics of photoexcited carriers in WS$_{2}$
at the W O$_3$ edge (37-45 eV) and carrier-induced modifications of
core-exciton absorption at the W N$_{6,7}$ edge (32-37 eV). Access to
continuous core-to-conduction band absorption features and discrete
core-exciton transitions in the same XUV spectral region in a semiconductor
provides a novel means to investigate the effect of carrier excitation on
core-exciton dynamics. The core-level transient absorption spectra, measured
with either pulse arriving first to explore both core-level and valence carrier
dynamics, reveal that core-exciton transitions are strongly influenced by the
photoexcited carriers. A $1.2\pm0.3$ ps hole-phonon relaxation time and a
$3.1\pm0.4$ ps carrier recombination time are extracted from the XUV transient
absorption spectra from the core-to-conduction band transitions at the W
O$_{3}$ edge. Global fitting of the transient absorption signal at the W
N$_{6,7}$ edge yields $\sim 10$ fs coherence lifetimes of core-exciton states
and reveals that the photoexcited carriers, which alter the electronic
screening and band filling, are the dominant contributor to the spectral
modifications of core-excitons and direct field-induced changes play a minor
role. This work provides a first look at the modulations of core-exciton states
by photoexcited carriers and advances our understanding of carrier dynamics in
metal dichalcogenides.
