Coupled valence carrier and core-exciton dynamics in WS2 probed by few-femtosecond extreme ultraviolet transient absorption spectroscopy

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 WS2 at the W O3 edge (37-45 eV) and carrier-induced modifications of core-exciton absorption at the WN6,7 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 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±0.3 ps hole-phonon relaxation time and a 3.1±0.4 ps carrier recombination time are extracted from the XUV transient absorption spectra from the core-to-conduction-band transitions at the W O3 edge. Global fitting of the transient absorption signal at the W N6,7 edge yields ∼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.