  Femtosecond core-level transient absorption spectroscopy is utilized to
investigate photoinduced dynamics of the charge density wave in 1T-TiSe2 at the
Ti M2,3 edge (30-50 eV). Photoexcited carriers and phonons are found to
primarily induce spectral red-shifts of core-level excitations, and a carrier
relaxation time and phonon heating time of approximately 360 fs and 1.0 ps are
extracted, respectively. Pronounced oscillations in delay-dependent absorption
spectra are assigned to coherent excitations of the optical $A_{1g}$ phonon
(6.0 THz) and the $A_{1g}^*$ charge density wave amplitude mode (3.3 THz). By
comparing the measured spectra with time-dependent density functional theory
simulations, we determine the directions of the momentary atomic displacements
of both coherent modes and estimate their amplitudes. This work presents a
first look on charge density wave excitations with table-top core-level
transient absorption spectroscopy, enabling simultaneous access to electronic
and lattice excitation and relaxation.
