Electronic and Structural Fingerprints of Charge-Density-Wave Excitations in Extreme Ultraviolet Transient Absorption Spectroscopy

Abstract

Femtosecond core-level transient absorption spectroscopy is utilized to investigate photoinduced dynamics of the charge density wave in 1T-TiSe_{2} at the Ti M_{2,3} edge (32–50 eV). Photoexcited carriers and phonons are found to primarily induce spectral redshifts 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. Interestingly, the spectral fingerprint of the amplitude mode can be associated with many-body electron screening, strengthening the importance of electron-electron and simultaneous electron-phonon interaction for the stability of the charge density wave. In addition, we find that 10% and 13% of the absorbed energy is stored in the A_{1g} and A_{1g}^{*} coherent modes, respectively. This work presents a first look on charge density wave excitations with tabletop core-level transient absorption spectroscopy, enabling simultaneous access to electronic and lattice excitation and relaxation.