Carrier relaxation and displacive coherent phonon motion measured by XUV transient absorption spectroscopy

We report on the measurement of carrier relaxation and coherent phonon motion in Sb after excitation by a few-cycle NIR pulse measured with XUV transient absorption spectroscopy. Lifting of the Peierl’s distortion in Sb by optical excitation of carriers leads to a well-known oscillation of the lattice position in form of a coherent phonon motion. Following the delay-dependent change of core-level absorption, by using attosecond XUV bursts, permits following this motion and unraveling the interplay of optically excited carriers, their thermal and relaxed counterparts and the lattice motion. The observed dynamics by and large follow the Displacive Excitation of Coherent Phonon Motion model. But the results show that the relaxation of the optically excited carriers plays an influential role on the phase of the cosine-like phonon motion: We find great agreement with recently reported DFT calculations on impulsive forces caused by the optically excited carriers and their momentum relaxation via electron-phonon scattering [1]. The measured oscillation phases hereby allow to extract the energy-dependent lifetime of the momentum relaxation. We furthermore observe transient spectral reshaping of core-level transitions during temporal overlap of the NIR and XUV pulse, which allows us to estimate the coherence lifetime of Sb core-level excitations.

[1] O’Mahony, et al. PhysReLett 123, 8 (2019): 087401.