Time- and angle-resolved photoemission spectroscopy for the saddle-point excitons in GaAs

The electronic structure and dynamics of saddle-point excitons formed under E1-resonance excitation around 3.0 eV in GaAs have been studied using time- and angle-resolved photoemission spectroscopy. The momentum- and energy-resolved spectra of the exciton photoionization reveal that the exciton wave function is composed mainly of the electron-hole pair states at the wave number of 0.32 ± 0.04 Å−1 along the Γ-L direction of the bulk Brillouin zone. Time-resolved characteristics of the photoemission is featured by a bimodal decay in the femtosecond (fs) time frame with two different time constants of 11 ± 0.5 fs and 120 ± 10 fs . The short lifetime reflects the autoionization processes of coherently generated excitonic polarization to unbound electron-hole pair states, and the long time constant represents incoherent exciton formation by combination between unbound electron-hole pairs during relaxation. The momentum- and energy-resolved features of photoemission from the exciton are governed by recoil effects of the conjugated holes that restore the valence-band states.