Fig. 1: Time-resolved photoelectron spectroscopy of the ZnO(10–10) surface upon resonant excitation.
a Angle-integrated photoelectron (PE) spectra recorded with hνprobe = 6.3 eV with the pump laser pulse hνpump = 3.43 eV off or at negative pump-probe delays Δt. Inset: Repetition rate dependency of the signal. The dashed line is a guide to the eye. b Angle-resolved PE spectrum at negative pump-probe delays showing a dispersionless feature. Black dots: Peak maximum of the intensity distribution. c Energy level diagram and illustration of the discussed surface metal generation due to the photoexcitation of deep defects at the ZnO surface. Negative delays: Photodoped semiconductor due to long-lived defect excitons. Small positive delays: Formation of a metallic band. d Temporal evolution of the PE intensity in false colors as a function of pump-probe delay Δt and energy. Pump laser fluence: 27 μJ/cm2. The pump pulse induces an abrupt increase of the electron density below EF. The purple box (XC) indicates the energy-integration window for the evaluation shown in Fig. 2a. e Normalized, angle-integrated PE intensity for different positive delays (50–800 fs) with Fermi-Dirac distribution fits (solid lines). Inset: Electronic temperature versus Δt. Error bars represent standard deviations. f Angle-resolved PE intensity averaged from 4 to 8 ps showing a dispersive free electron-like band.
