Fig. 4: Signature of bulk-to-surface charge migration.

a, b Representative two-dimensional pseudo-color 2PPE spectra of (a) GaN and (b) Pt/GaN acquired under two-color excitation with a UV photon energy of 4.49 eV (276 nm) and a visible (Vis) photon energy of 2.58 eV (480 nm), after background subtraction using the signal recorded at –10 ps. The dashed arrow in b indicates the shift of the photoemission edge toward higher energies with increasing negative delay times. c Temporal evolution of the normalized photoemission intensity for excited-state carriers at a kinetic energy of 0.5 ± 0.2 eV. The gray line denotes the background level. d Normalized 2PPE spectra of GaN and Pt/GaN recorded at a delay time of –0.2 ps. e Temporal traces of the normalized photoemission intensity at a kinetic energy of 0.90 ± 0.20 eV for GaN and 0.65 ± 0.20 eV for Pt/GaN. The solid lines represent fits to the kinetic model to extract the characteristic time constants.