Excitation\nPhoton Energy-Dependent Carrier Multiplication\nin Graphite

Due\nto the enhanced carrier–carrier interaction, carrier\nmultiplication (CM) in graphitic materials is efficient and exhibits\nstrong wavelength dependence. However, there is still a lack of direct\nevidence for excitation photon energy-dependent impact ionization,\nthe origin of CM. In this work, time- and angle-resolved photoelectron\nspectroscopy has been used to measure the electron dynamics in graphite.\nWe find the unusual preceding growth of electron signal in 0–0.35\neV compared with that in 0.35–0.70 eV and 0.70–1.05\neV during the absorption of UV (3.10 eV) pulses, unambiguously suggesting\nthe occurrence of impact ionization. Such phenomena are much less\nsignificant under IR (1.55 eV) excitation. Taking into account the\nelectron–phonon scatterings during the electron thermalization,\nthe CM value is calculated to be 2.0 and 1.4 under UV and IR excitation,\nrespectively. This work provides direct evidence for excitation photon\nenergy-dependent impact ionization, which will benefit the potential\napplications of graphitic materials.