Excitation Photon Energy-Dependent Carrier Multiplication in Graphite

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