Ultrafast Multiphoton Thermionic Photoemission from Graphite

Abstract

Electronic heating of cold crystal lattices in nonlinear multiphoton excitation can transiently alter their physical and chemical properties. In metals where free electron densities are high and the relative fraction of photoexcited hot electrons is low, the effects are small, but in semimetals, where the free electron densities are low and the photoexcited densities can overwhelm them, the intense femtosecond laser excitation can induce profound changes. In semimetal graphite and its derivatives, strong optical absorption, weak screening of the Coulomb potential, and high cohesive energy enable extreme hot electron generation and thermalization to be realized under femtosecond laser excitation. We investigate the nonlinear interactions within a hot electron gas in graphite through multiphoton-induced thermionic emission. Unlike the conventional photoelectric effect, within about 25 fs, the memory of the excitation process, where resonant dipole transitions absorb up to eight quanta of light, is erased to produce statistical Boltzmann electron distributions with temperatures exceeding 5000 K; this ultrafast electronic heating causes thermionic emission to occur from the interlayer band of graphite. The nearly instantaneous thermalization of the photoexcited carriers through Coulomb scattering to extreme electronic temperatures characterized by separate electron and hole chemical potentials can enhance hot electron surface femtochemistry, photovoltaic energy conversion, and incandescence, and drive graphite-to-diamond electronic phase transition.

Observation of intercalation-driven zone folding in quasi-free-standing graphene energy bands

“…This selection rule for the image potential state is consistent with specific, previous measurements on graphite surfaces 61,64 . It is also consistent with the basic physical picture for dipole allowed transitions in ARPES.…”

“…AR-2PPE was used to probe the unoccupied bands E k at these interfaces with an energy range between the Fermi energy (E f ) and the vacuum energy (E v ) and with crystal momentum near the Brillouin zone center. [58][59][60][61] . For reference, the measured work function for each sample is added to the measured photoelectron kinetic energy.…”

Heat localization through reduced dimensionality

“…Among other effects, the electronic and lattice temperatures may be different, as has also been observed in graphite and van der Waals heterostructures [46][47][48], and deviations from the black body radiation law are possible. However, prior works have shown good agreement with the black body radiation law for CNTs [49,50].…”

Observations of Radiation‐Dominated Rapid Cooling of Structures Based on Carbon Nanotubes and Graphene

“…When mobile electrons in graphene or graphite are excited by intense incoming light, they thermalize with each other within tens of femtoseconds and form a hot‐electron bath at a temperature that can be substantially higher than that of the lattice because of weak screening and strong electron–electron interactions in the material . Remarkably, electronic temperatures as high as 5500 K have been reported . Electron energy loss to phonons takes picoseconds, and for the electrons to reach equilibrium with both optical phonons and acoustic phonons, it takes hundreds of picoseconds to nanoseconds .…”

“…[9][10][11] Remarkably, electronic temperatures as high as 5500 K have been reported. [11] Electron energy loss to phonons takes picoseconds, and for the electrons to reach equilibrium with both optical phonons and acoustic phonons, it takes hundreds of picoseconds to nanoseconds. [12] (In these relaxation processes, radiative loss plays a negligible role.)…”