Direct observation of optically induced transient structures in graphite using ultrafast electron crystallography

TL;DR

This study employs ultrafast electron crystallography to observe how femtosecond laser pulses induce transient structural changes in graphite, revealing nonthermal states with sp^3-like bonding driven by electronic structure modifications.

Contribution

It provides direct experimental evidence of transient nonthermal states in graphite and links these to electronic structure changes via ab initio calculations.

Findings

Lattice vibrations thermalize in ~8 ps at moderate fluences.

Higher fluences cause saturation of lattice vibration amplitudes.

Graphite transiently adopts sp^3-like bonding states after photo-excitation.

Abstract

We use ultrafast electron crystallography to study structural changes induced in graphite by a femtosecond laser pulse. At moderate fluences of ~< 21mJ/cm^2, lattice vibrations are observed to thermalize on a time scale of ~8ps. At higher fluences approaching the damage threshold, lattice vibration amplitudes saturate. Following a marked initial contraction, graphite is driven nonthermally into a transient state with sp^3-like character, forming interlayer bonds. Using ab initio density functional calculations, we trace the governing mechanism back to electronic structure changes following the photo-excitation.