Comment on "Structural Preablation Dynamics of Graphite Observed by Ultrafast Electron Crystallography"

TL;DR

This paper critiques prior ultrafast electron diffraction studies on graphite, revealing that transient electric fields caused beam deflections mistaken for lattice contractions and expansions, thus challenging previous interpretations of graphite dynamics.

Contribution

It demonstrates that transient electric fields significantly influence electron diffraction measurements, which was previously overlooked, leading to potential misinterpretations of lattice dynamics.

Findings

Transient electric fields cause beam deflections in UED measurements.

Previous lattice contraction and expansion observations may be artifacts of TEFs.

Correcting for TEFs alters understanding of graphite's ultrafast dynamics.

Abstract

In a recent letter (F. Carbone, P. Baum, P. Rudolf, et al., Physical Review Letters 100, 2008), graphite is reported to undergo a c-axis contraction on the time scale of few picoseconds (ps) after ultrafast pulsed laser excitation. The velocity of lattice contraction depended on the laser fluence. Furthermore, the lattice contraction is followed by large, non-thermal, lattice expansion of several picometers (pm) after few hundreds of ps. These results were interpreted based on the position of the (0014) diffraction spot in the glancing angle diffraction geometry off the graphite surface using ultrafast electron diffraction (UED). The lattice contraction and expansion corresponded to upward and downward movements of the diffraction spot respectively. Here, we show that ultrafast pulsed laser excitation of graphite produces large transient electric fields (TEFs). The TEFs deflect the electron beam in a manner similar to these reported and the neglect of TEFs likely led to erroneous conclusions.