  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.
