Direct Observation of Optically Induced Transient Structures in Graphite Using Ultrafast Electron Crystallography

We use ultrafast electron crystallography to study structural changes induced in graphite by a femtosecond laser pulse. At moderate fluences of $\le 21\mathrm{mJ}/{\mathrm{cm}}^2$, lattice vibrations are observed to thermalize on a time scale of $\approx 8\mathrm{ps}$. 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 $s{p}^3$-like character, forming interlayer bonds. Using ab initio density functional calculations, we trace the governing mechanism back to electronic structure changes following the photoexcitation.

Figure 1

Ultrafast Electron Crystallography (UEC) of highly oriented pyrolytic graphite. (a) The UEC pump-probe setup. (b) The layered structure of graphite. (c) Ground state diffraction pattern of graphite. (d) Ground state layer density distribution function (LDF), obtained via a Fourier transform of the central streak pattern in (c).

Figure 2

(a) Decay of the diffraction intensity. (b) Change in mean-squared atomic displacements perpendicular to the graphite planes, estimated from the Debye-Waller analysis of the intensity drops in (a). (c) Effective surface potential $V_s$ at the HOPG surface as a function of the applied fluence $F$. (d) Decay of the effective surface potential with fit to the drift-diffusion recombination model (see text). (e) Contraction of the interlayer spacing $\Delta d$ (left axis) and the corresponding increase in the effective surface potential (right axis) in the far-from equilibrium regime.

Figure 3

Signature of $s{p}^3$-like bonding in HOPG. (a) Molecular interference pattern $M(s)$ and the corresponding LDF curves at selected time instances following strong photoexcitation. The transient peak at 1.9 Å in the LDF indicates formation of an interlayer bond. (b) Ratio of peak intensities obtained from the diffraction patterns. The enhancement of the (0, 0, 12) with respect to the (0, 0, 10) peak cannot be explained by thermal vibrations and indicates a transient structure consistent with $s{p}^3$ bonding. (c) Ratio of peak intensities at $R=1.9\AA$ and 3.35 Å in the LDF.

Figure 4

Electronic structure changes during photoexcitations in graphite. (a) Schematic view of the structure. (b) Electronic density of states (solid line) and the Fermi-Dirac distribution of graphite at $T=0$ (dashed line) and $k_{B}T=1.0\mathrm{eV}$ (dotted line). (c) Total pseudocharge density $\rho (\mathbf{r})$ and (d) change in the pseudocharge density $\Delta \rho (\mathbf{r})$ due to an electronic temperature increase to $k_{B}T=1.0\mathrm{eV}$. The plane used in (c) and (d) intersects the graphene layers along the dashed lines in (a).