Ultrafast Transitions from Solid to Liquid and Plasma States of Graphite Induced by X-Ray Free-Electron Laser Pulses

We used photon pulses from an x-ray free-electron laser to study ultrafast x-ray-induced transitions of graphite from solid to liquid and plasma states. This was accomplished by isochoric heating of graphite samples and simultaneous probing via Bragg and diffuse scattering at high time resolution. We observe that disintegration of the crystal lattice and ion heating of up to 5 eV occur within tens of femtoseconds. The threshold fluence for Bragg-peak degradation is smaller and the ion-heating rate is faster than current x-ray-matter interaction models predict.

Figure 1

Experimental setup to characterize the response of the graphite crystal simultaneously through elastic and inelastic diffuse scattering and Bragg scattering.

Figure 2

Measured pulse-averaged Bragg intensity with $1\sigma$ error bars as a function of the peak fluence for two different pulse lengths.

Figure 3

Scattering spectra for 60 and 80 fs pulses.

Figure 4

(a) Integrated elastic diffuse scattering signal per incoming XFEL pulse energy for different pulse lengths and peak intensities, and a linear fit to the data. The inset shows the full data set. (b) Integrated elastic diffuse scattering signal for an intensity of $1.6\times {10}^{16}\mathrm{W}/{\mathrm{cm}}^2$.

Figure 5

Ion temperature for different times. The left (low $Z_f$) boundary is due to the constraint $T_e\le 10\mathrm{eV}$, and the right boundary is due to $T_i\le T_e$. The upper and lower boundaries are based on error in the Rayleigh strength in Fig. 4a.