Ultrafast Structural Dynamics of Nanoparticles in Intense Laser Fields

Femtosecond laser pulses have opened new frontiers for the study of ultrafast phase transitions and nonequilibrium states of matter. In this Letter, we report on structural dynamics in atomic clusters pumped with intense near-infrared (NIR) pulses into a nanoplasma state. Employing wide-angle scattering with intense femtosecond x-ray pulses from a free-electron laser source, we find that highly excited xenon nanoparticles retain their crystalline bulk structure and density in the inner core long after the driving NIR pulse. The observed emergence of structural disorder in the nanoplasma is consistent with a propagation from the surface to the inner core of the clusters.

Figure 1

Experimental setup. The x-ray pulses are focused with Be lenses [16] onto a multiple skimmed jet of Xe clusters and their WAXS patterns are recorded with a MPCCD detector. NIR pulses are overlapped in the interaction region in a close to collinear geometry with a prism mirror and pump the clusters into a nanoplasma state. Ion TOF spectroscopy is used to determine the temporal overlap between the two lasers.

Figure 2

Radial average of the observed Bragg spots for unpumped clusters. Bragg reflections at the positions of (111), (200), and (220) planes of the fcc lattice are detected. The inset shows the decrease in hit rate as a function of the delay time for pumped clusters. The data points shown in the inset were derived by averaging the whole data in each delay point.

Figure 3

Delay dependence of profiles of Bragg spots from (111) plane at the NIR power of $4\times {10}^{16}\mathrm{W}{\mathrm{cm}}^{-2}$. (a) Characteristic spot images. (b) Temporal evolution of the spot intensity (markers) was fitted by an exponential function (solid red line). Its decay constant was estimated to be around 200 fs. (c) Temporal evolution of the spot width (markers) was compared with surface melting model (solid magenta line). (d) The temporal evolution of the core radius calculated with the experimental (markers) and fitting [solid red line in (b)] data. The error bars of (b)–(d) show standard deviations. (e) A scheme of cluster disordering that proceeds from the surface and is consistent with our experiments.