Ultrafast X-Ray Scattering of Xenon Nanoparticles: Imaging Transient States of Matter

Femtosecond x-ray laser flashes with power densities of up to ${10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ at 13.7 nm wavelength were scattered by single xenon clusters in the gas phase. Similar to light scattering from atmospheric microparticles, the x-ray diffraction patterns carry information about the optical constants of the objects. However, the high flux of the x-ray laser induces severe transient changes of the electronic configuration, resulting in a tenfold increase of absorption in the developing nanoplasma. The modification in opaqueness can be correlated to strong atomic charging of the particle leading to excitation of ${\mathrm{Xe}}^{4+}$. It is shown that single-shot single-particle scattering on femtosecond time scales yields insight into ultrafast processes in highly excited systems where conventional spectroscopy techniques are inherently blind.

Figure 1

Experimental setup for the single-shot nanoparticle imaging experiments. Nanoclusters are injected into the focused x-ray pulse, and the scattered light is recorded with a high repetition rate detector. The x-ray photons are detected with an imaging quality multichannel plate (MCP) stack, converted into optical photons with a phosphorous screen, and digitized with an out-of-vacuum CCD camera. The multichannel plate stack is triggered to be active for only 50 ns during the arrival of the light pulse and then switched off prior to the arrival of any charged fragments from the explosion of the particle.

Figure 2

Single-shot scattering patterns of similarly sized single clusters with increasing power density from top to bottom. For large scattering angles, the signal increases more than linearly with respect to the variation of the incoming power density.

Figure 3

Single-shot scattering profiles (dotted curves). In (a), the profile of a $R=145\mathrm{nm}$ cluster and two Mie fits for low and high $qR$ as well as Porod’s law are shown. The three panels on the right-hand side correspond to the patterns shown in Fig. 2, demonstrating the power-density-dependent changes. The first maxima, in particular, for the highest power density shot in (d), suffer from detector saturation.

Figure 4

Particle absorption parameter $\beta$ deduced from the scattering patterns. Right: Modeled relative absorption contribution, i.e., relative abundance times cross section, of the dominant ions based on atom and cluster target tof spectra recorded with similar laser conditions.