Femtosecond Reduction of Atomic Scattering Factors Triggered by Intense X-Ray Pulse

X-ray diffraction of silicon irradiated with tightly focused femtosecond x-ray pulses (photon energy, 11.5 keV; pulse duration, 6 fs) was measured at various x-ray intensities up to $4.6\times {10}^{19}\mathrm{W}/{\mathrm{cm}}^2$. The measurement reveals that the diffraction intensity is highly suppressed when the x-ray intensity reaches of the order of ${10}^{19}\mathrm{W}/{\mathrm{cm}}^2$. With a dedicated simulation, we confirm that the observed reduction of the diffraction intensity can be attributed to the femtosecond change in individual atomic scattering factors due to the ultrafast creation of highly ionized atoms through photoionization, Auger decay, and subsequent collisional ionization. We anticipate that this ultrafast reduction of atomic scattering factor will be a basis for new x-ray nonlinear techniques, such as pulse shortening and contrast variation x-ray scattering.

Figure 1

A schematic illustration of the experiment.

Figure 2

X-ray diffraction intensity profiles of Si at high peak intensities: (a) $(2.8\pm 0.7)\times {10}^{17}$, (b) $(3.5\pm 0.9)\times {10}^{18}$, (c) $(4.6\pm 1.2)\times {10}^{19}\mathrm{W}/{\mathrm{cm}}^2$, and corresponding diffraction intensity profiles at low peak intensity ($\sim 2.1\times {10}^{16}\mathrm{W}/{\mathrm{cm}}^2$). Dotted curves in (c) represent the estimated background.

Figure 3

xmdyn simulation results for Si crystal during its exposure to 6-fs XFEL pulse with photon energy of 11.5 keV and peak intensity of $1.0\times {10}^{20}\mathrm{W}/{\mathrm{cm}}^2$. (a) Comparison of diffraction efficiency for 111, 220, 311, 400, 311 reflections of Si obtained from simulations (red circles) and from experiment (black squares). (b)–(d) (b) Root-mean-square atomic displacement, (c) relative ion population, and (d) average numbers of $K$, $L$, and $M$ holes per atom, for Si exposed to the XFEL pulse. Time zero corresponds to the intensity maximum of the XFEL pulse. Black dotted curves represent temporal intensity envelope of the XFEL pulse.