Clocking Femtosecond Collisional Dynamics via Resonant X-Ray Spectroscopy

Electron-ion collisional dynamics is of fundamental importance in determining plasma transport properties, nonequilibrium plasma evolution, and electron damage in diffraction imaging applications using bright x-ray free-electron lasers (FELs). Here we describe the first experimental measurements of ultrafast electron impact collisional ionization dynamics using resonant core-hole spectroscopy in a solid-density magnesium plasma, created and diagnosed with the Linac Coherent Light Source x-ray FEL. By resonantly pumping the $1s\to 2p$ transition in highly charged ions within an optically thin plasma, we have measured how off-resonance charge states are populated via collisional processes on femtosecond time scales. We present a collisional cross section model that matches our results and demonstrates how the cross sections are enhanced by dense-plasma effects including continuum lowering. Nonlocal thermodynamic equilibrium collisional radiative simulations show excellent agreement with the experimental results and provide new insight on collisional ionization and three-body-recombination processes in the dense-plasma regime.

Figure 1

Experimental setup: A ($54\pm 2$)-nm-thick Mg foil is irradiated by the focused x-ray pulse. The x-ray wavelength is tuned to the bound-bound resonance in a $7^{+}$ Mg ion, resonantly driving a $1s\to 2p$ transition creating a core-hole excited state. The excited state can then decay through Auger recombination or $K_{\alpha }$ emission.

Figure 2

Experimental and simulated (with the BCF, CAM, Lotz, and BC models) $K_{\alpha }$ emission spectrum resulting from resonantly driving the $K^2{L}^3\to K^1{L}^4$ transition centered around 1300 eV. After excitation, the states relax through collisional ionization (up in charge) and three-body recombination (down in charge). The numerals below each emission line denote the charge state of the emitting ion, III thus indicating the cold $K_{\alpha }$ line.

Figure 3

Simulated evolution of the pumped $K^2{L}^3$ (left) and $K^1{L}^4$ (right) populations in temperature-density space. Charge state populations are obtained from scfly simulations using the BCF model [Eq. (1)] that match the experimental conditions and are binned in a 2D temperature-density histogram across all points in time and space. Both populations peak near the LTE conditions, represented by the white dotted curve.

Figure 4

Collisional ionization cross section for the $K^1{L}^4$ ($7^{+}$) Mg ion. Theoretical and semiempirical models predict similar cross sections in the atomic limit. Accounting for increasing density via IPD lowers the threshold and raises the cross section; here the BCF model differs significantly from the Lotz and BC models. The range of cross sections consistent with our experimental results shown in Fig. 1 lies within the shaded band.