Unveiling and Driving Hidden Resonances with High-Fluence, High-Intensity X-Ray Pulses

We show that high fluence, high-intensity x-ray pulses from the world’s first hard x-ray free-electron laser produce nonlinear phenomena that differ dramatically from the linear x-ray–matter interaction processes that are encountered at synchrotron x-ray sources. We use intense x-ray pulses of sub-10-fs duration to first reveal and subsequently drive the $1s\leftrightarrow 2p$ resonance in singly ionized neon. This photon-driven cycling of an inner-shell electron modifies the Auger decay process, as evidenced by line shape modification. Our work demonstrates the propensity of high-fluence, femtosecond x-ray pulses to alter the target within a single pulse, i.e., to unveil hidden resonances, by cracking open inner shells energetically inaccessible via single-photon absorption, and to consequently trigger damaging electron cascades at unexpectedly low photon energies.

Figure 1

Revealing and driving a hidden resonance within a single SASE pulse. An x-ray pulse at 848 eV first strips a $2p$ electron from Ne to reveal and then excite the ${\mathrm{Ne}}^{+}1s\to 2p$ resonance. Stimulated emission competes with Auger decay to refill the $1s$ hole. Cycling is terminated by Auger decay which changes the resonance energy.

Figure 2

(a) Electron emission from Ne vs x-ray energy, observed at 90° to the x-ray polarization axis. Photoelectron lines disperse linearly with photon energy while Auger lines are independent of photon energy [31]. The data are normalized to represent electron yields from x-ray irradiation of $30\mathrm{Joules}/\mathrm{eV}$. (b) ${}^{1}D$ Auger electron yield as a function of x-ray photon energy. (c) Absorption resonances for higher charge states that can be produced by high-fluence x-ray pulses calculated by the Hartree-Fock-Slater (HFS) method. Neutral Ne resonances, with a maximum cross section of 1.5 Mb, are barely visible, compared to the many ionic resonances.

Figure 3

Electron kinetic energy spectra of the ${}^{1}D$ Auger line. (a) Nonresonant Auger, $E_x=930\mathrm{eV}$. (b) Resonant Auger, $E_x=848\pm 1\mathrm{eV}$. Solid lines are the simulations for the experimental conditions: resonant (blue), nonresonant (red). (c) Simulations of Auger line shape before convolution with the instrumental function. All curves in this figure are normalized to the integrals over the displayed energy region.

Figure 4

Theoretical simulations for resonant $1s\to 2p$ excitation of neon with FEL pulses of intensity $3.5\times {10}^{17}\mathrm{W}/{\mathrm{cm}}^2$. (a) Occupation probabilities for the [$1s$] and [$2p_0$] vacancy states of the ${\mathrm{Ne}}^{+}$ ion as a function of time for irradiation by the single SASE pulse shown in (b). (b) The degree of coherence between the [$1s$] and [$2p_0$] vacancy states and SASE pulse profile used in (a) and (b). (c) The resonant Auger line shape generated by an ensemble of SASE pulses (averaged Gaussian temporal profile of 8.5 fs FWHM, 6 eV bandwidth) and a longitudinally coherent Gaussian pulse (8.5 fs FWHM, transform-limited).