Mapping Resonance Structures in Transient Core-Ionized Atoms

The nature of transient electronic states created by photoabsorption critically determines the dynamics of the subsequently evolving system. Here, we investigate $K$-shell photoionized atomic neon by absorbing a second photon within the Auger-decay lifetime of 2.4 fs using the European XFEL, a unique high-repetition-rate, wavelength-tunable x-ray free-electron laser. By high-resolution electron spectroscopy, we map out the transient Rydberg resonances unraveling the details of the subsequent decay of the hollow atom. So far, ultra-short-lived electronic transients, which are often inaccessible by experiments, were mainly inferred from theory but are now addressed by nonlinear x-ray absorption. The successful characterization of these resonances with femtosecond lifetimes provides the basis for a novel class of site-specific, nonlinear, and time-resolved studies with strong impact for a wide range of topics in physics and chemistry.

Popular Summary

The Universe is full of ionic matter for many reasons, one of them being an abundance of x rays in space. The first step to becoming an ion via x rays is photoabsorption, which leads to a variety of subsequent relaxation processes in the interacting system (either an isolated atom or a compound of two or more atoms). Exploring how such a highly energy-loaded system looks before it relaxes is of great importance for many areas of x-ray physics, including investigations of ultrafast atomic processes. To that end, we explore, for the first time, the electronic structure of a neon atom directly after absorption of an x-ray photon.

For our experiments, we use the largest x-ray laser in the world, the European x-ray free-electron laser (XFEL), which provides an unprecedentedly large number of intense flashes of short-wavelength photons per second. With this facility, we are able to create a highly transient state, in which neon atoms lose a core electron, and to directly interrogate this system with a second x-ray photon within its ultrashort lifetime of about 2.4 fs, which maps out the elusive electronic structure via resonantly exciting the remaining core electron.

Our study benchmarks the unique photonic conditions available at the Small Quantum Systems instrument of the European XFEL and opens the door to the exploration of highly transient states of matter.

Figure 1

Scheme of the investigated process. Linear regime (left): The first photon absorption and resulting ionization of neon is typically followed by an Auger decay after approximately 2.4 fs (from top to bottom). Nonlinear regime (right): With short pulses of sufficiently high fluence, this sequence can be intercepted by a second photon that creates $1s^{-2}np$-Rydberg states in the transient ion. The relaxation of this double-core hole results in the ejection of an electron, which is the subject of the present investigation.

Figure 2

Resonance map of the transient Ne ion. (a) Interpolated false color representation of the electron spectra recorded by varying the photon energy across the region of the ${\mathrm{Ne}}^{+}{1s}^1{2s}^2{2p}^6\to {1s}^0{2s}^2{2p}^{6}np$ transitions with steps of 1 eV. (b) Total DCH Auger-electron yield (black line). The total yield is decomposed into individual contributions for the different Rydberg states by projecting the 2D map at selected electron energies [see colored arrows on the right side of (a)]: ${1s}^{0}3p$ excitation (green, electron energy $E_k=881\mathrm{eV}$), ${1s}^{0}4p$ excitation (red, $E_k=875.5\mathrm{eV}$), ${1s}^{0}5p$ excitation (magenta, $E_k=872.5\mathrm{eV}$), ${1s}^0ϵp$ excitation (blue, $E_k=870.5\mathrm{eV}$). The integration regions for the selected electron energies are 6, 3, 1, and 0.5 eV, respectively. The black arrows indicate the photon energies selected for the detailed analysis of the relaxation pathways (Fig. 3). A small contribution of direct valence photoionization of ${\mathrm{Ne}}^{2+}$ ions in their ground state can be seen as faint diagonal lines at high electron energies.

Figure 3

Electron spectra from individual resonances. The experimental electron spectra at (a) $h\nu =982\pm 0.5\mathrm{eV}$ (circles), (b) $h\nu =989\pm 0.5\mathrm{eV}$ (squares), (c) $h\nu =991\pm 0.5\mathrm{eV}$ (diamonds), and (d) $h\nu =1000\pm 0.5\mathrm{eV}$ (stars) are plotted in comparison to the calculated yield from all transitions that are possible within the specified photon-energy bandwidth (thick black curves). For each panel, the color code assigns blue to $3p$, red to $4p$, yellow to $5p$, green to $(>5)p$, and purple to continuum excitation states. The theory spectra are decomposed in the contribution from the photon-energy-selected resonant excitation (full thicker lines) and, for panels (b) and (c), the non-negligible yields from close lying resonances (dashed lines) labeled in gray. The photon-energy-selected excitation yields are further resolved in the contributions attributed to the different final states (full thick lines, spectator decay; full thin lines, shakeup or -down decays).

Figure 4

Electron spectrum above the DCH ionization threshold. High-resolution electron spectrum of neon ionized at $hv=1110\mathrm{eV}$ with a pulse energy of $1.1\pm 0.1\mathrm{mJ}$. The sharp feature at an electron energy 804.5 eV shaded in red is attributed to the ${\mathrm{KL}}_{23}{L}_{23}({}^{1}D)$ Auger line generated by the decay of the SCH state ${\mathrm{Ne}}^{+}{1s}^1{2s}^2{2p}^6$ after $1s$ ionization of the neutral. The broad structures at 1060 and 1090 eV result from the valence ionization of the neutral atom. The structure between 835 and 875 eV electron energy is mostly assigned to the DCH Auger decay, with final states as indicated in the legend. The spectroscopic assignments are taken from Ref. [24] and references therein. The two uncolored lines around 850 eV may be identified according to Ref. [24] as decays ${\mathrm{Ne}}^{2+}{1s}^0{2s}^2{2p}^{5}3s,3p\to {\mathrm{Ne}}^{3+}{1s}^1{2s}^1{2p}^{4}3s,3p$, which are excited from the ground state of Ne by two photons much stronger than by a single photon of the synchrotron radiation. Inset: pulse energy dependence of the SCH (red) and DCH (blue) contributions over more than an order of magnitude obtained by attenuating the pulse energy upstream the interaction region. The dots represent the experimental data obtained by integrating the electron yield from the shaded areas in the spectrum at given pulse energies; the lines represent power-law fits from the pulse energy range ${\mathrm{E}}_{\text{pulse}}<800\mu \mathrm{J}$, where the experiment is not affected by target depletion. The extracted power laws are consistent with a one-photon (linear) and a two-photon process, respectively.

Figure 5

Calculated photoabsorption cross section, resonant Auger spectrum, and resonance map of the transient Ne ion. (a) Interpolated false color representation of the theory-based electron spectra for different photon energies across the region of the ${\mathrm{Ne}}^{+}$ ${1s}^1{2s}^2{2p}^6\to {\mathrm{Ne}}^{+}$ ${1s}^0{2s}^2{2p}^6\mathrm{np}$ transitions. The map results from the convolution of the calculated photon-energy-dependent electron spectra with a two-dimensional Gaussian of widths taken from the experimental electron spectrum resolution 2.1 eV FWHM and the photon-energy bandwidth of 8.5 eV FWHM, as given in the text. (b) Left axis: calculated cross section of the transient Ne ion. Green: without inclusion of $np$-spectator decay channels. Blue: with inclusion of decay channels related to $3p$ and $4p$ spectator electrons. Black: with inclusion of all decay channels listed in the text. The scale for the right axis is the cross section in arbitrary units for the red solid line, which is obtained after convolution with an 8.5-eV-wide Gaussian. This line corresponds to the integral over the kinetic energy axis of the 2D map shown in (a). (c) Black: calculated resonant Auger spectrum at a photon energy of 981 eV corresponding to the calculated excitation energy of the $3p$ resonance. Red: the same spectrum after convolution with both the photon-energy bandwidth and the spectrometer response function. This line corresponds to a column at a given photon energy in the 2D map shown in (a).