Time-resolved soft X-ray absorption spectroscopy in transmission mode on liquids at MHz repetition rates

Abstract

We present a setup combining a liquid flatjet sample delivery and a MHz laser system for time-resolved soft X-ray absorption measurements of liquid samples at the high brilliance undulator beamline UE52-SGM at Bessy II yielding unprecedented statistics in this spectral range. We demonstrate that the efficient detection of transient absorption changes in transmission mode enables the identification of photoexcited species in dilute samples. With iron(II)-trisbipyridine in aqueous solution as a benchmark system, we present absorption measurements at various edges in the soft X-ray regime. In combination with the wavelength tunability of the laser system, the set-up opens up opportunities to study the photochemistry of many systems at low concentrations, relevant to materials sciences, chemistry, and biology.

FIG. 1.

(a) Schematic top view of the experimental layout for static and time resolved NEXAFS measurements of liquid samples. (b) Thickness variation across a water flatjet based on the transmitted intensity at a photon energy of 600 eV and tabulated values for the attenuation length. Used step sizes were Δy = 50 μm and Δz = 140 μm.

FIG. 2.

NEXAFS spectra recorded with the flatjet setup at different edges in the soft X-ray energy regime using the GaAs-diode. The average flux from the full BESSY II bunch pattern is used. The (a) C K-edge, (b) N K-edge and (d) Fe L₃-edge of $\text{Fe}{(\text{bpy})}_3^{2+}$ (aq, 30 mM; flow rate, 1.8 ml/min) and (c) O K-edge of H₂O (flow rate, 3.1 ml/min) at jet thicknesses of 2.5, 4.0, 1.3, and 0.8 μm. Incident energy bandwidths were 55 meV (9.8 × 10¹⁰ photons/s), 95 meV (6.6 × 10¹⁰ photons/s), 210 meV (3.7 × 10¹⁰ photons/s) and 45 meV (2.4 × 10¹⁰ photons/s), respectively. Counting times for the C K-edge and the N K-edge were 4 s per 0.05 eV step, for the Fe L-edge was 30 s per 0.1 eV step and for the O K-edge was 2 s per 0.25 eV step, including measurements of the energy dependent beamline flux I₀(E).

FIG. 3.

Transient NEXAFS spectra of $\text{Fe}{(\text{bpy})}_3^{2+}$ (aq, 30 mM) at the N K-edge (a) and the Fe L₃-edge (b) excited at a repetition rate of 208 kHz using laser pulses at a central wavelength of 343 nm and a power density of about 270 mJ/cm². Static measurements of the N K-edge and the Fe L₃-edge are shown in blue for comparison. The bandwidth of the incoming X-ray radiation was 130 meV at the N K-edge and 310 meV at the Fe L₃-edge. At these settings, the flux from the BESSY II hybrid bunch detected with an APD amounts to 2.6 × 10⁸ photons/s (∼1250 photons/pulse) at the N K-edge and 1.5 × 10⁸ photons/s (∼720 photons/pulse) at the Fe L₃-edge. The noise level of the transient spectra is estimated by the standard deviation of the background level and amounts to 0.1 mOD for a single scan. The counting time for the presented spectra was 1 s per 0.05 eV step at the N K-edge and 4 s per 0.1 eV step at the Fe L₃-edge. (c) and (d) show the corresponding temporal behavior of the NEXAFS signals at points indicated in the spectra. The counting time for all delay scans was 2 s per 10 ps step.

FIG. 4.

(a) Laser-fluence-dependent transient N K-edge absorption spectra measured at 150 ps delay. The excitation wavelength was 343 nm, and the fluence was varied between 5 and 280 mJ/cm². (b) The first positive amplitude of a two-Gaussian fit for energies below 400.4 eV is plotted against the corresponding excitation fluence. The data were taken under the same measurement conditions as in Fig. 3.

FIG. 5.

(a) Visible absorption spectrum of $\text{Fe}{(\text{bpy})}_3^{2+}$ in aqueous solution. (b)–(d) N K-edge picosecond excited state dynamics of $\text{Fe}{(\text{bpy})}_3^{2+}$ upon excitation into different optical absorption bands. Excitation densities were 210 mJ/cm², 270 mJ/cm², and 50 mJ/cm² for 515, 343, and 258 nm, respectively, at a repetition rate of 208 kHz. The incoming hybrid bunch X-ray flux at energies of 399 eV (colored) and 399.5 eV (black) and at a bandwidth of 130 meV amounts to 2.6 × 10⁸ photons/s (∼1250 photons/pulse). The counting times for delay scans in (b) and (c) were 1 s per 10 ps step and that for the scan in (d) is 6 s per 10 ps.