Ultrafast Time-Resolved Hard X-Ray Emission Spectroscopy on a Tabletop

Experimental tools capable of monitoring both atomic and electronic structure on ultrafast (femtosecond to picosecond) time scales are needed for investigating photophysical processes fundamental to light harvesting, photocatalysis, energy and data storage, and optical display technologies. Time-resolved hard x-ray ($>3\mathrm{keV}$) spectroscopies have proven valuable for these measurements due to their elemental specificity and sensitivity to geometric and electronic structures. Here, we present the first tabletop apparatus capable of performing time-resolved x-ray emission spectroscopy. The time resolution of the apparatus is better than 6 ps. By combining a compact laser-driven plasma source with a highly efficient array of microcalorimeter x-ray detectors, we are able to observe photoinduced spin changes in an archetypal polypyridyl iron complex $[\mathrm{Fe}(2,2^{\prime }\text{−}\text{bipyridine}{)}_3{]}^{2+}$ and accurately measure the lifetime of the quintet spin state. Our results demonstrate that ultrafast hard x-ray emission spectroscopy is no longer confined to large facilities and now can be performed in conventional laboratories with 10 times better time resolution than at synchrotrons. Our results are enabled, in part, by a 100- to 1000-fold increase in x-ray collection efficiency compared to current techniques.

Popular Summary

Chemical reactions driven by light are fundamental to biology and are the inspiration for tasks such as solar-energy harvesting and data storage. However, observing and understanding chemical photodynamics requires experimental tools capable of monitoring both atomic and electronic structure on ultrafast time scales. Time-resolved hard x-ray (> 3 keV) spectroscopy has proven to be valuable for these measurements because of its elemental specificity and sensitivity to both the geometrical and electronic configuration of atoms and molecules. However, intense x-ray beams are often required, and such measurements can only be performed at large facilities where access to instrumentation is highly competitive (e.g., synchrotrons and free-electron lasers). Here, we present a tabletop apparatus capable of performing time-resolved x-ray emission spectroscopy with a time resolution better than 6 ps.

We make use of intense 800-nm laser light and a water target to generate bremsstrahlung x-ray radiation from 3–15 keV. By combining our compact laser-driven plasma source with a highly efficient array of microcalorimeter x-ray detectors, we are able to observe photoinduced spin changes in an archetypal polypyridyl iron complex. We observe a photoinduced transition between singlet and quintet spin states and accurately measure the lifetime of the quintet state. Additionally, we demonstrate an x-ray collection efficiency that is 2 to 3 orders of magnitude higher than that of other teams. Our results reveal that ultrafast hard x-ray emission spectroscopy is no longer confined to large facilities: The characteristic size and power consumption of our apparatus are meters and kilowatts, respectively, compared with hundreds of meters and megawatts for synchrotrons and free-electron laser facilities.

We expect that our findings will make ultrafast time-resolved hard x-ray spectroscopy available to a much wider community of researchers.

Figure 1

(a) Energy-level diagram for x-ray emission after photoionization of a $1s$ core electron. (b) Energy-level schematic showing low-spin and high-spin states of the $3d$ orbital shell of Fe(II). (c) Schematic potential energy curves of Fe(II) complexes as a function of the Fe-N bond distance. (d) Experimental setup for time-resolved x-ray emission spectroscopy (TRXES) combining an ultrafast laser system, laser-driven x-ray plasma source, sample jet, and array of microcalorimeter detectors.

Figure 2

X-ray emission spectra from ${[\mathrm{Fe}{(\mathrm{bpy})}_3]}^{2+}$. The XES features are characteristic of a low-spin Fe state and are in agreement with reference data [19]. The insets display the $K\alpha$ and $K\beta$ regions. The energy resolution of the detector in the $K\alpha$ region is 5.2 eV FWHM. The detector resolution in the $K\beta$ region is less well determined, but is near 5.5 eV.

Figure 3

X-ray emission spectra of ${[\mathrm{Fe}{(\mathrm{bpy})}_3]}^{2+}$ less than 6 ps after photoexcitation. (a) Fe $K\alpha$. (b) Fe $K\beta$. Pumped and unpumped measurements are shown as red and blue markers, respectively. The dashed black lines are the reference high-spin spectra convolved with the detector response (DR) function. The solid red and blue curves are fits to the data that use a linear combination of high-spin and low-spin reference data convolved with the DR. The black markers show the difference between the pumped and unpumped data with 1-standard-deviation error bars. The solid black line is the difference between the two fit curves. The lower axes show residuals between the data and the two fit curves. Error values are calculated assuming Poisson statistics in each energy bin. This data set is acquired over 12.5 h of integration.

Figure 4

Time evolution of high-spin (HS) fraction. Values for the HS fraction in optically excited ${[\mathrm{Fe}{(\mathrm{bpy})}_3]}^{2+}$ obtained from measurements of Fe $K\alpha$ and $K\beta$ spectra plotted versus time. A single-exponential fit to the HS fraction yields a decay-time constant of $570\pm 100\mathrm{ps}$. Inset: Zoom into HS fraction at short time delays. Time zero is chosen to lie halfway between the two time points exhibiting the largest change in a measurement with 6-ps spacing between delays. These results show that our tabletop apparatus has a time resolution better than 6 ps.

Figure 5

X ray, pump, and sample interaction region. (a) Side view of the interaction region of the pump beam, x-ray beam, and the sample jet. The $100\text{−}\mu \mathrm{m}$-diameter sample jet is angled 10° with respect to the central axis of the x-ray beam. The x-ray focus has a nominal diameter of $80\mu \mathrm{m}$. The 400-nm pump beam is orthogonal to the other beams and therefore travels perpendicular to the plane of the page. The pump is focused using two orthogonal cylindrical lenses down to an ellipsoidal spot measuring $1000\times 300\mu \mathrm{m}$. (b) Top view of interaction region. Since the 10° angle between x rays and sample jet is measured with respect to the horizontal plane, the x rays and sample jet overlap in this view. The small insets show notional x-ray spectra in photons versus energy before and after the sample jet. The sharp drop in the upper inset is due to the Fe $K$ absorption edge. The observer symbol denotes the orientation of the microcalorimeter array.