X-ray probes of chemical dynamics

Since impor tant chemical processes are often irreversible, principal SwissFEL applications will use the pump probe method: An external fast trigger, e.g., an optical, IR, or THz pulse, initiates a reaction, typically involving vibration, relaxation, and/or dissociation, and the Swiss- FEL delivers a synchronized hard- or soft-X-ray pulse to probe the subsequent processes. Historically, ultrafast chemistry experiments have been per formed with fs laser pump pulses as the reaction initiator. The intriguing possibility of using non-ionizing THz radiation to initiate a surface catalytic reaction in the electronic ground state is the subject of an Infobox. As with the majority of high-intensity SwissFEL experiments, it will be necessary to refresh the sample between measurements, for example using a high-speed liquid jet for solutions (see Infobox) or a movable sur face for catalytic reactions. In a solution environment, optical pump pulses pose a problem in the ultrafast regime: As the desired time resolution improves, not only the probe pulse, but also the pump pulse must become shorter. Since it is the pump energy which determines the degree of photoexcitation in the sample, the peak intensity must increase as the pulse duration decreases. By delivering a large number of photons in such a short period of time, competing excitation processes in both the sample and the solvent become increasingly relevant. In order to avoid such potential complications as nonlinear sample excitation, multiphoton solvent ionization and, at longer pump wavelengths, tunneling ionization, a balance must be maintained between generating a sufficient level of sample excitation and ensuring that only the photoreaction of interest is being probed, and not a highly excited plasma. To achieve this balance, it will be necessary to perform laser spectroscopic pump-probe measurements in the optical regime prior to the SwissFEL measurements, using the same pump conditions as will be used with the X-ray probe.

Two basic methods are proposed to probe intermediate chemical states with X-rays: scattering and absorption/ emission spectroscopy.

X-ray scattering to probe chemical structure

Monochromatic hard X-rays (>3 keV) will easily penetrate a liquid jet (see Infobox) and are scattered into a 2ddetector by the solute and solvent molecules. Depending on the angular range collected, one speaks of smallangle (SAXS) or wide-angle (WAXS) X-ray scattering; there is an inverse relation (related to Bragg’s law for crystal diffraction) between the scattering angle and the length scale responsible for the scattering. Advantages of X-ray scattering for pump-probe investigations of chemical processes include the use of a fixed X-ray wavelength, a straightforward data analysis and the ability to probe large structural motions. Disadvantages of scattering are the necessity of an elaborate 2d-detector and the unavoidable background scattering from solvent molecules. The latter may be alleviated by choosing a resonant photon energy with enhanced scattering from the solute atoms. Although X-ray scattering is based on coherent inter ference, in the case of scattering from dissolved molecules, the long transverse coherence length of the SwissFEL radiation is not of par ticular advantage – indeed, the speckles resulting from coherent scattering by widely separated molecules will occur on an angular scale which is too fine to resolve. As an example of the information available from pumpprobe solution scattering, consider the simulated scattering by photoexcited molecular iodine dissolved in hexane, per formed by B. Zietz (EPFL, PSI). From the scattering profiles in Figures II.5a and b, one can appreciate the challenge to obtain an acceptable signalto- noise ratio. This task will be made still more difficult by pulse-to-pulse fluctuations in the SwissFEL intensity. Because of the linear polarization of the pump laser pulse (along the x-direction in the simulation), favorably- oriented I2 molecules will be preferentially excited, leading to the anisotropic X-ray scattering pattern in Figure II.5c. This anisotropy avoids the problem of the pulseto- pulse variations, since both excited and unexcited scattering is registered in a single measurement. Finally, the time-dependence of the anisotropic scattering, shown in Figure II.5d, includes an oscillating component, corresponding to the excited-state wave-packet vibration of I2, and a decrease in anisotropy, due to diffusional rotation. By controlling the pump laser wavelength and fluence, one can influence such processes as dissociation, solvent caging effects, geminate recombination, and non-geminate recombination, making pump-probe X-ray scattering a prolific source of chemical information.

A compact XANES / XES spectrometer

When performing pump-probe X-ray spectroscopy with the SwissFEL, it will be highly advantageous to avoid the requirement of tuning the incoming X-ray wavelength, hence allowing for single-shot data collection. This is possible using X-ray emission specroscopy (XES), where monochromatic X-rays cause characteristic fluorescence over a wide range of wavelengths. Alternatively, one can make use of the broadband emission mode of the SwissFEL, with detuned undulator modules, to per form single-shot X-ray near-edge absorption spectroscopy (XANES). Both of these alternatives require an efficient energy-dispersive X-ray detector. A possible design for such a detector, based on a deeply-etched, flexible silicon analyzer and the very successful MYTHEN detector from PSI [19] is shown in Fig. II.i4.

X-ray spectroscopy to probe electronic and geometric structure

X-ray spectroscopic signatures of the solute molecule contain a wealth of element-specific information about, e.g., valence, bonding configuration and local structure. The probing X-rays must either be tunable in energy around a resonant atomic absorption edge, ranging from the soft X-ray regime (e.g., C, N, O) to the hard X-ray regime (Fe, Ru, Pt), or be of sufficient bandwidth to cover the principal spectroscopic features of interest. Detection can be as simple as a transmission measurement or as complex as an energy-dispersed fluorescence measurement. In an X-ray absorption measurement, one differentiates between XANES (X-ray absorption nearedge structure) measurements, taken within 40 eV of the absorption edge, and EXAFS (extended X-ray absorption fine structure) measurements, extending up to 1 keV above the edge (see Fig. II.6). In an EXAFS measurement, the incoming X-ray photon energy is set to a value E = E0 + ΔE, above an absorption edge. This causes the emission, to first approximation, of a spherical photoelectron wave from the absorbing atom with kinetic energy ΔE, and hence a de Broglie wavelength \lambda_e = h/\sqrt{2m_e \triangle E} This spherical wave then reflects on the nearest-neighbor atoms, at the distance R1, returning (after a round-trip distance 2R1) to the absorbing atom with the phase shift Δφ = 4πR1e. Constructive inter ference at the absorbing atom of the (virtual) photoelectron waves will occur if Δφ is an integral multiple of 2π, causing a maximum in the quantum mechanical probability of the original photoelectron absorption. Hence, as the incoming photon energy is scanned above the absorption edge, λe will vary, and the photoabsorption probability will oscillate. A Fourier analysis of the absorption signal yields the distance to the nearest-neighbors, as well as that to more distant shells. In this way, EXAFS provides a sensitive signature of the local geometric structure around the absorbing atom and of the type of neighboring atoms. For the SwissFEL, per forming a pump-probe EXAFS experiment requires tuning the XFEL wavelength, by combined variation of the accelerated electron energy and of the undulator gap. Furthermore, careful shot-by-shot normalization of the incoming intensity is necessary. The full-bandwidth SwissFEL pulses will have an intrinsic spectral width of approximately 0.1%, which, at moderate X-ray energies, will provide sufficient bandwidth to per form single-shot measurements of par ticular XANES features. The SwissFEL will fur ther present the possiblity, through detuning of the individual undulator modules, of generating a tailored single-shot bandwidth of the order of 1–2%, at the cost of beam intensity. In either case, an energy-dispersive detetector is required (see Infobox). In contrast to scattering (and EXAFS), the interpretation of XANES is complicated and enriched by the combination of geometrical structure and electronic effects, involving consideration of a large number of unoccupied electronic states of the central atom and its neighbors (see Fig. II.7 and the Infobox) as well as more complicated multiple scattering pathways due to the long de Broglie wavelength of the photoelectron near the absorption edge. An example of the power of XANES to determine the atomic arrangement of a chemisorbed species is provided by the static measurements at the Pt K-edge of CO on the catalytic sur face of Pt nanopar ticles, shown in Figure II.8 [7].

An alternative single-shot X-ray spectroscopic method, suitable for use in a pump-probe SwissFEL experiment in chemical dynamics, is X-ray emission spectroscopy (XES). Here, a probe pulse of monoenergetic X-rays, with photon energy close to and greater than a resonant absorption edge, is applied to the laser-excited sample, and the X-ray fluorescence spectrum is measured in a wide-acceptance, energy-dispersive detector. XANES and XES yield complementary information: XANES on the unoccupied electronic levels and XES on the occupied levels (Infobox). A strong enhancement of the (inelastic) fluorescence yield occurs when the incoming radiation is resonant with an electronic transition. The measurement, called resonant inelastic X-ray spectroscopy (RIXS), is then sensitive to low-lying excitations of the absorbing ion, of its immediate neighborhood, or even of the entire sample (see Chapter V).

Finally, another related spectroscopic option to probe chemical systems is the non-resonant inelastic (Raman) scattering of hard X-rays. Because it is far from resonance, the scattered photon yield is very small. But the ability to observe low-energy absorption features using penetrating radiation is of great interest for phenomena occuring, for example, at interfaces covered with a water layer, such as the liquid-phase electrochemistry at a water-covered fuel-cell electrode.

The combination of scattering and spectroscopic techniques provided by the SwissFEL, over a wide range of impor tant elemental absorption edges and in a pumpprobe experimental arrangement, makes it an invaluable tool for understanding the shor t-lived molecular structures and electronic states involved in solution-phase and catalytic chemical reactions.

XES and RIXS

Whereas the XANES (X-ray absorption near-edge structure) technique yields information on the local coordination, valence and electronic structure of the absorbing atom, the emission spectroscopies XES (X-ray emission spectroscopy) and RIXS (resonant inelastic X-ray spectroscopy), being sensitive to electron- electron interactions in the occupied levels, yield complementary information: In addition to the local symmetry and coordination, XES gives orbital splittings and spin- and oxidation-states. The resonant technique, RIXS, provides access to low-energy excitations arising from local (e.g., d-d), nearest-neighbor (e.g., charge-transfer) and collective (e.g., plasmon) interactions and is described more fully in Chapter V.