International
Tables for
Crystallography
Volume I
X-ray absorption spectroscopy and related techniques
Edited by C. T. Chantler, F. Boscherini and B. Bunker

International Tables for Crystallography (2022). Vol. I. Early view chapter
https://doi.org/10.1107/S1574870720004760

Time-resolved optical pump/X-ray absorption spectroscopy probe

Majed Cherguia*

aLaboratoire de Spectroscopie Ultrarapide and Lausanne Centre for Ultrafast Science (LACUS), ISIC, Faculté des Sciences de Base, Ecole Polytechnique Fédérale de Lausanne, Station 6, CH-1015 Lausanne, Switzerland
Correspondence e-mail: majed.chergui@epfl.ch

The capabilities offered by time-resolved X-ray absorption spectroscopy, focusing on the picosecond to femtosecond time domain, are reviewed. Examples are provided in chemistry, biology and materials science.

Keywords: time-resolved XAS; pump–probe.

1. Introduction

Any system, be it a solid material, a molecule or a protein, which is activated by the absorption of light will undergo structural changes on the order of tens of femtoseconds to hundreds of picoseconds and even longer, depending on the scale of the changes. These changes in geometric structure are triggered and/or accompanied by changes in electronic structure. Ideally, one would therefore like to probe the geometric structure changes in real time, but also identify the electronic structure changes underlying them. In addition, most of (bio)chemistry occurs in liquids, and therefore the ideal tool should be able to probe the above changes in disordered media. Time-resolved X-ray absorption spectroscopy (XAS; Chen, 2004[link]; Bressler & Chergui, 2004[link], 2008[link]; Chergui & Zewail, 2009[link]) in a pump–probe configuration has emerged as the method of choice in recent years, as it offers the ability to probe electronic and structural changes with elemental selectivity.

An X-ray absorption spectrum, with its pre-edge, X-ray absorption fine structure (XANES) and extended X-ray absorption fine structure (EXAFS) (Koningsberger & Prins, 1988[link]), delivers information about the electronic structure of a specific element and the geometric structure around it (i.e. bond distances and angles to the nearest neighbours), as discussed elsewhere in this volume. Any photo-induced electronic and/or structural change in the system under study will affect the characteristic features of the X-ray absorption spectrum at the edge of choice, depending on the nature of the photoexcitation (ionization, charge transfer, dd excitation in transition-metal systems etc.).

The principle of time-resolved XAS in the femtosecond to nanosecond domain is based on the pump–probe scheme. The pump laser pulse triggers a process in the system under study, while a short X-ray pulse with a tunable time delay with respect to the pump pulse will probe the changes in the system.

2. The methodology

The first demonstration of time-resolved XAS was by Mills and coworkers, who investigated the recombination of carbon monoxide (CO) with myoglobin (Mb) after its photodissociation from the active centre (the Fe atom in the porphyrin) using a green nanosecond laser pulse (Mills, Lewis et al., 1984[link]). The sample was probed at the Fe K edge (7.12 keV) using synchrotron-radiation X-ray pulses, and the authors were able to monitor the photoexcited signal around the absorption edge (XANES) over the full range of sample relaxation (from a few microseconds to >10 ms) and the recovery of carboxymyoglobin (MbCO). As the authors pointed out, the data-collection time, and the resulting signal to noise, was controlled not by the sample or by the X-ray flux, but instead by the laser repetition rate (Mills, Pollock et al., 1984[link]).

It took another 17 years after these groundbreaking experiments for time-resolved XAS experiments to be implemented in the nanosecond time domain by Chen et al. (2001[link]), who investigated photo-induced changes in porphyrins. Shortly after this, the Chergui group carried out the first picosecond-resolved XAS experiment on a ruthenium complex in solution (Saes et al., 2003[link]) and also implemented the transient absorption data-acquisition scheme (pumped minus unpumped sample X-ray absorption) based on a pulse-to-pulse data acquisition (Fig. 1[link]). This approach took advantage of fast X-ray detectors in transmission and fluorescence to resolve the X-ray pulse structure from the synchrotron ring, allowing the photons from a single X-ray probe pulse to be selected electronically (Saes et al., 2004[link]). The initial experiments were carried out using a femtosecond laser operating at a 1 kHz repetition rate synchronized with the isolated `hybrid' X-ray pulse of the synchrotron source that is recorded at 2 kHz (once for the excited sample and once for the unexcited sample; Fig. 1[link]). Considering the megahertz repetition rate of synchrotrons, this implies that typically 103 X-ray pulses are not used. As a consequence, the measurements require significant acquisition times (typically tens of hours) to ensure meaningful signal-to-noise ratios (S/N), and the necessary sample concentrations were generally high (tens to hundreds of millimoles per litre), which excluded the investigation of biological systems that can be dissolved to at most a few millimoles per litre. Finally, given the 50–100 ps width of the synchrotron pulses, the use of femtosecond laser pulses for excitation does not provide additional advantages in terms of time resolution. In order to overcome the limitations due to the mismatch in repetition rate between the pump laser and the probe X-ray pulses, the Chergui group implemented a scheme using a high-repetition-rate picosecond pump laser that allowed improvements of orders of magnitude in the achievable S/N in time-resolved XAS measurements (Fig. 3; Lima et al., 2011[link]) and which was quickly adopted at other synchrotron centres (March et al., 2011[link]; Stebel et al., 2011[link]). Interestingly, this scheme confirms the above prediction by Mills, Pollock et al. (1984[link]). The high-repetition-rate scheme also offers the possibility of carrying out time-domain photon-in/photon-out spectroscopies such as XES, and resonant inelastic X-ray scattering (RIXS).

[Figure 1]

Figure 1

The high-repetition-rate pump–probe scheme for time-resolved X-ray spectroscopy at synchrotrons (here the case of the Swiss Light Source is shown; Lima et al., 2011[link]). The pump laser runs at half (520 kHz) the repetition rate of the isolated hybrid pulse of the synchrotron (1.04 MHz) with which it is synchronized. The time delay between the laser and X-ray pulses is tunable. The signal consists of the difference between the X-ray signal of the unpumped and the pumped sample recorded on a pulse-to-pulse basis.

The X-ray linear absorption coefficient μ(Ω) at a particular incident photon energy Ω is derived from the Beer–Lambert law,[A(\Omega) = \mu(\Omega)\cdot d = \ln {{I_0}\over {I_{\rm t}}}. \eqno (1)]

When extended to the time domain, the XAS signal at a particular X-ray energy and pump–probe time delay (Δt) is recorded twice, alternating between the signal of the excited sample (pumped) and that of the unexcited sample (unpumped). A zero measurement is also made by reading the detector signal in the gap where no X-rays are present, which is then subtracted from the corresponding X-ray signal to compensate for any drifts over time of the data-acquisition baseline. The transient spectrum is then expressed as the difference between the pumped (excited) and the unpumped (ground) state,[\Delta I(\Omega, t)= f\cdot [\Phi(t)I_{\rm pumped}({\Omega})-I_{\rm unpumped}(\Omega)],\eqno (2)]where f is the photolysis yield and Φ(t) represents the quantum yield of the product, the time dependence of which reflects decay processes. This general methodology also applies to photon-in/photon-out spectroscopies.

Thus far, the above was limited to the <100 ps resolution of synchrotrons. In order to get to subpicosecond time resolution, a slicing scheme was developed which allowed the extraction of ∼100 fs pulses from the synchrotrons, albeit with a dramatic loss in X-ray photon flux (Schoenlein et al., 2000[link]; Khan et al., 2006[link]; Beaud et al., 2007[link]). Nevertheless, using this scheme the Chergui group demonstrated femtosecond X-ray spectroscopy of a dilute molecular system for the first time (Bressler et al., 2009[link]). Soon after came the fourth-generation instruments, i.e. X-ray free-electron lasers (XFELs), which produce ten orders of magnitude more photons than the slicing scheme for the same temporal pulse width (Zangrando et al., 2011[link]; Zhu et al., 2012[link]; Feng et al., 2012[link]; Ishikawa et al., 2012[link]; Lemke et al., 2013[link]). XFELs are now the prime source of ultrashort X-ray pulses, with several examples of femtosecond XAS experiments being carried out on molecular complexes in solution and in the solid phase (Chergui & Collet, 2017[link]).

Most of the above examples concerned the hard X-ray domain, for which no vacuum is needed. Going into the soft X-ray domain offers the advantage of accessing the L edges of transition metals, which play a crucial role in coordination chemistry, biology and the physics of strongly correlated materials, as well as accessing the K edges of light elements such as C, N, O, S etc. that are central in biology. However, working in the 6–2000 eV range requires a vacuum, which poses problems with liquid samples. Ingenious solutions have been found such as using cells with SiN membrane windows, both static and flowing, free-liquid microjets and, recently, colliding flat jets (Ekimova et al., 2015[link]). These schemes are now routinely being implemented at synchrotrons (Fondell et al., 2017[link]) and free-electron lasers, and also at laboratory-based sources of vacuum-ultraviolet to soft X-ray sources, such as high-harmonic generation (HG) sources (Faubel et al., 2012[link]; Ojeda et al., 2016[link]).

3. Applications

3.1. Chemistry

The ability to read off spectra that yield information about photo-induced changes in metal complexes, such as oxidation shifts, spin transitions, structural changes etc., at the metal centre was a major incentive to implement time-resolved XAS. As a matter of fact, the first demonstration of pico­second XAS was on a ruthenium complex that undergoes a change in oxidation state and in structure upon photoexcitation (Saes et al., 2003[link]; Gawelda et al., 2006[link]). Several studies followed (and continue to do so) on a large diversity of transition-metal complexes (Chen, 2001[link], 2004[link], 2005[link]; Bressler & Chergui, 2004[link]; Chergui & Zewail, 2009[link]; Chen et al., 2010[link]; Bressler & Chergui, 2010[link]; Chergui, 2010[link]; Milne et al., 2014[link]; Chergui & Collet, 2017[link]). A good example of the type of information that can be extracted is the case of halogenated rhenium–carbonyl–bipyridine complexes, where an open question remained concerning the nature of the photo-induced intra­molecular charge-transfer excitation. Indeed, given the mixed character of the metal–halogen moiety, it was unclear whether the charge transfer occurs from the metal alone or from the metal and halogen atoms. This type of question is unambiguously addressed by time-resolved XANES by examining the pre-edge and edge features (Fig. 2[link]).

[Figure 2]

Figure 2

Left: ground-state Re L3 edge (blue) of [ReBr(CO)3(bpy)] in solution and transient XAS (red) recorded 630 ps after excitation at 355 nm. The inset clearly shows a first-derivative shape reflecting a blue shift of the edge, along with an increase in the intensity of the A and B features. Both of these observations reflect a loss of electronic charge by the Re atom. Right: the same is observed at the Br K edge, where the transient XAS (black) shows the appearance of a pre-edge feature, while the edge shows a first-derivative shape with respect to the ground-state spectrum (red). Reprinted with permission from El Nahhas et al. (2013[link]). Copyright 2013 American Chemical Society.

These studies were first extended to the femtosecond time domain using the slicing scheme at synchrotrons (Bressler et al., 2009[link]), but are now commonly carried out at X-ray free-electron lasers (Chergui & Collet, 2017[link]). In addition, the latter instruments have allowed the implementation of femtosecond X-ray emission and femtosecond resonant inelastic X-ray scattering (Zhang et al., 2014[link]; Wernet et al., 2015[link]).

3.2. Biology

As mentioned above, the first ever time-resolved XAS experiment was attempted on myoglobin with microsecond resolution (Mills, Lewis et al., 1984[link]). The first picosecond study was carried out by the Chergui group on myoglobin–NO, providing insight into the recombination kinetics of the NO ligand (Silatani et al., 2015[link]). Soon after, XAS experiments at XFELs took off and the first femtosecond XANES spectra were reported, probing the structural dynamics of vitamin B12 using polarized cobalt K-edge absorption (Miller et al., 2017[link], 2020[link]) and of ferrous (Mara et al., 2017[link]) and ferric (Bacellar et al., 2020[link]) cytochrome c and myoglobin–NO (Bacellar et al., 2021[link]).

3.3. Materials science

Time-resolved XAS was initially used with a resolution of tens of pico­seconds in the soft X-ray regime to probe the melting of solid materials (Johnson et al., 2003[link]). These experiments probed both the electronic and structural changes upon melting. More recently, electron trapping in photoexcited bare and sensitized transition-metal oxides (TMOs) has been probed by metal K-edge XANES studies with 70 ps resolution (Katz et al., 2010[link], 2012[link]; Rittmann-Frank et al., 2014[link]). TMOs are very important materials in solar energy conversion processes, both photovoltaics and photocatalysis, and the issue of identifying the nature of traps is essential, especially when it comes to operating conditions, in particular room temperature.

Fig. 3[link] shows the static Ti K-edge absorption spectra of anatase and amorphous titanium dioxide (TiO2) nanoparticles (NPs) and the transient XAS obtained upon photoexcitation of the anatase NPs above the band gap at 355 nm. The changes therein are rather well reproduced by obtaining the difference between the red-shifted (1 eV) amorphous steady-state spectrum and the anatase spectrum, yielding the red dashed trace. This implies that the electrons delivered to the conduction band become trapped at titanium centres, which they convert to Ti3+, and these centres are located at defects. In Rittmann-Frank et al. (2014[link]) and Santomauro et al. (2015[link]), these were identified as pentacoordinated defects. Indeed, anatase NPs are known to have an ordered core and a rather disordered shell, and the transients point to the electrons becoming trapped in the shell region.

[Figure 3]

Figure 3

(a) Edge region of the Ti K-edge steady-state spectra of anatase and amorphous TiO2 nanoparticles (pre-edge peaks A1, A2, A3 and B, and X-ray absorption near-edge structure). (b) Transient Ti K-edge absorption spectra of bare anatase TiO2 nanoparticles excited at 355 nm at a time delay of 100 ps (black) and the calculated difference spectrum (red dashed line) of the amorphous steady-state spectrum [green in (a)] shifted by −1 eV and the anatase-sample spectrum. Copyright 2014 Wiley. Reprinted with permission from Rittmann-Frank et al. (2014[link]).

The trapping time of the electron was measured by femtosecond XAS using the slicing scheme (Santomauro et al., 2015[link]) and was found to occur in <200 fs, which means that the electrons are trapped in the vicinity of where they are created. These experiments were recently repeated at an XFEL (Obara et al., 2017[link]), which thanks to the increased S/N revealed additional effects: namely, the shift of the edge occurs three times faster (∼100 fs) than the changes in the pre-edge and post-edge regions (∼330 fs). This shows that while the reduction of Ti4+ centres to Ti3+ at defects is within the time resolution of the experiment, the structural signatures (pre-edge and post-edge) take a longer time as the environment requires a finite time to structurally adapt to the change in electronic structure. The above studies mostly probed the trapping of electrons at defects (Katz et al., 2010[link], 2012[link]; Rittmann-Frank et al., 2014[link]; Obara et al., 2017[link]). More recently, the fate of charge carriers was investigated in inorganic CsPbBr3 perovskites by monitoring the response of each atom in these materials, leading to the conclusion that the holes are localized at Br atoms (Santomauro et al., 2017[link]; Cannelli et al., 2021[link]).

In the soft X-ray range, the femtosecond slicing scheme has been used to probe the photo-induced insulator-to-metal transition in VO2 by V L-edge and O K-edge spectroscopy (Cavalleri et al., 2004[link]). Subsequently, the ultrafast laser-induced demagnetization of ferromagnetic films was also probed at the Ni L edge (Boeglin et al., 2010[link]).

The above has outlined some examples of the exceptional capabilities of picosecond/femtosecond X-ray absorption spectroscopy. We have mostly mentioned X-ray absorption spectroscopy, but with the advent of picosecond/femtosecond X-ray emission spectroscopy (XES), another level of insight is gained into the spin and electronic structure of intermediate species (Zhang et al., 2014[link]; Wernet et al., 2015[link]; Kinschel et al., 2020[link]).

Note: This chapter was originally written in 2017 and was subsequently updated by the inclusion of a number of new references in 2021.

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