Monitoring Ultrafast Chemical Dynamics by Time-Domain X-ray Photo

Article pubs.acs.org/accounts

Monitoring Ultrafast Chemical Dynamics by Time-Domain X‑ray Photo- and Auger-Electron Spectroscopy Published as part of the Accounts of Chemical Research focus issue “Understanding Heterogeneous Chemical Processes Using X-ray Techniques”. Oliver Gessner*,† and Markus Gühr‡,§ †

Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States PULSE Institute, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States § Institut für Physik und Astronomie, Universität Potsdam, 14476 Potsdam, Germany ‡

CONSPECTUS: The directed flow of charge and energy is at the heart of all chemical processes. Extraordinary efforts are underway to monitor and understand the concerted motion of electrons and nuclei with ever increasing spatial and temporal sensitivity. The element specificity, chemical sensitivity, and temporal resolution of ultrafast X-ray spectroscopy techniques hold great promise to provide new insight into the fundamental interactions underlying chemical dynamics in systems ranging from isolated molecules to application-like devices. Here, we focus on the potential of ultrafast X-ray spectroscopy techniques based on the detection of photo- and Auger electrons to provide new fundamental insight into photochemical processes of systems with various degrees of complexity. Isolated nucleobases provide an excellent testing ground for our most fundamental understanding of intramolecular coupling between electrons and nuclei beyond the traditionally applied Born−Oppenheimer approximation. Ultrafast electronic relaxation dynamics enabled by the breakdown of this approximation is the major component of the nucleobase photoprotection mechanisms. Transient X-ray induced Auger electron spectroscopy on photoexcited thymine molecules provides atomic-site specific details of the extremely efficient coupling that converts potentially bond changing ultraviolet photon energy into benign heat. In particular, the time-dependent spectral shift of a specific Auger band is sensitive to the length of a single bond within the molecule. The X-ray induced Auger transients show evidence for an electronic transition out of the initially excited state within only ∼200 fs in contrast to theoretically predicted picosecond population trapping behind a reaction barrier. Photoinduced charge transfer dynamics between transition metal complexes and semiconductor nanostructures are of central importance for many emerging energy and climate relevant technologies. Numerous demonstrations of photovoltaic and photocatalytic activity have been performed based on the combination of strong light absorption in dye molecules with charge separation and transport in adjacent semiconductor nanostructures. However, a fundamental understanding of the enabling and limiting dynamics on critical atomic length- and time scales is often still lacking. Femtosecond time-resolved X-ray photoelectron spectroscopy is employed to gain a better understanding of a short-lived intermediate that may be linked to the unexpectedly limited performance of ZnO based dye-sensitized solar cells by delaying the generation of free charge carriers. The transient spectra strongly suggest that photoexcited dye molecules attached to ZnO nanocrystals inject their charges into the substrate within less than 1 ps but the electrons are then temporarily trapped at the surface of the semiconductor in direct vicinity of the injecting molecules. The experiments are extended to monitor the electronic response of the semiconductor substrate to the collective injection from a monolayer of dye molecules and the subsequent electron−ion recombination dynamics. The results indicate some qualitative similarities but quantitative differences between the recombination dynamics at moleculesemiconductor interfaces and previously studied bulk-surface electron−hole recombination dynamics in photoexcited semiconductors.

1. INTRODUCTION

showcase examples to illustrate opportunities and challenges associated with X-ray chemical dynamics studies. In the first case, we elaborate on an isolated molecular system undergoing pure intramolecular relaxation. The second case discusses a molecular system coupled to a semiconductor surface. Both

Chemistry is driven by the directed flow of charge and energy on intra- and intermolecular scales. The element specificity, chemical sensitivity, and temporal resolution of emerging ultrafast X-ray spectroscopy techniques hold great promise to provide new insight into the fundamental interactions underlying chemical dynamics in systems ranging from single molecules to application-like devices. Here, we discuss two © XXXX American Chemical Society

Received: August 4, 2015

A

DOI: 10.1021/acs.accounts.5b00361 Acc. Chem. Res. XXXX, XXX, XXX−XXX

Article

Accounts of Chemical Research

Figure 1. UV excitation and molecular relaxation scheme in (a) a potential energy representation and (b) occupation of orbitals. (a) Radiationless electronic relaxation pathways from the photoexcited ππ* state into to the nπ* and the ground state are opened by crossings (conical intersections). Dynamics simulations suggest that the initially populated Franck−Condon region is separated from the crossing region by a barrier. (b) UV pump− soft X-ray probe scheme, showing a resonant X-ray probe as in absorption spectroscopy or a nonresonant probe as in photoelectron or Augerelectron spectroscopy.

2. SMALL SYSTEMS: PHOTOPROTECTION OF NUCLEOBASES A rich body of theoretical simulations on isolated nucleobases identifies the breakdown of the Born−Oppenheimer approximation at conical intersections as the crucial process for the relaxation to lower lying electronic states that ultimately dissipates the absorbed photon energy into heat.11−18 Previous time-resolved ion19−21 and photoelectron22 measurements in the isolated nucleobase thymine determined time constants in the femtosecond and picosecond range. The assignment to specific nuclear or electronic relaxation channels, however, is not feasible purely on experimental grounds. Although transient absorption experiments in the UV to IR range could contribute to our understanding of the dynamics from a different point of view, the required optical densities in an absorption cell are beyond practical limits for these molecules. Figure 1a shows a simplified sketch of the ππ* state UV excitation and the important potential energy surfaces of thymine with a corresponding orbital scheme in Figure 1b. A full dynamics simulation predicts an initial ultrafast nuclear relaxation into a ππ* state minimum along a Franck−Condon active bond alternation mode with a strong C−O stretch component.17 The simulations predict a reaction barrier in the ππ* state that traps the population for several picoseconds on the way to the conical intersections with the nπ* and the ground state. In this context, the experimentally determined picosecond decay would be interpreted as the limiting time scale in electronic relaxation. Other simulations based on linear interpolation14 or minimal energy paths13 predict a barrierless and thus very fast decay from the Franck−Condon region to the electronic ground state implying that experimentally determined femtosecond decays are due to electronic relaxation. We approach this controversy by adding information through element selective X-ray probes. In contrast to optical pulses, X-ray pulses probe the electronic structure at particular elements or sites inside the molecule. From this local information one can deduce a more detailed picture of the important nuclear motion and electronic changes. Element specificity relies on large differences in the core electron binding energies, which are on the order of 100 eV when comparing carbon, nitrogen, and oxygen. Site specificity for atoms of the same element is based on much smaller core binding energy shifts in sites featuring different binding partners. These shifts become visible as a splitting of near edge absorption or photoelectron emission features on a single to few eV scale. Additional spatial information as

experiments employ electron detection, taking advantage of the high surface sensitivity of photoelectron spectroscopy and bond length sensitivity in Auger spectroscopy. In both cases, the atomic scale perspective provided by core-level transitions is critical to provide novel insight into intra- and intermolecular dynamics. One of the most fundamental building blocks of life, nucleobases are remarkably resilient to photodamage despite their strong UV absorption. The absorbed energy is mostly dissipated into heat mediated by ultrafast electronic relaxation out of the initially photoexcited state into lower lying electronic states.1−4 While the determination of the time scales for intramolecular relaxation (∼fs to ps) has been remarkably successful using ultrafast spectroscopy techniques with ultraviolet probe pulses, a description of the underlying physics by a particular model is often underdetermined by the measurement. We discuss the additional information from atomic sitespecific probing of unimolecular relaxation dynamics in UVexcited thymine gained by femtosecond time-resolved Auger electron spectroscopy. In particular, we demonstrate how the results provide strong evidence against a previously proposed potential energy barrier and a clearer picture of the associations between observed time scales and underlying molecular dynamics. Many emerging energy and climate relevant technologies are based on hybrid materials that contain multiple interfaces between molecules, semiconductors (SCs), and liquid electrolytes. In particular, new concepts for photovoltaic (PV) and photoelectrochemical (PEC) applications often rely on heterogeneous designs with distinct sites for efficient photon absorption, charge generation, and/or chemical function.5−10 The complexity of the heterostructures poses a tremendous challenge for a fundamental understanding of the electronic and chemical dynamics that underlie the very concepts of their design strategies. Using the showcase example of a film of dyesensitized semiconductor nanocrystals, we demonstrate how ultrafast X-ray photoelectron spectroscopy may be employed to characterize a short-lived intermediate state that plays a major role in delaying the free charge carrier generation in ZnO based dye-sensitized PV cells. The method is also used to simultaneously monitor the response of the semiconductor band structure to the interfacial charge transfer dynamics. B


Article


difference spectrum at short times is given in blue in Figure 2c. The spectrum reflects a UV-induced shift of the entire ground state Auger band toward higher Auger kinetic energies. The immediate Auger shift with UV excitation reflects the creation of a vibrational wave packet in the ππ* excited state. The blueshift of the Auger spectrum is closely linked to the nuclear dynamics of the molecule. Simulations and intuitive electron density arguments based on the π and π* orbital shapes predict that the C(4)−O(8) bond (which is the bond close to the methyl group) stretches in the ππ* state after UV excitation.17,18 The simulated spectra in Figure 2b contain many Auger channels, which are broadened by their lifetimes. The energy of each channel is calculated by a configuration interaction singles (CIS) approach while the rates have been calculated evaluating the Auger matrix elements with the final (dicationic) state being represented by an electron-molecule scattering wave function obtained using the complex-Kohn method.26 The simulated Auger spectrum of the ππ* state is based on a nuclear geometry after initial nuclear relaxation out of the Franck−Condon region with an extended C(4)−O(8) bond (blue in Figure 2b). It exhibits a clear shift toward higher kinetic energies relative to the ground state spectrum (black in Figure 2b). Intuitive energy conservation arguments involving the characteristic element and local site sensitivity of X-ray transitions explain the trend. After Auger decay of the localized O 1s hole, the molecule misses two valence electrons around the initially ionized oxygen, reducing the screening between oxygen and the nearest carbon atom. Thus, a strong Coulomb repulsion acts between C and O, which stores molecular energy on the cost of Auger kinetic energy. If, however, the C−O distance is increased, less energy is stored in Coulomb repulsion and instead may be released as a higher kinetic energy of the Auger electron. The high local sensitivity of the 1s Auger decay therefore allows to identify dynamics at individual bonds. The fast C−O nuclear relaxation occurs immediately after UV excitation, and the experimental time resolution is insufficient to resolve the gradual shift. About 200 fs after photoexcitation, the difference spectrum changes shape. The shift to higher kinetic energies has turned into a shift to lower kinetic energies (red in Figure 2c). Simulations predict that the molecule is trapped in a minimum of the ππ* state for some picoseconds due to a reaction barrier in the path to reach the conical intersection.17,18 The shift to higher kinetic energies would persist in this minimum. Thus, these simulations predict a blueshifted spectrum for the next couple of picoseconds after excitation. The experimentally observed decay of the blue-shifted channel with 200 fs time constant, however, indicates that the minimum configuration only lives for a short time (≤200 fs). Thus, we find that the majority of the photoexcited ππ* population is not blocked by a barrier and has quick access to the conical intersection.

accessed, for example, in scanning X-ray microscopy techniques, is not sought after in the presented studies. The measurements below average over an ensemble of molecules; however, each molecule can be considered as fairly well isolated. Probing by UV-ionization of molecular valence orbitals is generally not site selective. The electronic wave functions of core electrons, on the other hand, are extremely well localized. Their high binding energies of several 100 eV are indicative of electronic wave functions that are confined to a very narrow range of the Coulomb potential of a particular atom. Any transition involving core electrons is then necessarily also confined in space. We conducted an experiment using Auger probing of molecular dynamics, which applies the element selectivity of X-rays in a more subtle way than X-ray absorption or photoelectron spectroscopy. Figure 1b shows a sketch of the Auger probing principle. A nonresonant X-ray pulse is used to create a 1s core hole on the molecule, which decays with participation from two valence electrons.23,24 As a confined core hole is involved, the valence transitions are necessarily localized at the core site. The oxygen Auger decay in thymine delivers a several 10 eV wide, relatively unstructured spectrum with the strongest Auger yield around 500 eV. The Auger electron kinetic energy is independent of the photon energy, thus the method is ideally suited for studies at X-ray Free Electron Lasers (X-FELs) based on self-amplified stimulated emission (SASE), which exhibit large spectral fluctuations.25 The experiment was performed at the Linac Coherent Light Source (LCLS). A femtosecond UV (266 nm) pulse excited the evaporated molecules, a delayed soft X-ray pulse (∼570 eV) probed the molecular dynamics by Auger emission. Figure 2a shows the experimental data of the UV pump−Xray probe scan spanning ∼3 ps. The subtraction of unexcited

Figure 2. (a) Time-resolved difference Auger spectrum (excited minus nonexcited Auger spectra). The false color code indicates UV enhanced Auger decay (red) and UV reduced Auger decay (blue). (b) Calculations based on a configuration interaction singles (CIS) model26 indicate a shift of the Auger spectrum toward higher kinetic energies when the molecule is excited to the ππ* state. (c) Cuts through the experimental difference spectrum at early times replicate this trend. Around 200 fs, the shift toward higher kinetic energies reverses into a shift to lower kinetic energies. The simulated Auger spectra have some shortcomings, as a comparison to the measured ground state spectrum indicates.26 This explains the differences between theoretically expected and measured difference spectra at a delay of 75 fs. Reproduced from ref 26.

3. ATOMIC SCALE VIEW OF INTERFACIAL CHARGE TRANSFER Ultrafast X-ray techniques may also be used to study dynamics in much more complex systems. Several groups have performed pioneering time-resolved X-ray absorption spectroscopy studies to identify short-lived intermediates in systems such as solvated molecules27−32 and dye-sensitized SC nanocrystals.33,34 Here, we concentrate on a recent series of time-resolved X-ray photoelectron spectroscopy (tr-XPS) studies of interfacial charge transfer (CT) dynamics in building blocks of emerging

spectra from UV excited spectra in Figure 2a reveals the UV induced changes. One can clearly identify a signal decrease (blue) in the kinetic energy region around 500 eV that lasts for the full delay range. Around 509 eV kinetic energy, we identify a short-lived positive feature (red). A more detailed view of the C


Article

Accounts of Chemical Research PV and PEC technologies.5,6 In particular, we demonstrate how tr-XPS may be employed to gain a more complete picture of crucial intermediates during charge injection from molecular dyes into films of SC nanocrystals and of the electronic response of the SC substrate. Figure 3c,d illustrates the spectroscopic insight provided by XPS for a prototypical interface of N3 dye molecules

as indicated. The chemical sensitivity of XPS becomes particularly apparent in the magnified section shown in Figure 3d. The region contains inner-shell photolines from the Ru metal center and ligand carbon atoms as indicated by the colorcoded photoemission lines and the corresponding atomic sites in Figure 3b.36,37 This sensitivity of inner-shell binding energies to local valence electronic structures makes tr-XPS a promising tool to monitor electronic and chemical dynamics from the perspective of well-defined atomic sites within complex interfacial configurations.38,39 Figure 4 illustrates a recent example of a femtosecond tr-XPS experiment performed at the LCLS.40 The experiment aimed at providing a better defined picture of the short-lived intermediate that is associated with an approximately 1000 times longer appearance time of free charge carriers in ZnO based PVs compared to TiO2 based cells (∼100 ps compared to ∼50 fs).41,42 This delay is widely regarded as a possible root cause for the limited performance of ZnO based cells since it may increase the yield of parasitic relaxation channels that do not produce an electric current. Two models have been proposed to explain the observed injection delay, each of which is associated with a different intermediate electronic configuration. In the two-state injection model it is assumed that electronic coupling between the dye molecule and the SC substrate is not efficient enough to compete with intramolecular relaxation from the initially excited, singlet metal-toligand charge-transfer state (1MLCT) to its triplet counterpart (3MLCT), leading to a temporary trapping of the electron on the molecule.43,44 In the intermediate interfacial state model, the electron leaves the dye very fast but then gets trapped in an interfacial complex (IC).41,45,46 The implications of both scenarios for improved interface design strategies are significantly different but determining the electron−hole separation length scale that is representative of the short-lived intermediate electronic configuration has been challenging. In the LCLS experiment, a film of N3-sensitized, sintered ZnO nanocrystals was excited by a ≤ 50 fs long optical (535 nm) pulse and the charge transfer dynamics were monitored by a delayed ≤100 fs long X-ray pulse (850 eV). Two spectra are

Figure 3. (a) Schematic of dye-sensitized PV cell design. Reproduced with permission from ref 6. Copyright 2005 American Chemical Society. (b) Illustration of N3 dye adsorbed on a ZnO substrate with distinct color codes for atoms in different chemical environments. (c) XPS spectrum of an N3-sensitized film of fused ZnO nanocrystals. (d) Magnified view of the C 1s/Ru 3d section of (c) with spectral contributions marked by color codes according to (b). Adapted from ref 35. Copyright 2015, with permission from Elsevier.

(bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II), Figure 3b) adsorbed on a sintered film of ZnO nanocrystals.35 Each of the annotated peaks in Figure 3c corresponds to an inner-shell photoline of a different element

Figure 4. Identifying a short-lived intermediate during charge-injection at a dye-SC interface. (a) Transient XPS spectra of an N3-sensitized film of ZnO nanocrystals before (blue dashed) and 500 fs after (red solid) optical excitation. (b) Difference of spectra in (a) due to a transient chemical shift of the Ru 3d doublet. Possible intermediates are shown in (c), corresponding predicted chemical shifts (vertical colored bars) in (d). The pink and purple orbitals in (c) correspond to electron densities, the red and green orbitals to hole densities. The gray bar in (d) represents the experimental result. Adapted with permission from ref 40. Copyright 2014 American Chemical Society). D


Article


4. COUPLED ELECTRONIC DYNAMICS OF ADSORBATE MOLECULES AND SEMICONDUCTOR SUBSTRATES An important next step in the study of molecule-SC CT dynamics is the simultaneous probing of the coupled electronic dynamics on both sides of the interface with site-specificity. The electronic structure of a SC near its surface is usually marked by an upward or downward bending of the SC electronic bands toward the interface, which is a consequence of a local charge imbalance (Figure 5a).35,48−50 The extent and

shown in Figure 4a. The blue dashed spectrum was recorded before the pump pulse reached the sample while the solid red spectrum corresponds to X-rays delayed by 500 fs relative to the optical pulse. At this delay the initially excited 1MLCT state has already relaxed but no free charge carriers are detected in the SC substrate.41,42 A systematic trend is observed in the difference between the two spectra as shown in Figure 4b. The optically induced spectral change (circles) can be well described by a two-parameter fit (dashed green line) that assigns the observed effect to a transient shift of the Ru 3d doublet to higher binding energies. The two free fit parameters are the fraction of molecules excited by the pump pulse ((4 ± 1) %) and the extent of the transient chemical shift ((2.3 ± 0.2) eV). The observed chemical shift can be qualitatively understood based on electronic orbital considerations. The highest occupied molecular orbital (HOMO) of the dye is predominantly concentrated on the Ru center and the NCS groups.28,40 Removing an electron from this orbital by optical excitation will lead to a reduced screening of the Ru core holes upon X-ray photoemission and, therefore, a blueshift of the corresponding inner-shell binding energies. A quantitative interpretation of the experiments is facilitated by ab initio calculations employing constrained density functional theory (CDFT) as illustrated in Figure 4c,d. The dye/SC interface is described by an N3 dye molecule chemisorbed on a nonpolar ZnO (101̅0) surface, which is modeled by a 150 atom cluster of ZnO. The ranges of expected chemical shifts are estimated by employing three different functionals (BLYP, B3LYP, CAM-B3LYP) with differing degrees of Hartree−Fock exchange, corresponding to different levels of core-hole screening by the surrounding valence charges and, consequently, different core-level shifts. The vertical red, green, and blue bars in Figure 4d represent the predicted ranges of chemical shifts for the three electronic configurations in Figure 4c. From left to right, these are a 3 MLCT state where the excited electron is retained on the dye, an IC where the electron is trapped at the SC surface next to the dye, and an ionized dye where the electron has left the immediate vicinity of the interface. As intuitively expected, the predicted chemical shift increases with increasing electron− hole distance (left to right) due to a concomitant reduction in core hole screening. Despite the uncertainty in the theoretically predicted chemical shifts (vertical bars), the experimentally determined value (gray bar) indicates that 500 fs after optical excitation, the electron has already left the dye molecule but is still located in its immediate vicinity, strongly supporting the model that during the period of delayed charge injection into the SC substrate the electron is temporarily trapped in an interfacial complex. A second measurement at 1 ps pump− probe delay confirms this conclusion.40 We note that more experimental and theoretical work is required to determine the exact character of the transient IC. Its current modeling predominantly establishes an average electron−hole distance that is consistent with the experimentally determined chemical shift of the Ru 3d doublet. Future high repetition rate X-FELs will enable significantly increased data rates and the coverage of much larger series of pump−probe delays within a single experiment. In particular, the space charge limited usable X-ray pulse energies can then be compensated by much higher pulse repetition rates.47

Figure 5. (a) Band bending and transient band flattening at an N3/ ZnO interface due to photoinduced charge injection. (b,c) SPV response of a ZnO substrate due to interfacial CT dynamics. The green crosses in (c) are the result of a control experiment with a bare nanoporous ZnO film, demonstrating that the observed transient SPV in the dye-sensitized film is indeed the result of interfacial CT. Adapted from ref 35. Copyright 2015, with permission from Elsevier.

dynamics of electronic band bending may significantly affect CT and chemical activity at an interface, in particular, due to the potential gradient’s impact on local charge carrier densities.49 Figure 5b,c illustrates how the transient interfacial band structure of a sintered film of N3-sensitized ZnO nanocrystals may be monitored by tr-XPS. The experiment was performed at the Advanced Light Source (ALS) using optical (532 nm, 10 ps) pump pulses to induce the interfacial CT and X-ray (850 eV, 70 ps) probe pulses to monitor the interfacial band bending.35 A novel time-stamping technique was employed to record the tr-XPS spectra. It makes use of the entire ALS bunch train without the need to gate on specific optical/X-ray pump− probe delays.35 As shown in Figure 5b (30 ps nominal delay), the entire XPS spectrum is subject to a rigid shift of several hundred meV to higher binding energies, a clear signature of a transient surface photovoltage (SPV) resulting from interfacial band flattening (Figure 5a).50,51 The temporal evolution of the SPV (Figure 5c, blue crosses), which reflects the electron−hole recombination dynamics across the dye−SC interface, exhibits at least two different characteristic time scales. Biphasic SPV relaxation dynamics have been observed before for direct, above bandgap excited SCs and described within a parametrized thermionic emission model.52−54 It is noteworthy that the heterogeneous recombination dynamics that involve CT across the dye−SC interface can be described by the same functional E


Article

Accounts of Chemical Research form but the corresponding model parameters are notably different, which may reflect the differences in the underlying physics.35 Future studies will take full advantage of the site-specificity of core-level transitions and monitor SPV dynamics from different sites of the molecule−SC interface. This may reveal, for example, transient dipole layers that could play an important role for interfacial CT. First-principles descriptions of CTinduced band bending dynamics at molecule/SC interfaces are barely within reach.55−57 Given the ubiquity of their anticipated applications in energy and climate relevant technologies, a concerted effort on this front would be highly desirable in order to enable a more predictive understanding driven support for future design strategies. We emphasize that studies exploring both ultrafast (X-FEL) and slower (synchrotron) temporal domains are required to gain a comprehensive understanding of interfacial charge transfer dynamics in complex systems.

Markus Gühr is on the faculty of the physics and astronomy department at the University of Potsdam, Germany, and a former senior staff scientist at SLAC National Accelerator Laboratory. He is interested in molecule−light interactions and fundamentals of photon energy conversion. He performs ultrafast experiments probing with high photon energies from the extreme ultraviolet to the soft X-ray using high harmonic and free electron laser sources.

■

ACKNOWLEDGMENTS O.G. and M.G. were supported by the Department of Energy Office of Science Early Career Research Program. Portions of this research were carried out at the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory. LCLS is an Office of Science User Facility operated for the U.S. Department of Energy Office of Science by Stanford University. Experiments were also performed at beamline 11.0.2 of the Advanced Light Source (ALS). ALS is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC0205CH11231.

5. CONCLUSION The field of ultrafast X-ray chemical dynamics studies evolves at an astounding pace, which is strongly supported by significant investments into new generations of accelerator-based X-ray light sources. Among the different techniques, time-resolved Xray photo- and Auger-electron spectroscopy have so far played a less prominent role than, for example, transient X-ray absorption. However, high detection efficiency, surface specificity, and sensitivity to both intramolecular and macroscopic electronic dynamics make photoemission and Auger detection based experiments an important component of the ultrafast X-ray toolbox. The examples discussed herein provide a first impression of emerging opportunities. The next generation of pulsed X-ray light sources such as diffractionlimited storage rings and high repetition rate X-ray Free Electron Lasers (X-FELs) will enable particularly significant advances for photoemission experiments as their operating parameters will be much better geared toward efficient, highrepetition rate pump−probe photo- and Auger-electron spectroscopy studies than currently available light sources. With increased spectral stability, time-resolved X-ray absorption spectroscopy and resonant Auger spectroscopy in isolated molecules will become feasible. While the interpretation of transient Auger spectroscopy for larger systems will always rely on excited state Auger calculations because of spectral congestion, the changes in a clearly resolved absorption spectrum might be interpreted more intuitively. Some significant challenges, however, will also have to be addressed to take full advantage of the new light sources. For condensed phase samples, controlling space charge effects as well as sample damage will require particular attention and further technical developments.

■

■

REFERENCES

(1) Crespo-Hernández, C. E.; Cohen, B.; Hare, P. M.; Kohler, B. Ultrafast Excited-State Dynamics in Nucleic Acids. Chem. Rev. 2004, 104, 1977−2020. (2) Kleinermanns, K.; Nachtigallová, D.; de Vries, M. S. Excited state dynamics of DNA bases. Int. Rev. Phys. Chem. 2013, 32, 308−342. (3) Middleton, C. T.; de La Harpe, K.; Su, C.; Law, Y. K.; CrespoHernández, C. E.; Kohler, B. DNA Excited-State Dynamics: From Single Bases to the Double Helix. Annu. Rev. Phys. Chem. 2009, 60, 217−239. (4) Schreier, W. J.; Schrader, T. E.; Koller, F. O.; Gilch, P.; CrespoHernandez, C. E.; Swaminathan, V. N.; Carell, T.; Zinth, W.; Kohler, B. Thymine Dimerization in DNA Is an Ultrafast Photoreaction. Science 2007, 315, 625−629. (5) O’Regan, B.; Grätzel, M. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 1991, 353, 737− 740. (6) Grätzel, M. Solar Energy Conversion by Dye-Sensitized Photovoltaic Cells. Inorg. Chem. 2005, 44, 6841−6851. (7) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Photoinduced Electron Transfer from a Conducting Polymer to Buckminsterfullerene. Science 1992, 258, 1474−1476. (8) Bardeen, C. Exciton Quenching and Migration in Single Conjugated Polymers. Science 2011, 331, 544−545. (9) Sivula, K.; Le Formal, F.; Grätzel, M. Solar Water Splitting: Progress Using Hematite (α-Fe2O3) Photoelectrodes. ChemSusChem 2011, 4, 432−449. (10) Lin, Y.; Yuan, G.; Sheehan, S.; Zhou, S.; Wang, D. Hematitebased solar water splitting: challenges and opportunities. Energy Environ. Sci. 2011, 4, 4862−4869. (11) Asturiol, D.; Lasorne, B.; Worth, G. A.; Robb, M. A.; Blancafort, L. Exploring the sloped-to-peaked S2/S1 seam of intersection of thymine with electronic structure and direct quantum dynamics calculations. Phys. Chem. Chem. Phys. 2010, 12, 4949−4958. (12) Asturiol, D.; Lasorne, B.; Robb, M. A.; Blancafort, L. Photophysics of the π,π* and n,π* States of Thymine: MS-CASPT2 minimum-Energy Paths and CASSCF on-the-Fly Dynamics. J. Phys. Chem. A 2009, 113, 10211−10218. (13) Merchán, M.; González-Luque, R.; Climent, T.; Serrano-Andrés, L.; Rodríguez, E.; Reguero, M.; Peláez, D. Unified Model for the Ultrafast Decay of Pyrimidine Nucleobases. J. Phys. Chem. B 2006, 110, 26471−26476. (14) Perun, S.; Sobolewski, A. L.; Domcke, W. Conical Intersections in Thymine. J. Phys. Chem. A 2006, 110, 13238−13244.

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest. Biographies Oliver Gessner is a senior scientist in the Chemical Sciences Division of Lawrence Berkeley National Laboratory. His research concentrates on dynamics in molecules, clusters, and interfacial systems, which he investigates with a variety of ultrafast XUV and X-ray spectroscopy and imaging techniques. F


Article

Accounts of Chemical Research (15) Perun, S.; Sobolewski, A. L.; Domcke, W. Photostability of 9Hadenine: mechanisms of the radiationless deactivation of the lowest excited singlet states. Chem. Phys. 2005, 313, 107−112. (16) Perun, S.; Sobolewski, A. L.; Domcke, W. Ab Initio Studies on the Radiationless Decay Mechanisms of the Lowest Excited Singlet States of 9H-Adenine. J. Am. Chem. Soc. 2005, 127, 6257−6265. (17) Hudock, H. R.; Levine, B. G.; Thompson, A. L.; Satzger, H.; Townsend, D.; Gador, N.; Ullrich, S.; Stolow, A.; Martínez, T. J. Ab Initio Molecular Dynamics and Time-Resolved Photoelectron Spectroscopy of Electronically Excited Uracil and Thymine. J. Phys. Chem. A 2007, 111, 8500−8508. (18) Szymczak, J. J.; Barbatti, M.; Soo Hoo, J. T.; Adkins, J. A.; Windus, T. L.; Nachtigallová, D.; Lischka, H. Photodynamics Simulations of Thymine: Relaxation into the First Excited Singlet State. J. Phys. Chem. A 2009, 113, 12686−12693. (19) Canuel, C.; Mons, M.; Piuzzi, F.; Tardivel, B.; Dimicoli, I.; Elhanine, M. Excited states dynamics of DNA and RNA bases: Characterization of a stepwise deactivation pathway in the gas phase. J. Chem. Phys. 2005, 122, 074316. (20) Kang, H.; Lee, K. T.; Jung, B.; Ko, Y. J.; Kim, S. K. Intrinsic Lifetimes of the Excited State of DNA and RNA Bases. J. Am. Chem. Soc. 2002, 124, 12958−12959. (21) Matsika, S. Radiationless Decay of Excited States of Uracil through Conical Intersections. J. Phys. Chem. A 2004, 108, 7584−7590. (22) Ullrich, S.; Schultz, T.; Zgierski, M. Z.; Stolow, A. Electronic relaxation dynamics in DNA and RNA bases studied by time-resolved photoelectron spectroscopy. Phys. Chem. Chem. Phys. 2004, 6, 2796− 2801. (23) Burhop, E. H. S. The Auger Effect and Other Radiationless Transitions; R. E. Krieger Publishing Co.: Huntington, NY, 1980. (24) Siegbahn, H.; Asplund, L.; Kelfve, P. The Auger electron spectrum of water vapour. Chem. Phys. Lett. 1975, 35, 330−335. (25) Emma, P.; Bane, K.; Cornacchia, M.; Huang, Z.; Schlarb, H.; Stupakov, G.; Walz, D. Femtosecond and Subfemtosecond X-Ray Pulses from a Self-Amplified Spontaneous-Emission−Based FreeElectron Laser. Phys. Rev. Lett. 2004, 92, 074801. (26) McFarland, B. K.; Farrell, J. P.; Miyabe, S.; Tarantelli, F.; Aguilar, A.; Berrah, N.; Bostedt, C.; Bozek, J. D.; Bucksbaum, P. H.; Castagna, J. C.; Coffee, R. N.; Cryan, J. P.; Fang, L.; Feifel, R.; Gaffney, K. J.; Glownia, J. M.; Martinez, T. J.; Mucke, M.; Murphy, B.; Natan, A.; Osipov, T.; Petrović, V. S.; Schorb, S.; Schultz, T.; Spector, L. S.; Swiggers, M.; Tenney, I.; Wang, S.; White, J. L.; White, W.; Gühr, M. Ultrafast X-ray Auger probing of photoexcited molecular dynamics. Nat. Commun. 2014, 5, 4235. (27) Huse, N.; Cho, H.; Hong, K.; Jamula, L.; de Groot, F. M. F.; Kim, T. K.; McCusker, J. K.; Schoenlein, R. W. Femtosecond Soft Xray Spectroscopy of Solvated Transition-Metal Complexes: Deciphering the Interplay of Electronic and Structural Dynamics. J. Phys. Chem. Lett. 2011, 2, 880−884. (28) Van Kuiken, B. E.; Huse, N.; Cho, H.; Strader, M. L.; Lynch, M. S.; Schoenlein, R. W.; Khalil, M. Probing the Electronic Structure of a Photoexcited Solar Cell Dye with Transient X-ray Absorption Spectroscopy. J. Phys. Chem. Lett. 2012, 3, 1695−1700. (29) Bressler, C.; Milne, C.; Pham, V. T.; ElNahhas, A.; van der Veen, R. M.; Gawelda, W.; Johnson, S.; Beaud, P.; Grolimund, D.; Kaiser, M.; Borca, C. N.; Ingold, G.; Abela, R.; Chergui, M. Femtosecond XANES Study of the Light-Induced Spin Crossover Dynamics in an Iron(II) Complex. Science 2009, 323, 489−492. (30) Chen, L. X. Probing Transient Molecular Structures In Photochemical Processes Using Laser-Initiated Time-Resolved X-Ray Absorption Spectroscopy. Annu. Rev. Phys. Chem. 2005, 56, 221−254. (31) Zhang, W.; Alonso-Mori, R.; Bergmann, U.; Bressler, C.; Chollet, M.; Galler, A.; Gawelda, W.; Hadt, R. G.; Hartsock, R. W.; Kroll, T.; Kjær, K. S.; Kubiček, K.; Lemke, H. T.; Liang, H. W.; Meyer, D. A.; Nielsen, M. M.; Purser, C.; Robinson, J. S.; Solomon, E. I.; Sun, Z.; Sokaras, D.; van Driel, T. B.; Vankó, G.; Weng, T.-C.; Zhu, D.; Gaffney, K. J. Tracking excited-state charge and spin dynamics in iron coordination complexes. Nature 2014, 509, 345−348.

(32) Wernet, P.; Kunnus, K.; Josefsson, I.; Rajkovic, I.; Quevedo, W.; Beye, M.; Schreck, S.; Grübel, S.; Scholz, M.; Nordlund, D.; Zhang, W.; Hartsock, R. W.; Schlotter, W. F.; Turner, J. J.; Kennedy, B.; Hennies, F.; de Groot, F. M. F.; Gaffney, K. J.; Techert, S.; Odelius, M.; Föhlisch, A. Orbital-specific mapping of the ligand exchange dynamics of Fe(CO)5 in solution. Nature 2015, 520, 78−81. (33) Zhang, X.; Smolentsev, G.; Guo, J.; Attenkofer, K.; Kurtz, C.; Jennings, G.; Lockard, J. V.; Stickrath, A. B.; Chen, L. X. Visualizing Interfacial Charge Transfer in Ru-Dye-Sensitized TiO2 Nanoparticles Using X-ray Transient Absorption Spectroscopy. J. Phys. Chem. Lett. 2011, 2, 628−632. (34) Rittmann-Frank, M. H.; Milne, C. J.; Rittmann, J.; Reinhard, M.; Penfold, T. J.; Chergui, M. Mapping of the Photoinduced Electron Traps in TiO2 by Picosecond X-ray Absorption Spectroscopy. Angew. Chem., Int. Ed. 2014, 53, 5858−5862. (35) Neppl, S.; Gessner, O. Time-resolved X-ray Photoelectron Spectroscopy Techniques for the Study of Interfacial Charge Dynamics. J. Electron Spectrosc. Relat. Phenom. 2015, 200, 64−77. (36) Shirley, D. A. High-Resolution X-Ray Photoemission Spectrum of the Valence Bands of Gold. Phys. Rev. B: Condens. Matter 1972, 5, 4709−4714. (37) Mayor, L. C.; Saywell, A.; Magnano, G.; Satterley, C. J.; Schnadt, J.; O’Shea, J. N. Adsorption of a Ru(II) dye complex on the Au(111) surface: Photoemission and scanning tunneling microscopy. J. Chem. Phys. 2009, 130, 164704. (38) Dachraoui, H.; Michelswirth, M.; Siffalovic, P.; Bartz, P.; Schäfer, C.; Schnatwinkel, B.; Mattay, J.; Pfeiffer, W.; Drescher, M.; Heinzmann, U. Photoinduced Reconfiguration Cycle in a Molecular Adsorbate Layer Studied by Femtosecond Inner-Shell Photoelectron Spectroscopy. Phys. Rev. Lett. 2011, 106, 107401. (39) Bauer, M.; Lei, C.; Read, K.; Tobey, R.; Gland, J.; Murnane, M. M.; Kapteyn, H. C. Direct Observation of Surface Chemistry Using Ultrafast Soft-X-Ray Pulses. Phys. Rev. Lett. 2001, 87, 025501. (40) Siefermann, K. R.; Pemmaraju, C. D.; Neppl, S.; Shavorskiy, A.; Cordones, A. A.; Vura-Weis, J.; Slaughter, D. S.; Sturm, F. P.; Weise, F.; Bluhm, H.; Strader, M. L.; Cho, H.; Lin, M.-F.; Bacellar, C.; Khurmi, C.; Guo, J.; Coslovich, G.; Robinson, J. S.; Kaindl, R. A.; Schoenlein, R. W.; Belkacem, A.; Neumark, D. M.; Leone, S. R.; Nordlund, D.; Ogasawara, H.; Krupin, O.; Turner, J. J.; Schlotter, W. F.; Holmes, M. R.; Messerschmidt, M.; Minitti, M. P.; Gul, S.; Zhang, J. Z.; Huse, N.; Prendergast, D.; Gessner, O. Atomic Scale Perspective of Ultrafast Charge Transfer at a Dye-Semiconductor Interface. J. Phys. Chem. Lett. 2014, 5, 2753−2759. (41) Němec, H.; Rochford, J.; Taratula, O.; Galoppini, E.; Kužel, P.; Polívka, T.; Yartsev, A.; Sundström, V. Influence of the ElectronCation Interaction on Electron Mobility in Dye-Sensitized ZnO and TiO2 Nanocrystals: A Study Using Ultrafast Terahertz Spectroscopy. Phys. Rev. Lett. 2010, 104, 197401. (42) Anderson, N. A.; Ai, X.; Lian, T. Electron Injection Dynamics from Ru Polypyridyl Complexes to ZnO Nanocrystalline Thin Films. J. Phys. Chem. B 2003, 107, 14414−14421. (43) Anderson, N. A.; Lian, T. Ultrafast Electron Transfer At The Molecule-Semiconductor Nanoparticle Interface. Annu. Rev. Phys. Chem. 2005, 56, 491−519. (44) Benkö, G.; Kallioinen, J.; Myllyperkiö, P.; Trif, F.; KorppiTommola, J. E. I.; Yartsev, A. P.; Sundström, V. Interligand Electron Transfer Determines Triplet Excited State Electron Injection in RuN3−Sensitized TiO2 Films. J. Phys. Chem. B 2004, 108, 2862− 2867. (45) Furube, A.; Katoh, R.; Hara, K.; Murata, S.; Arakawa, H.; Tachiya, M. Ultrafast Stepwise Electron Injection from Photoexcited Ru-Complex into Nanocrystalline ZnO Film via Intermediates at the Surface. J. Phys. Chem. B 2003, 107, 4162−4166. (46) Tiwana, P.; Docampo, P.; Johnston, M. B.; Snaith, H. J.; Herz, L. M. Electron Mobility and Injection Dynamics in Mesoporous ZnO, SnO2, and TiO2 Films Used in Dye-Sensitized Solar Cells. ACS Nano 2011, 5, 5158−5166. (47) Dell’Angela, M.; Anniyev, T.; Beye, M.; Coffee, R.; Föhlisch, A.; Gladh, J.; Kaya, S.; Katayama, T.; Krupin, O.; Nilsson, A.; Nordlund, G


Article

Accounts of Chemical Research D.; Schlotter, W. F.; Sellberg, J. A.; Sorgenfrei, F.; Turner, J. J.; Ö ström, H.; Ogasawara, H.; Wolf, M.; Wurth, W. Vacuum space charge effects in sub-picosecond soft X-ray photoemission on a molecular adsorbate layer. Struct. Dyn. 2015, 2, 025101. (48) Lüth, H. Surfaces and Interfaces of Solids; Springer: Berlin, 1993. (49) Zhang, Z.; Yates, J. T. Band Bending in Semiconductors: Chemical and Physical Consequences at Surfaces and Interfaces. Chem. Rev. 2012, 112, 5520−5551. (50) Kronik, L.; Shapira, Y. Surface photovoltage phenomena: theory, experiment, and applications. Surf. Sci. Rep. 1999, 37, 1−206. (51) Long, J. P.; Sadeghi, H. R.; Rife, J. C.; Kabler, M. N. Surface space-charge dynamics and surface recombination on silicon (111) surfaces measured with combined laser and synchrotron radiation. Phys. Rev. Lett. 1990, 64, 1158−1161. (52) Bröcker, D.; Gießel, T.; Widdra, W. Charge carrier dynamics at the SiO2/Si(1 0 0) surface: a time-resolved photoemission study with combined laser and synchrotron radiation. Chem. Phys. 2004, 299, 247−251. (53) Spencer, B. F.; Graham, D. M.; Hardman, S. J. O.; Seddon, E. A.; Cliffe, M. J.; Syres, K. L.; Thomas, A. G.; Stubbs, S. K.; Sirotti, F.; Silly, M. G.; Kirkham, P. F.; Kumarasinghe, A. R.; Hirst, G. J.; Moss, A. J.; Hill, S. F.; Shaw, D. A.; Chattopadhyay, S.; Flavell, W. R. Timeresolved surface photovoltage measurements at n-type photovoltaic surfaces: Si(111) and ZnO(1010̅ ). Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 88, 195301. (54) Yukawa, R.; Yamamoto, S.; Ozawa, K.; Emori, M.; Ogawa, M.; Yamamoto, S.; Fujikawa, K.; Hobara, R.; Kitagawa, S.; Daimon, H.; Sakama, H.; Matsuda, I. Electron-hole recombination on ZnO(0001) single-crystal surface studied by time-resolved soft X-ray photoelectron spectroscopy. Appl. Phys. Lett. 2014, 105, 151602. (55) Xu, Y.; Hofmann, O. T.; Schlesinger, R.; Winkler, S.; Frisch, J.; Niederhausen, J.; Vollmer, A.; Blumstengel, S.; Henneberger, F.; Koch, N.; Rinke, P.; Scheffler, M. Space-Charge Transfer in Hybrid Inorganic-Organic Systems. Phys. Rev. Lett. 2013, 111, 226802. (56) Kilin, D. S.; Micha, D. A. Relaxation of Photoexcited Electrons at a Nanostructured Si(111) Surface. J. Phys. Chem. Lett. 2010, 1, 1073−1077. (57) Sinai, O.; Hofmann, O. T.; Rinke, P.; Scheffler, M.; Heimel, G.; Kronik, L. Multiscale approach to the electronic structure of doped semiconductor surfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91, 075311.

H


Monitoring Ultrafast Chemical Dynamics by Time-Domain X-ray Photo

Recommend Documents