l0pt Figure Femtosecond and attosecond XUV spectroscopy was used to study of electron dynamics stemming from many-body interactions, including the coupling between the electronic and nuclear degrees of freedom, electronic correlations, external light fields, or a combination thereof. This work was supported by the U. S. Army Research Laboratory and the U. S. Army Research Office under grant number W911NF-14-1-0383 and the National Science Foundation (NSF) award number PHY-1505556.onical intersections are an important topic of investigation because they serve as nature’s energy funnels in many biochemical processes, e.g. vision, light harvesting, etc. We focused on nuclear motion mediated evolution of an electron hole near a conical intersection in a CO₂ ion. Using pump-probe photodissociation spectroscopy, we made quantitative measurements of electronic couplings and monitored the role of decoherence in such dynamics, thereby probing the fundamental mechanisms responsible for the charge and energy redistribution in molecules. In another study, time resolved XUV photoelectron spectroscopy was applied to identify the role of multi electron excitations in the ultrafast Rydberg state dissociation of highly excited states in O₂. The talk will also discuss new opportunities arising the application of attosecond soft-x-ray sources.

---

Footnotes:

This work was supported by the U. S. Army Research Laboratory and the U. S. Army Research Office under grant number W911NF-14-1-0383 and the National Science Foundation (NSF) award number PHY-1505556.C

Metal oxides are stable, earth abundant semiconductors for the photochemical conversion of sunlight into chemical energy. Delafossite CuFeO2 is a promising material for photochemical energy conversion due to its small band gap (1.5 eV) and p-type conductivity. CuFeO2 has also shown promise for catalyzing both the hydrogen evolution and CO2 reduction reactions. Despite significant work in this area, important questions remain regarding the complex defect chemistry in copper-iron oxides and the effect of various defects on the carrier lifetime and photoelectrochemical efficiency. Of the various defects possible, here we investigate the role of type II heterojunction structures with interfaces of CuO and CuFeO2, Cu vacancies, and O interstitials on the photocarrier dynamics of this material. To elucidate the effects of carrier lifetimes on the photochemical efficiency of mixed phase CuFeO2, we probe the photocarrier dynamics using optical transient absorption spectroscopy. First, we consider the role of grain boundaries between CuO and CuFeO2 in mixed phase systems, which have been hypothesized to facilitate charge separation across this type II heterostructure. Transient absorption measurements suggest that photoexcited electrons in the most active materials reside on Fe 3d conduction band states, and we do not observe evidence for electron transfer to CuO, which indicates interfacial charge transfer from CuFeO2 to CuO is not responsible for enhanced carrier lifetimes in the catalysts studied here. We find that Cu vacancies appear to improve the efficiency of CuFeO2 due to fast charge separation as holes thermalize from O 2p to Cu 3d valence band states. This is confirmed by DFT calculations demonstrating a lowering of the Cu 3d band center with the introduction of Cu vacancies. In contrast, DFT calculations show that the valence band maximum of CuFeO2 changes from Cu 3d to O 2p states with the introduction of interstitial O atoms which tends to inhibit charge separation. Based on our results as well as DFT calculations, we conclude that Cu vacancies are primarily responsible for charge separation in this material.

Time-resolved photoelectron spectroscopy (TRPES) has been used to study the ultrafast relaxation of electronically excited thymine, thymidine, and thymidine monophosphate on femtosecond time scales in liquid water. Pump-probe experiments have been carried out using tunable UV (4.7-5.2 eV) and 200 nm (6.2 eV) pulses, enabling the observation of relaxation dynamics of excited state populations from the S₁(¹ππ*) excited state as well as a higher lying S_n(¹ππ*) excited state. Relaxation lifetimes from the S₁(¹ππ*) excited state have been obtained in reasonable agreement with previous workBuchner, F.; Nakayama, A.; Yamazaki, S., et al., Journal of the American Chemical Society 2015, 137 (8), 2931-2938.nd show no evidence of relaxation to the S₂(¹nπ*) excited state, in contrast to transient absorption studies.Hare, P. M.; Crespo-Hernández, C. E.; Kohler, B., Proceedings of the National Academy of Sciences 2007, 104 (2), 435-440.^,Kwok, W.−M.; Ma, C.; Phillips, D. L., Journal of the American Chemical Society 2008, 130 (15), 5131−5139.dditionally, relaxation from the higher lying S_n(^1

π

Kwok, W.-M.; Ma, C.; Phillips, D. L., Journal of the American Chemical Society 2008, 130 (15), 5131-5139.A

Ultrafast extreme ultraviolet (XUV) absorption spectroscopy can be used to probe the dynamics of excited states in first-row transition metals complexes with sensitivity to the oxidation state, spin state, and ligand field of the metal center. This technique can be performed with a tabletop instrument and probes the M-edge transitions from the 3p to the 3d orbital of a metal center. This technique is analogous to K-, and L-edge spectroscopy, which are performed at synchrotron or x-ray free electron lasers. These user-based facilities offer high photon counts and ultrafast time resolution, however beamtime is limited. Fortunately, the use of tabletop sources to generate XUV light with femtosecond time resolution has become a more widely available source of x-ray spectroscopy. XUV absorption spectroscopy has until now been limited to studying solid-state or gas-phase samples due to short penetration depth of XUV photons. To study solutions with XUV absorption, I used a microfluidic chip to generate free flowing liquid sheets of nonpolar, XUV transmissive solutions. I have characterized the thickness and stability of chloroform liquid sheets under vacuum and have adapted our tabletop high-harmonic instrument to maintain high vacuum. This sample delivery method now makes ultrafast XUV absorption spectroscopy available to study an array of transition-metal complexes in the solution phase.

Hematite is an earth abundant material that has the potential to be used as a photoanode for oxygen evolution from water using solar energy. Modifications such as surface functionalization, surface doping, use of a co-catalyst and preparation of layered heterojunctions have each been explored in an effort to increase the efficiency of the hematite electrode with varying degrees of success. However, due to the complexity of this material and the challenges associated with probing electron and hole dynamics with surface specificity and chemical state resolution, the fundamental processes governing carrier transport and trapping in surface states is still not well understood. In particular, recent transient studies carried out on hematite using extreme ultraviolet (XUV) absorption spectroscopy and XUV reflection absorption (RA) spectroscopy show key differences in the dynamics of small polaron formation in bulk hematite versus at the surface. To better understand the origin of these differences, we have functionalized hematite surfaces with a series of small organic molecules, including phenyl phosphonic acid (PPA), 4-Cyano PPA and 4-methoxy PPA. These molecularly functionalized surfaces have been characterized using x-ray photoelectron spectroscopy and sum frequency generation vibrational spectroscopy. Linear sweep voltammetry measurements is used to explore the effect of these systematic surface modifications on the photoelectrochemical efficiency of this material. To better understand the mechanism by which surface modification influences catalytic efficiency, transient XUV-RA spectroscopy is used to measure changes in the rate of small polaron formation and surface trapping in these materials. This direct observation of electron and hole dynamics in systematically modified hematite surfaces provides a better understanding of the material properties responsible for mediating energy conversion efficiency in these and related materials.

To improve the efficiency of first row transition metal catalysts to match their precious metal counterparts, a method must be used that can look at snapshots of the catalytic reaction and identify the salient qualities of key intermediates. Ultrafast tabletop extreme ultraviolet (XUV) spectroscopy is well suited to this purpose because it is sensitive to a metal’s oxidation state, spin state, and ligand field. In addition, it can be performed in the lab rather than at a largescale facility like a synchrotron or X-FEL. To demonstrate sensitivity of XUV spectroscopy to the electronic structure of nickel-centered complexes, I measure the photophysics of a nickel dithiocarbamate complex, nickel bis(diethyldithiocarbamate), chosen due to its similarities to catalytically-active nickel dithiolenes. I show how the electron density moves across the highly covalent nickel-sulfur bonds after excitation of an LMCT transition. Examining the excited state dynamics with XUV spectroscopy allows for clearer determination of the metal spin state and electron density, filling in gaps in the picture drawn by transient optical spectroscopy. Transient XUV spectroscopy is shown to measure both the electronic structure of the metal and the electron density on the ligand, providing a powerful tool to study charge flow between metal and ligand.