In photocatalytic transition metal oxides, the surface electronic structure and carrier kinetics are complicated by both charge trapping at defects and self-trapping by small polaron formation. These effects are specific to both the material and the type of defect or surface states involved. Here we review recent studies on ultrafast photoexcited charge carrier dynamics at the surfaces of α-Fe 2 O 3 (hematite) and NiO and at the interface of a α-Fe 2 O 3 /NiO heterojunction using extreme ultraviolet (XUV) reflection−absorption spectroscopy. We study the dynamics of small polaron formation at hematite surfaces showing that the rate of carrier self-trapping is slower than in the bulk due to a greater lattice reorganization energy required for surface polaron formation. We also show that surface dynamics of hematite can be systematically tuned to control carrier transport properties through surface functionalization. To better understand the impact of defects on carrier events, we study charge carrier recombination of NiO. It is shown that O vacancies have no detrimental effect on carrier lifetime while grain boundaries cause fast recombination. Finally, we study a model heterojunction consisting of α-Fe 2 O 3 and NiO and find that interfacial charge transfer across these two materials occurs by a two-step mechanism where the interfacial electric field drives fast exciton dissociation followed by hole injection from α-Fe 2 O 3 to NiO. These examples illustrate future opportunities to observe surface electron dynamics in photocatalytic materials with element and chemical state resolution using ultrafast XUV spectroscopy.

Anatase TiO 2 is an efficient water splitting photocatalyst using UV light, but solar energy harvesting requires the presence of midgap states to increase visible light absorption. Despite numerous studies, important questions remain regarding the photophysics in O vacancy doped TiO 2 . By employing extreme ultraviolet reflection−absorption (XUV-RA) spectroscopy at the Ti M 2,3 -edge, spectral signatures of both large and small polaron states are identified, allowing ultrafast electron and hole dynamics in these states to be independently resolved. Results show that visible light absorption occurs via promotion of an electron from the small polaron state to the TiO 2 conduction band. In contrast, absorption of UV light results in direct band gap excitation followed by carrier relaxation during which hot holes trap as small polarons in 45 ± 42 fs, and hot electrons couple to polar optical phonons leading to vibrational coherence and large polaron formation in 945 ± 92 fs.

CuO is often employed as a photocathode for H2 evolution and CO2 reduction, but observed efficiency is still far below the theoretical limit. To bridge the gap requires understanding the CuO electronic structure; however, computational efforts lack consensus on the orbital character of the photoexcited electron. In this study, we measure the femtosecond XANES spectra of CuO at the Cu M2,3 and O L1 edges to track the element-specific dynamics of electrons and holes. Results show that photoexcitation represents an O 2p to Cu 4s charge transfer state indicating the conduction band electron has primarily Cu 4s character. We also observe ultrafast mixing of Cu 3d and 4s conduction band states mediated by coherent phonons, with Cu 3d character of the photoelectron reaching a maximum of 16%. This is the first observation of the photoexcited redox state in CuO, and results provide a benchmark for theory where electronic structure modeling still relies heavily on model-dependent parametrization.

In time resolved spectroscopy of molecular systems, spectral signatures are directly correlated with processes such as charge migration, intra and intermolecular vibrational relaxation, internal conversion, and intersystem crossing. However, the challenge of probing these analogous processes in material systems with surface sensitivity and ultrafast time resolution motivates the goal to extend a molecular-level understanding to dynamics at surfaces and interfaces.In this talk, we describe the recent ability to directly observe spin-polarized electron transport at semiconductor surfaces using XUV Magnetic Circular Dichroism (XUV-MCD) in a reflection geometry. The ability to produce spin polarized currents at interfaces underlies many promising applications ranging from spintronics to enantioselective photocatalysis, but designing materials capable of these applications requires an improved understanding of spin-dependent electron dynamics at interfaces. Towards this goal, XUV-MCD reflection-absorption spectroscopy provides direct observation of spin dynamics in magnetic materials with ultrafast time resolution and surface sensitivity.Yttrium iron garnet (Y 3 Fe 5 O 12 , YiG) is a ferrimagnetic semiconductor, consisting of two sub-lattices based on octahedrally and tetrahedrally coordinated Fe(III) centers. A combination of linearly and circularly polarized XUV measurements at the Fe M 2,3 -edge of YiG provides a detailed picture of these lattice-dependent electron dynamics, which give rise to spin polarized current at the YiG surface upon band gap excitation. These findings have important applications towards the development of spin selective photocatalysts as well as new platforms for light-induced control of ultrafast spin polarization at material interfaces.

Using XUV reflection-absorption spectroscopy, we study the ultrafast surface electron dynamics in TiO2 with emphasis on the role of O vacancies and excitation wavelength. We identify unique photoexcited states as a function of excitation wavelength.