This talk will provide an introduction to this mini-symposium, which features advances in high-harmonic generation and the spectroscopy it enables. This strong-field process is a sensitive probe of small-molecule electronic structure, as the spectrum of the extreme ultraviolet (XUV) photons produced by the ionization-recollision process measures the shape and energy of the molecular orbitals. Furthermore, the XUV pulse can be used as an element-specific probe of electron dynamics in molecules and materials at femtosecond to attosecond timescales.

Hybrid organic-inorganic halide perovskites, such as methylammonium lead iodide have emerged as outstanding light absorbing and emitting materials in recent years. Quasi-2D/layered perovskites have also gained significant attention owing to enhanced ambient stability, high luminescence quantum yield and strong excitonic effects. However, our current understanding of the fundamental photophysics in these materials is limited by the overlap of spectral features in the energy ranges studied using traditional methods of time-resolved spectroscopy. Ultrafast extreme ultraviolet (XUV) absorption was used to investigate electron and hole dynamics in perovskites by observing transitions from core level (I 4d and Br 3d ) to the valence and conduction bands. Using a table-top instrument, ultrashort (30 fs) pulses of XUV radiation with a broad spectrum (40-75 eV) were generated via high-harmonic generation. Transient absorption measurements using visible pump and XUV probe directly observed carrier and element specific relaxation dynamics in mixed-halide and layered perovskites for above band edge excitation in the femtosecond and picosecond time scales.

A key feature of monolayer semiconductors, such as transition-metal dichalcogenides, is the poorly screened Coulomb potential, which leads to large exciton binding energy (E_b) and strong renormalization of the quasiparticle bandgap (E_g) by carriers. The latter has been difficult to determine due to cancellation in changes of E_b and E_g, resulting in little change in optical transition energy at different carrier densities. Here we quantify bandgap renormalization in macroscopic single crystal MoS₂ monolayers on SiO₂ using time and angle resolved photoemission spectroscopy (TR-ARPES) with femtosecond extreme UV (EUV) probe. At excitation density above the Mott threshold, E_g decreases by as much as 360 meV. We compare the carrier density dependent E_g with previous theoretical calculations and show the necessity of knowing both doping and excitation densities in quantifying the bandgap.

Methylammonium lead iodide perovskite is a promising candidate for next-generation photovoltaics. One application for this perovskite is in hot-carrier collection devices. In a standard cell any excess energy from absorbed photons is lost as heat, but a cell can be designed to extract carriers before they cool to increase its efficiency above the Shockley-Queisser limit. In order to achieve this, the cooling rate of carriers must be sufficiently slower than the extraction time. Perovskite may fit this criterion due to the presence of a hot-phonon bottleneck for carrier cooling. Time-resolved XUV absorption from the core I4d level to the valence band (45-50 eV) after optical excitation (3.1 eV) was used to probe the hole distribution of photoexcited perovskite. The holes were found to cool rapidly (cooling time shorter than 400 fs) at high carrier density (10¹⁹ cm⁻³). In comparison, time-resolved optical absorption (1.5-2.5 eV) was used to probe the electron distribution, which was found to cool slowly (cooling time longer than 5 ps) for the same excitation density. This indicates that a hot-carrier collection device using perovskite should be designed to only extract hot electrons, not hot holes.

The ability to follow electron dynamics at surfaces is necessary to identify the material properties and surface defect states, which mediate carrier lifetime and ultimately determine energy conversion efficiency. Toward this goal we have recently developed extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy as a surface specific analog of XUV transient absorption. Unlike absorption measurements, which sample only the imaginary component of the refractive index, reflection measurements probe both the real and imaginary components of the material’s complex refractive index. We find that the imaginary component is sensitive to the chemical state of the material as reported by the core-hole resonant spectrum while the real component is additionally sensitive to the physical morphology of the material. Using semi-empirical spectral simulations we show that XUV-RA spectroscopy retains the element and chemical state specificity of XUV absorption spectroscopy. Accordingly, this technique extends the benefits of element specific x-ray absorption spectroscopy to the study of surface electron dynamics by measuring core-hole resonances with a probe depth of less than 3 nm and a time resolution of less than 100 fs. Here we describe the application of XUV-RA spectroscopy to the study of ultrafast electron trapping and defect-mediated recombination at the surface of NiO. Direct observation of ultrafast electron trapping and subsequent recombination shows that grain boundaries rather than oxygen vacancies are responsible for fast electron-hole pair recombination. This result clarifies the design parameters for NiO water oxidation catalysts by showing that oxygen vacancies, which enhance catalytic activity, have no detrimental effect on carrier lifetime. Rather, carrier lifetime can be dramatically extended by the elimination of near-surface grain boundaries even in the presence of chemically active oxygen vacancies.

Materials with improved catalytic efficiency can be designed based on a more thorough understanding of the excited state dynamics that mediate energy transfer and drive charge separation. High harmonic generation can produce ultrafast pulses in the extreme ultraviolet (XUV) region, enabling the use of XUV reflection-absorption (XUV-RA) spectroscopy to analyze signatures of photoexcited electrons and holes separately. From XUV-RA experiments on anatase TiO, it is shown that, for the first 800 fs after photoexcitation there is an apparent quantum beating at 417 cm⁻¹. This frequency is consistent with coupling between a free carrier state and a large polaron state, which has been previously observed in TiO₂. Following the initial dynamic equilibrium between these two states, the electron localizes to a more energetically stable Ti³⁺ defect state 1 eV below the conduction band within 1.22 ps. We hypothesize that the strong electron-phonon coupling leading to the observed beating may be the result of Ti³⁺ defect states which we show are prevalent in this material. To investigate this hypothesis, we are now performing measurements on stoichiometric TiO₂ with Ti³⁺ defects removed. These ongoing experiments on defect-free TiO₂ will provide a more complete understanding of the role the Ti³⁺ defects play in the charge carrier kinetics of this catalytically relevant material.

A passivating oxide layer is critical for the stability and the performance of solar-fuel photoelectrodes. While the semiconductor surface can be passivated by a few nanometer oxide film, the best performance often correlates with a thicker and defect-rich amorphous TiO₂ layer. The defect states are suggested to facilitate hole transport between the semiconductor and metal catalyst. In this presentation, transient extreme ultraviolet (XUV) absorption spectroscopy quantifies the electron and hole transport between each element of a photoexcited Ni-TiO₂-Si photoelectrode. A ballistic hole tunneling from the p-type Si to the Ni metal is measured in 100 fs after photoexcitation of the Si. The measured hole injection efficiency is 26%. The transient hole population is then measured to back-diffuse through the TiO₂ on a picoseconds timescale, followed by an increased electron-hole recombination at the Si-TiO₂ interface. By temporally resolving the population of electrons and holes in each layer of the junction, the hole transport velocity in the TiO₂, the hole mobility in the Si, the diffusion constant of holes in the TiO₂, and the surface recombination velocity at the Si/TiO₂ interface are quantized.

Understanding the mechanism of interfacial hole transfer and the chemical nature of the hole acceptor state in NiO based solar materials are important for the rational design of devices with improved efficiency. NiO is widely utilized as a hole transport layer in solar energy devices. However, due to its complex electronic structure, the chemical nature of the hole acceptor state has remained an open question, despite the fact that hole localization in NiO significantly influences device efficiency. To comment on this, we present results of ultrafast charge carrier dynamics in a NiO based model heterojunction (/NiO) using extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy. Element specific XUV-RA demonstrates the formation of transient Ni³⁺ within a few ps following selective photoexcition of the underlying Fe₂O₃ substrate. This indicates that fast hole transfer in this system occurs to NiO valence band states composed of significant Ni 3d character. Additionally, we show that this hole injection process proceeds via a two-step sequential mechanism where fast field driven exciton dissociation occurs in Fe₂O₃ in 680 ± 60 fs, followed by subsequent hole injection to NiO in 9.2 ± 2.9 ps. These results reveal the chemical nature of the hole acceptor state in widely used NiO hole transport layers and provides a direct observation of exciton dissociation and interfacial hole transfer in this system.

Time-resolved inner shell absorption spectroscopy is a sensitive and localized probe of valence electronic dynamics. By measuring the temporal evolution of near-edge absorption features of multiple atoms simultaneously, complex dynamics can be separated into the individual underlying processes. In the condensed phase, the migration of both electrons and holes can be tracked during photoinduced interfacial electron transfer between a Ruthenium (Ru) metal complex and a copper oxide (CuO) substrate via transient absorption measurements at the various molecular and substrate specific atomic edges. A femtosecond water window source spanning the Carbon, Nitrogen, and Oxygen K-edges can also be used to follow the valence electronic dynamics and concerted nuclear motion of UV excited molecules such as nitrobenzene. We report progress on our development of such a source based on high harmonic generation (HHG) driven by a 100 kHz, > 100 W optical chirped pulse parametric amplifier and transient absorption measurements of the above systems.