Electron Transfer Dynamics at Dye-Sensitized SnO2/TiO2 Core/Shell Electrodes in Aqueous/Nonaqueous Electrolyte Mixtures.

The dynamics of photoinduced electron transfer were measured at dye-sensitized photoanodes in aqueous (acetate buffer), nonaqueous (acetonitrile), and mixed solvent electrolytes by nanosecond transient absorption spectroscopy (TAS) and ultrafast optical-pump terahertz-probe spectroscopy (OPTP). Higher injection efficiencies were found in mixed solvent electrolytes for dye-sensitized SnO2/TiO2 core/shell electrodes, whereas the injection efficiency of dye-sensitized TiO2 electrodes decreased with the increasing acetonitrile concentration. The trend in injection efficiency for the TiO2 electrodes was consistent with the solvent-dependent trend in the semiconductor flat band potential. Photoinduced electron injection in core/shell electrodes has been understood as a two-step process involving ultrafast electron trapping in the TiO2 shell followed by slower electron transfer to the SnO2 core. The driving force for shell-to-core electron transfer increases as the flat band potential of TiO2 shifts negatively with increasing concentrations of acetonitrile. In acetonitrile-rich electrolytes, electron injection is suppressed due to the very negative flat band potential of the TiO2 shell. Interestingly, a net negative photoconductivity in the SnO2 core is observed in mixed solvent electrolytes by OPTP. We hypothesize that an electric field is formed across the TiO2 shell from the oxidized dye molecules after injection. Conduction band electrons in SnO2 are trapped at the core/shell interface by the electric field, resulting in a negative photoconductivity transient. The overall electron injection efficiency of the dye-sensitized SnO2/TiO2 core/shell photoanodes is optimized in mixed solvents. The ultrafast transient conductivity data illustrate the crucial role of the electrolyte in regulating the driving forces for electron injection and charge separation at dye-sensitized semiconductor interfaces.

Time-Resolved X-ray Emission Spectroscopy and Resonant Inelastic X-ray Scattering Spectroscopy of Laser Irradiated Carbon.

The existence of liquid carbon as an intermediate phase preceding the formation of novel carbon materials has been a point of contention for several decades. Experimental observation of such a liquid state requires nonthermal melting of solid carbon materials at various laser fluences and pulse properties. Reflectivity experiments performed in the mid-1980s reached opposing conclusions regarding the metallic or insulating properties of the purported liquid state. Time-resolved X-ray absorption studies showed shortening of C-C bonds and increasing diffraction densities, thought to evidence a liquid or glassy carbon state, respectively. Nevertheless, none of these experiments provided information on the electronic structure of the proposed liquid state. Herein, we report the results of time-resolved resonant inelastic X-ray scattering (RIXS) and time-resolved X-ray emission spectroscopy (XES) studies on amorphous carbon (a-C) and ultrananocrystalline diamond (UNCD) as a function of delay time between the irradiating pulse and X-ray probe. For both a-C and UNCD, we attribute decreases in RIXS or XES signals to transition blocking, relaxation, and finally, ablation. Increased signal at 20 ps following the irradiation of the UNCD is attributed to the probable formation of nanoscale structures in the ablation plume. Differences in the amount of signal observed between a-C and UNCD are explained by the difference in sample thickness and, specifically, incomplete melting of the UNCD film. Comparisons to spectral simulations based on MD trajectories at extreme conditions indicate that the carbon state in our experiments is crystalline. Normal mode analysis confirmed that symmetrical bending or stretching of the C-C bonds in the diamond lattice results in XES spectra with small intensity differences. Overall, we observed no evidence of melting to a liquid state, as determined by the lack of changes in the spectral properties for up to 100 ps delays following the melting pulses.

Mechanistic Investigation, Wavelength-Dependent Reactivity, and Expanded Reactivity of N-Aryl Azacycle Photomediated Ring Contractions.

Under mild blue-light irradiation, α-acylated saturated heterocycles undergo a photomediated one-atom ring contraction that extrudes a heteroatom from the cyclic core. However, for nitrogenous heterocycles, this powerful skeletal edit has been limited to substrates bearing electron-withdrawing substituents on nitrogen. Moreover, the mechanism and wavelength-dependent efficiency of this transformation have remained unclear. In this work, we increased the electron richness of nitrogen in saturated azacycles to improve light absorption and strengthen critical intramolecular hydrogen bonding while enabling the direct installation of the photoreactive handle. As a result, a broadly expanded substrate scope, including underexplored electron-rich substrates and previously unsuccessful heterocycles, has now been achieved. The significantly improved yields and diastereoselectivities have facilitated reaction rate, kinetic isotope effect (KIE), and quenching studies, in addition to the determination of quantum yields. Guided by these studies, we propose a revised ET/PT mechanism for the ring contraction, which is additionally corroborated by computational characterization of the lowest-energy excited states of α-acylated substrates through time-dependent DFT. The efficiency of the ring contraction at wavelengths longer than those strongly absorbed by the substrates was investigated through wavelength-dependent rate measurements, which revealed a red shift of the photochemical action plot relative to substrate absorbance. The elucidated mechanistic and photophysical details effectively rationalize empirical observations, including additive effects, that were previously poorly understood. Our findings not only demonstrate enhanced synthetic utility of the photomediated ring contraction and shed light on mechanistic details but may also offer valuable guidance for understanding wavelength-dependent reactivity for related photochemical systems.

A solid-state high harmonic generation spectrometer with cryogenic cooling.

Solid-state high harmonic generation (sHHG) spectroscopy is a promising technique for studying electronic structure, symmetry, and dynamics in condensed matter systems. Here, we report on the implementation of an advanced sHHG spectrometer based on a vacuum chamber and closed-cycle helium cryostat. Using an in situ temperature probe, it is demonstrated that the sample interaction region retains cryogenic temperature during the application of high-intensity femtosecond laser pulses that generate high harmonics. The presented implementation opens the door for temperature-dependent sHHG measurements down to a few Kelvin, which makes sHHG spectroscopy a new tool for studying phases of matter that emerge at low temperatures, which is particularly interesting for highly correlated materials.

Probing lithium mobility at a solid electrolyte surface.

Solid-state electrolytes overcome many challenges of present-day lithium ion batteries, such as safety hazards and dendrite formation1,2. However, detailed understanding of the involved lithium dynamics is missing due to a lack of in operando measurements with chemical and interfacial specificity. Here we investigate a prototypical solid-state electrolyte using linear and nonlinear extreme-ultraviolet spectroscopies. Leveraging the surface sensitivity of extreme-ultraviolet-second-harmonic-generation spectroscopy, we obtained a direct spectral signature of surface lithium ions, showing a distinct blueshift relative to bulk absorption spectra. First-principles simulations attributed the shift to transitions from the lithium 1 s state to hybridized Li-s/Ti-d orbitals at the surface. Our calculations further suggest a reduction in lithium interfacial mobility due to suppressed low-frequency rattling modes, which is the fundamental origin of the large interfacial resistance in this material. Our findings pave the way for new optimization strategies to develop these electrochemical devices via interfacial engineering of lithium ions.

Signatures of Multiband Effects in High-Harmonic Generation in Monolayer MoS_{2}.

High-harmonic generation (HHG) in solids has been touted as a way to probe ultrafast dynamics and crystal symmetries in condensed matter systems. Here, we investigate the polarization properties of high-order harmonics generated in monolayer MoS_{2}, as a function of crystal orientation relative to the mid-infrared laser field polarization. At several different laser wavelengths we experimentally observe a prominent angular shift of the parallel-polarized odd harmonics for energies above approximately 3.5 eV, and our calculations indicate that this shift originates in subtle differences in the recombination dipole strengths involving multiple conduction bands. This observation is material specific and is in addition to the angular dependence imposed by the dynamical symmetry properties of the crystal interacting with the laser field, and may pave the way for probing the vectorial character of multiband recombination dipoles.

Saturable Absorption of Free-Electron Laser Radiation by Graphite near the Carbon K-Edge.

The interaction of intense light with matter gives rise to competing nonlinear responses that can dynamically change material properties. Prominent examples are saturable absorption (SA) and two-photon absorption (TPA), which dynamically increase and decrease the transmission of a sample depending on pulse intensity, respectively. The availability of intense soft X-ray pulses from free-electron lasers (FELs) has led to observations of SA and TPA in separate experiments, leaving open questions about the possible interplay between and relative strength of the two phenomena. Here, we systematically study both phenomena in one experiment by exposing graphite films to soft X-ray FEL pulses of varying intensity. By applying real-time electronic structure calculations, we find that for lower intensities the nonlinear contribution to the absorption is dominated by SA attributed to ground-state depletion; our model suggests that TPA becomes more dominant for larger intensities (>1014 W/cm2). Our results demonstrate an approach of general utility for interpreting FEL spectroscopies.

Light-induced dimension crossover dictated by excitonic correlations.

In low-dimensional systems with strong electronic correlations, the application of an ultrashort laser pulse often yields novel phases that are otherwise inaccessible. The central challenge in understanding such phenomena is to determine how dimensionality and many-body correlations together govern the pathway of a non-adiabatic transition. To this end, we examine a layered compound, 1T-TiSe2, whose three-dimensional charge-density-wave (3D CDW) state also features exciton condensation due to strong electron-hole interactions. We find that photoexcitation suppresses the equilibrium 3D CDW while creating a nonequilibrium 2D CDW. Remarkably, the dimension reduction does not occur unless bound electron-hole pairs are broken. This relation suggests that excitonic correlations maintain the out-of-plane CDW coherence, settling a long-standing debate over their role in the CDW transition. Our findings demonstrate how optical manipulation of electronic interaction enables one to control the dimensionality of a broken-symmetry order, paving the way for realizing other emergent states in strongly correlated systems.

Polarization-Resolved Extreme-Ultraviolet Second-Harmonic Generation from LiNbO_{3}.

Second harmonic generation (SHG) spectroscopy ubiquitously enables the investigation of surface chemistry, interfacial chemistry, as well as symmetry properties in solids. Polarization-resolved SHG spectroscopy in the visible to infrared regime is regularly used to investigate electronic and magnetic order through their angular anisotropies within the crystal structure. However, the increasing complexity of novel materials and emerging phenomena hampers the interpretation of experiments solely based on the investigation of hybridized valence states. Here, polarization-resolved SHG in the extreme ultraviolet (XUV-SHG) is demonstrated for the first time, enabling element-resolved angular anisotropy investigations. In noncentrosymmetric LiNbO_{3}, elemental contributions by lithium and niobium are clearly distinguished by energy dependent XUV-SHG measurements. This element-resolved and symmetry-sensitive experiment suggests that the displacement of Li ions in LiNbO_{3}, which is known to lead to ferroelectricity, is accompanied by distortions to the Nb ion environment that breaks the inversion symmetry of the NbO_{6} octahedron as well. Our simulations show that the measured second harmonic spectrum is consistent with Li ion displacements from the centrosymmetric position while the Nb─O bonds are elongated and contracted by displacements of the O atoms. In addition, the polarization-resolved measurement of XUV-SHG shows excellent agreement with numerical predictions based on dipole-induced SHG commonly used in the optical wavelengths. Our result constitutes the first verification of the dipole-based SHG model in the XUV regime. The findings of this work pave the way for future angle and time-resolved XUV-SHG studies with elemental specificity in condensed matter systems.

Angstrom-Resolved Interfacial Structure in Buried Organic-Inorganic Junctions.

Charge transport processes at interfaces play a crucial role in many processes. Here, the first soft x-ray second harmonic generation (SXR SHG) interfacial spectrum of a buried interface (boron-Parylene N) is reported. SXR SHG shows distinct spectral features that are not observed in x-ray absorption spectra, demonstrating its extraordinary interfacial sensitivity. Comparison to electronic structure calculations indicates a boron-organic separation distance of 1.9 Å, with changes of less than 1 Å resulting in easily detectable SXR SHG spectral shifts (ca. hundreds of milli-electron volts).

Extreme Ultraviolet Second Harmonic Generation Spectroscopy in a Polar Metal.

The coexistence of ferroelectricity and metallicity seems paradoxical, since the itinerant electrons in metals should screen the long-range dipole interactions necessary for dipole ordering. The recent discovery of the polar metal LiOsO3 was therefore surprising [as discussed earlier in Y. Shi et al., Nat. Mater. 2013, 12, 1024]. It is thought that the coordination preferences of the Li play a key role in stabilizing the LiOsO3 polar metal phase, but an investigation from the combined viewpoints of core-state specificity and symmetry has yet to be done. Here, we apply the novel technique of extreme ultraviolet second harmonic generation (XUV-SHG) and find a sensitivity to the broken inversion symmetry in the polar metal phase of LiOsO3 with an enhanced feature above the Li K-edge that reflects the degree of Li atom displacement as corroborated by density functional theory calculations. These results pave the way for time-resolved probing of symmetry-breaking structural phase transitions on femtosecond time scales with element specificity.

Table-top extreme ultraviolet second harmonic generation.

The lack of available table-top extreme ultraviolet (XUV) sources with high enough fluxes and coherence properties has limited the availability of nonlinear XUV and x-ray spectroscopies to free-electron lasers (FELs). Here, we demonstrate second harmonic generation (SHG) on a table-top XUV source by observing SHG near the Ti M2,3 edge with a high-harmonic seeded soft x-ray laser. Furthermore, this experiment represents the first SHG experiment in the XUV. First-principles electronic structure calculations suggest the surface specificity and separate the observed signal into its resonant and nonresonant contributions. The realization of XUV-SHG on a table-top source opens up more accessible opportunities for the study of element-specific dynamics in multicomponent systems where surface, interfacial, and bulk-phase asymmetries play a driving role.