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 (\u3e1014 W/cm2). Our results demonstrate an approach of general utility for interpreting FEL spectroscopies

Sterically Induced Binding Selectivity of Single m-Terphenyl Isocyanide Ligands

Sterically encumbering m-terphenyl isocyanides are a class of metal-binding group that foster low-coordinate metal-center environments in coordination chemistry by exerting considerable intermolecular steric pressures between neighboring ligands. In the context of metal surfaces, the encumbering steric properties of the m-terphenyl isocyanides are shown to weaken the interaction between the metal-binding group and a planar substrate, leading to a preference for molecular adsorption at sites with convex curvature, such as the step edges and herringbone elbow sites on Au(111). Here, we investigate the site-selective binding of individual m-terphenyl isocyanide ligands on a Au(111) surface through scanning tunneling microscopy (STM) and inelastic electron tunneling spectroscopy (IETS). The site-dependent steric pressure alters the vibrational fingerprint of the m-terphenyl isocyanides, which is characterized with single-molecule precision through joint experimental and theoretical approaches. This study for the first time provides molecular-level insights into the steric-pressure-enabled surface binding selectivity as well as its effect on the chemical properties of individual m-terphenyl isocyanide ligands, thereby highlighting the potential to control the physical and chemical properties of metal surfaces through tailored ligand design

Polarization-Resolved Extreme-Ultraviolet Second-Harmonic Generation From Linbo3

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 LiNbO3, 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 LiNbO3, which is known to lead to ferroelectricity, is accompanied by distortions to the Nb ion environment that breaks the inversion symmetry of the NbO6 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

{\AA}ngstr\"om-resolved Interfacial Structure in Organic-Inorganic Junctions

Charge transport processes at interfaces which are governed by complex interfacial electronic structure play a crucial role in catalytic reactions, energy storage, photovoltaics, and many biological 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 {\AA}, wherein changes as small as 0.1 {\AA} result in easily detectable SXR-SHG spectral shifts (ca. 100s of meV). As SXR-SHG is inherently ultrafast and sensitive to individual atomic layers, it creates the possibility to study a variety of interfacial processes, e.g. catalysis, with ultrafast time resolution and bond specificity.Comment: 19 page

Polarization-Resolved Extreme Ultraviolet Second Harmonic Generation from LiNbO3_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 orders through their angular anisotropies within the crystal structure. However, the increasing complexity of novel materials and emerging phenomena hamper 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 non-centrosymmetric 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 by $\sim$0.5 Angstrom while the Nb-O bonds are elongated/contracted by displacements of the O atoms by $\sim$0.1 Angstrom. 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. This 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

First-Principles Investigation of Core Level Second Harmonic Generation as Interfacial Probe

Interfacial probe at elemental selectivity is highly sought after experimental tool. Chemical phenomena happen at the interface and our understanding these processes is often challenging. Second harmonic generation (SHG) spectroscopy technique can selectively probe the interface due to symmetry breaking. Coupling this technique with core electron XUV/X-ray spectroscopy allows elemental selectivity. In this regard, the development of novel core level second harmonic generation spectroscopy demonstrates a powerful tool at atomic resolution. Theoretical framework for simulation of XUV/X-ray SHG is developed to study different interfaces and interpret the experimental result. In this research, we explored Li ion displacement in ferroelectric perovskite LiOsO3 is studied via XUV-SHG, boron electronics structure at inorganic/organic junction and Li mobility in solid-state electrolyte lithium lanthanum titanate. In all cases, the bulk responses were indistinguishable between different interfaces while the surface sensitive SHG technique demonstrate different responses. Our approach shows a promising result toward elemental selective interfacial probe