24 research outputs found
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 LiNbO
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, 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, which is
known to lead to ferroelectricity, is accompanied by distortions to the Nb ion
environment that breaks the inversion symmetry of the NbO octahedron as
well. Our simulations show that the measured second harmonic spectrum is
consistent with Li ion displacements from the centrosymmetric position by
0.5 Angstrom while the Nb-O bonds are elongated/contracted by
displacements of the O atoms by 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
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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