Session D40: Probing Structure and Dynamics with XUV and X-Ray Light: Ultrafast Surface Dynamics

Ultrafast optical spectroscopy of thermodynamics & kinetics of reaction steps at an electrode surface

Computationally, often the energetics of intermediate reaction steps differentiate the efficiency of heterogeneous catalysts for product evolution.  Yet, when compared to experiment, kinetic models are applied.  For example, a material’s activity measured by one rate of product evolution is plotted as a function of the calculated formation energies of intermediate chemical forms.  A critically important reaction for which this dichotomy between experiment and theory exists is the oxygen evolution reaction (OER) from water.  In the laboratory group, we employ time-resolved electronic (visible) and vibrational (infrared) spectroscopy to deconstruct OER into its individual reaction steps on an electrode surface. In the presentation, I will describe the experimental methodology and the chosen model system, the n-doped SrTiO₃/aqueous interface.  I’ll show how these experiments identify both the rates and energetics of the first reaction step, the release of a proton and electron from an absorbed water species, denoted by OH* or O*. A recent success was to isolate a Langmuir isotherm of the intermediate population arising within < 2 ps on the SrTiO₃ surface.  This < 2 ps population also engenders interfacial strain that leads to coherences in the visible spectroscopy.  Current work connects the decay of these intermediates at microsecond timescales, presumably related to later reaction steps of OER, to their pH-dependent formation.  During the course of the presentation, connections between ultrafast optical and x-ray probes of this interface will be suggested. A growing area is to apply the particular experimental methodology to a surface of similar electronic structure but diverse crystal geometries, namely the photo-electrochemistry of rutile TiO₂.   

Dynamics of Nonequilibrium Charge Density Waves in 1T-TiSe2: Unraveling the Interplay of Dimensionality, Excitonic Correlations, Electron-Phonon Couplings, and Topological Defects

Despite being a prototypical charge-density-wave compound, the layered transition metal dichalcogenide, 1T-TiSe₂, has been the subject of intense study for its exotic properties both in and out of thermal equilibrium, such as the possibility of excitonic condensation, spontaneous gyrotropic ordering, as well as anomalous light-induced states such as potential energy gap opening and a metastable metallic phase. Central to this plethora of fascinating states in 1T-TiSe₂ is a unique combination of the dimensions of the crystal and the CDW, the strong excitonic correlations, the mode-selective electron-phonon coupling, and the susceptibility to topological defects formation. In this talk, I will discuss how we unravel the interplay of these different aspects via state-of-the-art attosecond transient extreme-ultraviolet (XUV) absorption spectroscopy and mega-electron-volt ultrafast electron diffraction (MeV-UED). I will show how photoexcitation leads to a 3D-to-2D dimension crossover in the CDW order parameter, a process dictated by the excitonic correlations in the system. The excitonic effect is further evidenced in the initial response of selected core-level absorption edges, and these observations help pinpoint the specific role of excitonic correlations in the CDW transition. I will also illustrate the hidden 1D nature of the CDW and its implication on the formation mechanism of domain-wall-like topological defects, which are found to emerge well under one picosecond following photoexcitation. Lastly, I will illustrate how coherent phonon spectroscopy in the XUV regime is a powerful way to uncover band-selective electron-phonon coupling strengths for the unoccupied electronic bands, which are not accessible by other probes but play an essential role in the nonequilibrium dynamics of 1T-TiSe₂ and other strongly-correlated materials.   

Determining Quasi-Equilibrium Electron and Hole Distributions of Plasmonic Photocatalysts using Photomodulated X-ray Absorption Spectroscopy

Most photocatalytic and photovoltaic devices operate under broadband, constant illumination. Electron and hole dynamics in these devices are usually measured using ultrafast pulsed lasers, and the quasi-equilibrium properties are then estimated from the carrier rates and lifetimes. In this work, we prove that steady-state, photomodulated x-ray spectra from a non-time-resolved synchrotron beamline can be used to directly measure electron and hole distributions. A set of plasmonic metal core-shell nanoparticles is designed to systematically isolate photothermal, hot electron, and thermalized electron-hole pairs in a TiO₂ shell. Steady-state changes in the Ti L_2,3 edge are measured with and without continuous-wave illumination of the nanoparticle’s localized surface plasmon resonance. Ab initio excited-state x-ray theory is then applied to model the experimental spectra and extract the steady-state carrier distributions and lattice temperature. For example, we measure that the quasi-equilibrium hot electron distribution exists up to 0.3 eV above the TiO₂ conduction band minimum. The ability to separate heating and quasi-equilibrium carrier distributions from mixed excited-state phenomena opens new avenues for in-situ and operando measurements from non-time-resolved beamlines.    

Revealing Molecular Optomechanics Induced Hybrid Properties in Soft Materials Filled Plasmonic Nanocavities

Recent advances indicate that enhanced light-matter interaction in plasmonic nanocavities can create hybrid properties in integrated plasmonic metal nanostructures and soft materials. Here, by integrating polyelectrolytes and surface ligands in gold nanorod-on-mirror nanocavities and detecting the nanocavity resonance and vibrational Raman scattering simultaneously, we found that the plasmon-vibration interaction modifies both the nanocavity and molecular responses. Large enhancement of Raman scattering accompanied by the plasmon resonance linewidth broadening are observed as the laser-plasmon detuning approaches the CH vibrational frequency of the molecular systems in the nanocavities. The experimental observations are consistent with the molecular optomechanics theory that predicts dynamical backaction amplification of the vibrational modes and high sensitivity of Raman scattering when the plasmon resonance overlaps with the Raman emission frequency. The results presented here suggests that molecular optomechanics coupling may be manipulated for creating hybrid properties based on quantum mechanical interaction of molecular oscillators and nanocavity electromagnetic optical modes.   

Exploring the role of disorder on organic exciton-polaritons

Exciton polaritons, arising from the interaction of electronic transitions with confined electromagnetic fields, have emerged as a powerful tool to manipulate the properties of organic materials. However, standard experimental and theoretical approaches overlook the significant energetic disorder present in most materials now studied. Using the conjugated polymer P3HT as a model platform, we systematically tune the degree of energetic disorder and observe a corresponding redistribution of photonic character within the polariton manifold. Based on these subtle spectral features, we develop a more generalized approach to describe strong light-matter coupling in disordered systems that captures the key spectroscopic observables and provides a description of the rich manifold of states intermediate between bright and dark. Applied to a wide range of organic systems, our method challenges prevailing notions about ultrastrong coupling and whether it can be achieved with broad, disordered absorbers.   

Tracking triplet pair state in singlet fission using exciton-polariton: A search for light in the dark

Exciton-polaritons, by virtue of mixed photon and exciton character, possess the potential to modify the properties of organic materials by manipulating their energetics under ambient conditions. Here we explore that potential using singlet fission (SF) in organic photovoltaics as a model process, where two individual excited triplets can be generated at the expense of one photoexcited singlet. This conversion is enabled by an intermediate triplet pair state, which remains enigmatic despite the recent wide-ranging efforts in developing new fission materials. The triplet pair states are generally non-emissive or "dark", hence traditional optical spectroscopic techniques have permitted determination of the triplet-pair energy in only a small subset of materials. Hence, we investigate the microcavities of TIPS-pentacene, a popular SF agent with uncharacterized triplet-pair energy. By analyzing the temperature-dependence and lifetime of lower polariton emission as a function of detuning, we identify contributions to the polariton population from two distinct intra-reservoir states. The lower of these tracks the reported behavior of triplet pairs in transient absorption, permitting direct assignment of the triplet-pair energy from the polariton emission. This finding opens a new avenue of using exciton-polaritons to map the energetic structure of functionally important but poorly characterized dark electronic states and turning them on in the process towards a promising direction.   

Nonlinearities and Electrical Field Effects on Merocyanine Exciton-Polariton Cavities

Recently a significant attention has been given to organic exciton-polaritons due to their promise to alter chemical reactions and realization of Bose-Einstein condensates at higher temperatures. Organic exciton-polaritons are quasiparticles emerging from hybridization of localized Frenkel excitons and photon modes in Fabry-Perot cavities. In resonance, this light and matter state appears as Rabi-split modes called upper and lower polaritons. Described by the Tavis-Cummings model, the splitting of this two level system increases with the concentration of the active molecule, however, the high level of disorder in organic thin films has raised concerns on the aplicability of this model. Alongside the additional high number of degrees of freedom of single molecules, disorder results in broadened absorption peaks. Here, we address one category of disorder, transition dipole moment orientational disorder, through the use of merocyanine dyes known for their hypsochromic/electrochromic effects and high dipole moments. Owing to their non-centrosymmetric donor and acceptor structure, these dyes possess large second-order hyperpolarizability values. We apply capacitance electrical fields in Fabry-Perot microcavities and perform steady-state reflectivity as well as electroreflectance measurements to assess the degree to which polariton Rabi-splitting deviates from the zero-field behavior and the extent to which these quasiparticles inherit the nonlinear properties of molecular excitons.   

Oral: Resonance Theory of Vibrational Polariton Chemistry

We perform numerically exact quantum dynamics simulations using the hierarchical equation of motion (HEOM) approach to investigate the resonance modification of chemical reaction rate constants due to the vibrational strong coupling (VSC) in polariton chemistry. The results reveal that the cavity mode acts like a ``rate-promoting vibrational mode" that enhances the ground state chemical reaction rate constant when the cavity mode frequency matches the vibrational transition frequency. The VSC-modified rate constant will first increase quadratically, then quickly saturate and decay as the Rabi splitting $Omega_R$ increases. When changing the cavity lifetime $ au_c$ from the lossy to the lossless limit, the numerical results show there will be a turnover of the rate constant. With given $Omega_R$, the resonance enhancement of VSC rate is proportional to $ au_c$ in the lossy limit of $ au_c ll 1 / Omega_R$, and to $1 / au_c$ in the lossless limit of $ au_c gg 1 / Omega_R$. We further present an analytic rate theory based on Fermi's golden rule to explain the observed behaviors of VSC rates, including the sharp resonance peak and origin of its broadening, the effects of $Omega_R$ and $ au_c$, resonance condition at the normal incidence, etc. To the best of our knowledge, this is the first analytic theory that clearly illustrates the reaction mechanism under VSC modifications, and is able to explain the sharp resonance behavior of the VSC-modified rate profile with quantitative accuracy.   

MXenes as Hydrogen Storage Materials

The ever-increasing need for electricity will require foremost increased efficiency in the uses of electric energy, more secure and sustainable energy resources and storages.  To be better tailored to these challenges, novel materials for electrochemical and chemical energy storages that can efficiently store and deliver electric energy is highly important. In this case, hydrogen fuel stands out as a promising energy solution, and able offering a clean alternative to conventional energy sources while exhibiting the highest specific energy among many alternatives. Due to their high aspect ratio and tunable surface, slit-shape ion/mass transport channels, MXenes is a promising candidate in hydrogen storage materials. To address this issue, I will share with you our recent efforts based on theoretical studies with experimental supports from our collaborators. Based on our findings, some key avenues for future research that may help overcome the challenges and enable MXenes materials attain its full potential in this problem will be discussed.   

Nanoscale X-ray Tomography of Mesoporous Particle Improved via Adaptive Multidistance Coherent Diffraction Imaging

Mesoporous nanoparticles provide rich platforms to devise functional materials by customizing the three-dimensional (3D) structures of nanopores. With the pore network as a key tuning parameter, the noninvasive and quantitative characterization of these 3D structures is crucial for the rational design of functional materials. This has prompted researchers to develop versatile nanoprobes with a high penetration power to inspect various specimens sized a few micrometers at nanoscale 3D resolutions. Here, with adaptive phase retrievals on independent data sets with different sampling frequencies, we introduce multidistance coherent X-ray tomography as a noninvasive and quantitative nanoprobe to realize high-resolution 3D imaging of micrometer-sized specimens. The 3D density distribution of an entire mesoporous silica nanoparticle was obtained at 13 nm 3D resolution for quantitative physical and morphological analyses of its 3D pore structure. The morphological features of the whole 3D pore network and pore connectivity were examined to gain insight into the potential functions of the particles. The proposed multidistance tomographic imaging scheme with quantitative structural analyses is expected to advance studies of functional materials by facilitating their structure-based rational design.