Session B40: Probing Structure and Dynamics with XUV and X-Ray Light: Chemical Dynamics in Molecular and Plasmonic Systems

Multipulse Femtosecond X-ray Spectroscopy in Solution

The development of commercial femtosecond laser systems has resulted in pump-probe spectroscopy becoming an essential tool to discover and understand complex electronic and structural dynamics in solvated molecular, biological, and material systems. Here we use two femtosecond hard X-ray pulses from LCLS to perform a two-color X-ray pump X-ray probe transient absorption experiment in solution. The X-ray pump creates a localized excitation by removing a 1s electron from an Fe atom in solvated ferro- and ferricyanide complexes. Following the ensuing Auger–Meitner cascade in the complexes, novel electronic excited states with 3p holes are generated. The second X-ray probe pulse interrogates the Fe 1s→3p transitions. The experimental results are compared with theory to extract +2 eV shifts in transition energies per valence 3d hole in the iron complexes. We gain insight into correlated interactions of valence 3d with 3p and deeper-lying electrons.   

Characterizing Transient Species after Photodissociation of Ironpentacarbonyl in Solution

Following the ultra-fast dynamics in the photodissociation of ironpentacarbonyl [1], the system can through interactions in solution open channels to new intermediatary species as shown in ongoing and previous spectroscopic studies [2]. Through ab initio molecular dynamics simulations [3], we are investigating the solvation of the iron tetracarbonyl species in different spin states, which shows a distinct difference in how ethanol interacts with the iron center, yield a strong Fe-O bond in the singlet state. Also the energetics of different configurations in the singlet state is discussed. The inter-system crossing leading to decay of the triplet species is discussed. Also, the link to experimental validation of the theoretical models are discussed.   

Tracking photoinduced electron and proton transfer and the coupled solvent reorganization with femtosecond X-rays

Light-induced electron transfer (ET) and proton-coupled electron-transfer (PCET) reactions are ubiquitous in chemistry, playing an important role in both natural and artificial solar energy conversion. To advance these applications, it is vital to achieve a molecular-level understanding of this important class of reactions. I will present investigations with time-resolved X-ray methods of light-triggered ET and PCET reactions in metal-based donor-acceptor complexes, with focus in considering explicitly the interaction of such molecular systems with the surrounding solvent.

As previously demonstrated, time-resolved X-ray solution scattering (XSS) can directly probe solvation dynamics and is uniquely sensitive to photoinduced changes in solute-solvent hydrogen bonding. Simultaneously, time-resolved X-ray spectroscopy can probe charge reorganization with element specificity as well as provide direct evidence of protonation. I have utilized these methods to capture ultrafast ET and coupled solvent reorganization in a tri-metallic mixed-valence complex in a series of solvent environments with varying hydrogen bonding ability. I will also show femtosecond XSS and N K-edge X-ray absorption data reporting on PCET in a Ru-polypyridyl complex in aqueous environment and coupled solvent reorganization. The experimental data will be shown together with theoretical models necessary to their interpretation, including molecular dynamics and time-dependent density functional theory calculations.      

Photodissociation of Fe(CO)5: Insights from femtosecond core-level spectroscopy and theory

After 266 nm excitation, gas-phase Fe(CO)₅, a prototypical photocatalyst, undergoes sequential CO dissociation to form Fe(CO)₃. The initial photoexcitation is ascribed to metal-to-ligand charge transfer states, which rapidly relax to dissociative metal-centered states. However, spectroscopic detection of the intricate excited state pathways responsible for sequential dissociation have long been elusive. Using femtosecond extreme ultraviolet transient absorption spectroscopy near the Fe M_2,3-edge, we have achieved the first spectroscopic characterization of the electronic dynamics during Fe(CO)₅ photodissociation. Using non-orthogonal configuration interaction singles calculations, which can treat core-to-valence transitions of two-electron open-shell states, we uncover the spectroscopic signatures of the intertwined structural and electronic evolution among the metal-centered excited states during the first CO loss from Fe(CO)₅ on a 100-fs time scale. Furthermore, the evolution of spectroscopic signals associated with the formation of Fe(CO)₄ in its closed-shell singlet surface in C_2v and C_3v geometries, and its subsequent picosecond dissociation into Fe(CO)₃ in its C_s geometry, are corroborated with electron-affinity time-dependent density functional theory.   

Tabletop M-edge XANES reveals hidden states in molecular transition metal photocatalysts

X-ray absorption near edge spectroscopy (XANES) is a powerful technique for electronic structure determination.  Recent developments in extreme ultraviolet (XUV) light sources using the laser-based technique of high-harmonic generation have enabled core-level spectroscopy to be performed on femtosecond to attosecond timescales.  We have extended the scope of tabletop XUV spectroscopy and demonstrated that M_2,3-edge XANES, corresponding to 3p→3d transitions, can reliably measure the electronic structure of first-row molecular transition metal complexes with femtosecond time resolution.  In the same 40-100 eV energy range, the 5p→5d and 4f→5d transitions give similar information about short-lived states in third-row metal complexes. We use this ability to track the excited-state relaxation pathways of photocatalysts and chromophores, uncovering hidden loss mechanisms and providing new design principles for transition metal photochemistry.   

Modeling Atomistic Dynamics in Complex Environments

The ensemble charge transfer embedded atom method (ECT-EAM)¹ is a physics-based interaction potential designed for atomistic simulations of molecules and materials with strong directional features. Examples include defected materials (surfaces, dislocations, interstitials), compositionally-complex alloys, and compounds with multivalent elements. The quantum mechanical effects of charge distortion and charge transfer are described by ensemble representations of the total electron density in terms of atomic basis densities with dynamically-evolving weights, and corresponding ensemble embedding and electrostatic potential energy components. We describe the implementation of ECT-EAM and its application to the potential energy surfaces of paradigm molecular systems exemplifying the diverse, localized bonding patterns of solid state materials.

[1] S. R. Atlas. “Embedding quantum statistical excitations in a classical force field,” J. Phys. Chem. A, 125, 3760 (2021);  K. Muralidharan, S. M. Valone, and S. R. Atlas. “Environment dependent charge potential for water,” arXiv:0705.0857 [cond-mat.mtrl-sci] (2007).   

Electron Dynamics of Plasmonic Light Harvesting Studied by Ultrafast Time-Resolved Ambient-Pressure X-ray Photoelectron Spectroscopy

Heterogeneous light harvesting systems consisting of metal nanoparticles interfaced with transition metal semiconductors are among the most studied platforms for solar photon based approaches to sustainable energy supplies and climate change mitigation. Yet, it remains challenging to disentangle the fundamental electronic dynamics and mechanisms that drive the targeted photocatalytic activity, such as solar fuels production or CO₂ reduction. To address this challenge, we translate the atomic-scale sensitivity of X-ray photoemission spectroscopy (XPS) to interfacial electronic and chemical configurations into the ultrafast time-domain. Utilizing picosecond time-resolved ambient pressure XPS (TRAPXPS) at the Advanced Light Source synchrotron and femtosecond TRXPS measurements at the FLASH Free Electron Laser, we study photoinduced charge transfer dynamics in gold nanoparticle sensitized TiO₂ under ultrahigh vacuum conditions as well as under exposure to water. Dramatic differences between dynamics at dry and water-exposed interfaces indicate that conditions for photocatalytic activity of the interface improve substantially with the introduction of the reactant. First steps have been undertaken to complement the experiments by ab initio calculations and reveal the underlying mechanisms.   

Probing strong exciton-plasmon coupling via hot-electron electroluminescence

In electrically driven plasmonic tunnel junctions, electroluminescent emission can take place through the radiative recombination of hot electrons and holes, via the plasmon-modified photonic density of states.  We use this electroluminescence to probe plasmon- exciton coupling in hybrid structures, each consisting of a nanoscale plasmonic tunnel junction and few-layer WSe₂ flake transferred onto the junction. The coupling between the junction local surface plasmons and the excitons in the semiconductor leads to a photonic density of states with a strong plexcitonic splitting and polarization-dependent emission.  We use the above-threshold hot carrier electroluminescence to extract the photonic density of states and find Rabi splittings exceeding 50meV in strong coupling regime. Electromagnetic simulations explain the emergence of plexciton polaritons as well as the polarization characteristics of their far-field emission in the presence of other plasmon modes. Electroluminescence modulated by plexciton coupling is a promising approach for compact electrically driven light sources.   

Modification of chemical reactivity via light–matter coherence

Experimental evidence of chemical reactivity in cavity quantum electrodynamics shows modifications of reaction rates under strong light-matter coupling in contrast with out-of-cavity scenario [1,2].  However, the development of general theories to understand and reproduce experimental measurements remains a challenge. We propose a quantum mechanical model that describes the reaction-rate suppression of up to 80% observed in experiments of phenyl isocyanate alcoholysis with cyclohexanol in a Fabry-Perot cavity [3]. We implement a Lindblad quantum master equation that describes the reactive vibrational NCO mode for an ensemble of phenyl isocyanate molecules coupled to a resonant electromagnetic cavity vacuum. Rate suppressions are predicted as a consequence of the resonant depopulation of the reactive vibrational mode, relative to a canonical Boltzmann distribution. We show that energetic disorder protects the light-matter coherences of an ensemble of molecules from many-body dilution of correlations, in agreement with recent work on disorder-assisted entanglement protection [4]. Our findings extend the understanding of cavity-modified chemistry and suggests connections with quantum science that have yet to be explored.

[1] F. Herrera & J. Owrutsky, J. Chem. Phys., 152(10) (2020).

[2] J. A. Hutchison, et al., Angew. Chem., 51(7), 1592-1596 (2012).

[3] W. Ahn, et al., Science, 380(6650), 1165-1168 (2023).

[4] D. Wellnitz, et. al., Commun. Phys., 5(1), 120 (2022).