Ultrafast Intersystem Crossing in Acetylacetone via Femtosecond X-ray Transient Absorption at the Carbon K-Edge

Capturing Ring Opening in Photoexcited Enolic Acetylacetone upon Hydrogen Bond Dissociation by Ultrafast Electron Diffraction

Photoinduced biological and chemical reactions are often based on key structural transformations of a molecule driven across multiple electronic states. Acetylacetone (AcAc) is a prototypical system for complex chemical pathways involving several conical intersections (CI) and singlet-triplet intersystem crossings (ISC) characterized by distinct geometries. In the gas phase, AcAc is predominantly in a planar ring-like enolic form stabilized by a strong intramolecular O-H···O hydrogen bond. Following excitation into the S2 (ππ*) state at 266 nm, acetylacetone undergoes rapid internal conversion followed by intersystem crossing. Such relaxation pathways are associated with structural changes including ring opening, deplanarization, and bond elongation. In this work, ultrafast electron diffraction (UED) at the SLAC MeV-UED setup is employed as a direct structural probe with a time resolution of 160 fs. Together with trajectory surface hopping simulations, analysis of the UED data provides a new perspective on the early time nuclear dynamics in acetylacetone. Specifically, AcAc is observed to undergo ring opening, deplanarization, and bond elongation all within the first 700 fs after photoexcitation. The monitored dynamics is associated mainly with the nuclear motion on the S1 potential energy surface, formed after very rapid transfer from S2 to S1, allowing AcAc to reach the conical intersection to intersystem crossing. Such time scales of nuclear motion are contrasted with the time scales of electronic transitions in AcAc that were previously characterized with spectroscopic methods, specifically internal conversion (<100 fs) and intersystem crossing (∼1.5 ps).

Ultrafast Nonadiabatic Electron Relaxation Dynamics in Photoexcited C60 Molecules

Fullerene molecules, being attractive for fundamental research and key building blocks in materials of energy harvesting, are important for ultrafast electron transfer studies. The nonradiative electron-relaxation dynamics in a C60 molecule is investigated after chosen initial photoexcitations. The methodology includes nonadiabatic molecular simulation combined with time-dependent density functional theory and a semiclassical surface hopping approach. Results treating the exchange-correlation by using hybrid functionals, Becke three-parameter Lee-Yang-Parr (B3LYP) and Perdew-Burke-Ernzerhof (PBE0), are presented. Both approaches produce similar unoccupied band structures in the ground state that qualitatively agree with our many-electron excited state calculation. The model-dependent differences in the ultrafast population dynamics, including the transient entrapment of the population, are studied systematically. The trend of the results demonstrates a universal dependence on the structure of the unoccupied band offering a spectroscopic route to probe the structure. Predictions can be assessed by comparison with ultrafast transient absorption or time-resolved photoelectron spectroscopy measurements. By selectively comparing with inexpensive nonempirical PBE results, the study facilitates method optimization for future studies of technologically important and larger fullerene complexes.

Local dynamics drive the C-CX3 (X = H and F) bond photodissociation in acetylacetones

The primary photodissociation events of acetylacetone and its fluorinated analogs reveal that the translational energy distribution profiles of the CH3 and CF3 radicals follow a barrier-impulsive model for the C-C bond cleavage. Analysis based on the one-dimensional potential energy surfaces in the T1 state, as well as dynamics simulations using on-the-fly semi-empirical potentials, suggest that the C-C bond cleavage proximal to the OH group, in general, is accompanied by proton migration. Interestingly, the near identical fragment translational energy distribution profiles of CH3 radical release from acetylacetone and trifluoroacetylacetone, as well as CF3 radical release from trifluoroacetylacetone and hexafluoroacetylacetone, suggest that the dynamics of formation of CH3/CF3 radicals in acetylacetones appears to be independent of the nature of the substituent on the other end of the molecule. In the case of acetylacetones, the C-C bond cleavage is governed by the local intramolecular vibrational redistribution along the complex reaction coordinate, which appears to be statistical for CF3 release and non-statistical for CH3 release; however, it remains non-statistical over the entire molecular framework.

Low Energy Electron Induced Dissociation of Acetylacetone

We report a detailed investigation of the dissociative electron attachment (DEA) to acetylacetone, an important molecule for industrial and atmospheric chemistry. We find that the DEA process leads to several dissociation channels, leading to the formation of H - , CH 3 - , O - , OH - and other anions with m/z 39, 41, 43, 57, 83, and 85, with the H- and OH- being the most dominant channels. The absolute cross sections for both of these channels peak around 8.8 and 9 eV with cross-section values of 3.1 × 10-18 and 3.7 × 10-19 cm2, respectively. We also report the DEA dynamics for these channels based on the Velocity Slice Images that were obtained. We also compare the dissociation pattern from DEA with that from photodissociation reported for 248 and 266 nm wavelengths and propose the possibility of the neutral excited state's role as the underlying parent state for the negative ion resonance.

Exploring the Influence of Approximations for Simulating Valence Excited X-ray Spectra

First-principles simulations of excited-state X-ray spectra are becoming increasingly important to interpret the wealth of electronic and geometric information contained within femtosecond X-ray absorption spectra recorded at X-ray Free Electron Lasers (X-FELs). However, because the transition dipole matrix elements must be calculated between two excited states (i.e., the valence excited state and the final core excited state arising from the initial valence excited state) of very different energies, this can be challenging and time-consuming to compute. Herein using two molecules, protonated formaldimine and cyclobutanone, we assess the ability of n-electron valence-state perturbation theory (NEVPT2), equation-of-motion coupled-cluster theory (EOM-CCSD), linear-response time-dependent density functional theory (LR-TDDFT) and the maximum overlap method (MOM) to describe excited state X-ray spectra. Our study focuses in particular on the behavior of these methods away from the Franck-Condon geometry and in the vicinity of important topological features of excited-state potential energy surfaces, namely, conical intersections. We demonstrate that the primary feature of excited-state X-ray spectra is associated with the core electron filling the hole created by the initial valence excitation, a process that all of the methods can capture. Higher energy states are generally weaker, but importantly much more sensitive to the nature of the reference electronic wave function. As molecular structures evolve away from the Franck-Condon geometry, changes in the spectral shape closely follow the underlying valence excitation, highlighting the importance of accurately describing the initial valence excitation to simulate the excited-state X-ray absorption spectra.

Unravelling structural features of small molecules for photochemical transformation of environmental contaminants

Small molecules, including natural metabolites, organic matter decomposition products, and engineered oxidation byproducts, are widespread in aquatic environment. However, the limited understanding of the photochemical interactions of these small molecules with water pollutants hampers the development of effective environmental protection strategies. This study explores the structural features governing the photochemical transformation of toxic oxyanions by α- and β-dicarbonyl compounds. By integrating experimental observations with quantum chemical calculations, a robust correlation network was constructed. The correlation network reveals that the reactivity of small organic molecules with oxyanions could be quantitively predicted by their intrinsic properties, such as electronic transition energy, bond dissociation energy, molecular softness, molecular orbital gap, atomic charge, and molecular surface local ionization energy. This network maps the relationship between the molecular architecture of chemicals and their photochemical behaviors. This perspective offers fresh insights into the photochemical behaviors of small molecules in diverse environmental and chemical contexts and are helpful for developing advanced water treatment strategies toward a sustainable future.

Mapping Hydrogen Positions along the Proton Transfer Pathway in an Organic Crystal by Computational X-ray Spectra

Understanding proton transfer (PT) dynamics in condensed phases is crucial in chemistry. We computed a 2D map of N 1s X-ray photoelectron/absorption spectroscopy (XPS/XAS) for an organic donor-acceptor salt crystal against two varying N-H distances to track proton motions. Our results provide a continuous spectroscopic mapping of O-H···N↔O-··· H+-N processes via hydrogen bonds at both nitrogens, demonstrating the sensitivity of N 1s transient XPS/XAS to hydrogen positions and PT. By reducing the O-H length at N1 by only 0.2 Å, we achieved excellent theory-experiment agreement in both XPS and XAS. Our study highlights the challenge in refining proton positions in experimental crystal structures by periodic geometry optimizations and proposes an alternative scaled snapshot protocol as a more effective approach. This work provides valuable insights into X-ray spectra for correlated PT dynamics in complex crystals, benefiting future experimental studies.

WFOT: A Wave Function Overlap Tool between Single- and Multi-Reference Electronic Structure Methods for Spectroscopy Simulation

We report the development of a novel diagnostic tool, named wave function overlap tool (WFOT), designed to evaluate the overlap between wave functions computed at single-reference [i.e., time-dependent density functional theory or configuration interaction singles (CIS)] and multireference (i.e., CASSCF/CASPT2) electronic structure levels of theory. It relies on truncating the single- and multireference WFs to CIS-like expansions spanning the same configurational space and maximizing the molecular orbital overlap by means of a unitary transformation. To demonstrate the functionality of the tool, we calculate the transient spectrum of acetylacetone by evaluating excited state absorption signals with multireference quality on top of single-reference on-the-fly dynamics simulations. Semiautomatic spectra generation is facilitated by interfacing the tool with the COBRAMM package, which also allows one to use WFOT with several quantum chemistry codes such as Gaussian, NWChem, and OpenMolcas. Other exciting possibilities for the utilization of the code beyond the simulation of transient absorption spectroscopy are eventually discussed.

Reinvented: An Attosecond Chemist

Attosecond science requires a substantial rethinking of how to make measurements on very short timescales; how to acquire the necessary equipment, technology, and personnel; and how to build a set of laboratories for such experiments. This entails a rejuvenation of the author in many respects, in the laboratory itself, with regard to students and postdocs, and in generating funding for research. It also brings up questions of what it means to do attosecond science, and the discovery of the power of X-ray spectroscopy itself, which complements the short timescales addressed. The lessons learned, expressed in the meanderings of this autobiographical article, may be of benefit to others who try to reinvent themselves.

EuIII and TbIII upconversion intermediated by interparticle energy transfer in functionalized NaLnF4 nanoparticles

Lanthanide (LnIII)-doped sodium gadolinium tetrafluoride (NaGdF4) nanoparticles have been excelled as attractive upconversion systems for anti-counterfeiting or energy conversion for instance, with a special interest in the visible upconversion of EuIII and TbIII. The core@shell architecture has enabled the bright upconversion of EuIII and TbIII in this matrix by interfacial energy transfer sensibilized by the TmIII/YbIII pair. Another approach to enable EuIII and TbIII upconversion could be the interparticle energy transfer (IPET) between LnIII-doped sensitizer and acceptor nanoparticles. Yet, the low molar absorptivity of the LnIII through 4f ↔ 4f electronic transitions and the large distance between the nanoparticles are shortcomings that should decrease the energy transfer efficiency. On the other hand, it is feasible to predict that the association of organic ligands displaying large molar absorptivity on the acceptor nanoparticle surface could help to overcome the absorption limitation. Inspired by this exciting possibility, herein, we present the EuIII/TbIII upconversion intermediated by IPET between the donor TmIII, YbIII-doped NaGdF4 nanoparticle and the acceptor LnIII-doped NaGdF4 (Ln = Eu and/or Tb) nanoparticles functionalized with a series organic ligands on the surface (tta- = thenoyltrifluoroacetonate, acac- = acetylacetonate, or 3,5-bbza- = 3,5-dibromebenzoate). Either in solid state or in suspension, upon excitation at 980 nm, visible EuIII/TbIII upconversion could be observed. This emission comes from the absorption of the TmIII, YbIII pair in the donor nanoparticle, followed by IPET from the TmIII excited levels to the ligand singlet/triplet states on the acceptor nanoparticle surface, ligand-to-EuIII/TbIII energy transfer, and upconversion emission. Spectroscopic evidences from the analysis of the donor level lifetimes indicate the contribution of non-radiative energy transfer for the IPET mechanism; the radiative mechanism also contributes for the IPET. Moreover, the design herein introduced enables the development of luminescence temperature probes with relative thermal sensitivity as high as 1.67% K-1 at 373 K. Therefore, this new upconversion pathway opens an avenue of possibilities in an uncharted territory to tune the visible upconversion of LnIII ions.

Time-resolved x-ray spectroscopy of nucleobases and their thionated analogs

The photoinduced relaxation dynamics of nucleobases and their thionated analogs have been investigated extensively over the past decades motivated by their crucial role in organisms and their application in medical and biochemical research and treatment. Most of these studies focused on the spectroscopy of valence electrons and fragmentation. The advent of ultrashort x-ray laser sources such as free-electron lasers, however, opens new opportunities for studying the ultrafast molecular relaxation dynamics utilizing the site- and element-selectivity of x-rays. In this review, we want to summarize ultrafast experiments on thymine and 2-thiouracil performed at free-electron lasers. We performed time-resolved x-ray absorption spectroscopy at the oxygen K-edge after UV excitation of thymine. In addition, we investigated the excited state dynamics of 2-tUra via x-ray photoelectron spectroscopy at sulfur. For these methods, we show a strong sensitivity to the electronic state or charge distribution, respectively. We also performed time-resolved Auger-Meitner spectroscopy, which shows spectral shifts associated with internuclear distances close to the probed site. We discuss the complementary aspects of time-resolved x-ray spectroscopy techniques compared to optical and UV spectroscopy for the investigation of ultrafast relaxation processes.

Spin Coupling Effect on Geometry-Dependent X-Ray Absorption of Diradicals

We theoretically investigate the influence of diradical electron spin coupling on the time-resolved X-ray absorption spectra of the photochemical ring opening of furanone. We predict geometry-dependent carbon K-edge signals involving transitions from core orbitals to both singly and unoccupied molecular orbitals. The most obvious features of the ring opening come from the carbon atom directly involved in the bond breaking through its transition to both the newly formed singly occupied and the available lowest unoccupied molecular orbitals (SOMO and LUMO, respectively). In addition to this primary feature, the singlet spin coupling of four unpaired electrons that arises in the core-to-LUMO states creates additional geometry dependence in some spectral features with both oscillator strengths and relative excitation energies varying observably as a function of the ring opening. We attribute this behavior to a spin-occupancy-induced selection rule, which occurs when singlet spin coupling is enforced in the diradical state. Notably, one of these geometry-sensitive core-to-LUMO transitions excites core electrons from a backbone carbon not involved in the bond breaking, providing a novel nonlocal X-ray probe of chemical dynamics arising from electron spin coupling.

Generalization of One-Center Nonorthogonal Configuration Interaction Singles to Open-Shell Singlet Reference States: Theory and Application to Valence-Core Pump-Probe States in Acetylacetone

We formulate a one-center nonorthogonal configuration interaction singles (1C-NOCIS) theory for the computation of core excited states of an initial singlet state with two unpaired electrons. This model, which we refer to as 1C-NOCIS two-electron open-shell (2eOS), is appropriate for computing the K-edge near-edge X-ray absorption spectra (NEXAS) of the valence excited states of closed-shell molecules relevant to pump-probe time-resolved (TR) NEXAS experiments. With the inclusion of core-hole relaxation effects and explicit spin adaptation, 1C-NOCIS 2eOS requires mild shifts to match experiment, is free of artifacts due to spin contamination, and can capture the high-energy region of the spectrum beyond the transitions into the singly occupied molecular orbitals (SOMOs). Calculations on water and thymine illustrate the different key features of excited-state NEXAS, namely, the core-to-SOMO transitions as well as shifts and spin-splittings in the transitions analogous to those of the ground state. Simulations of the TR-NEXAS of acetylacetone after excitation to its π → π* singlet excited state at the carbon K-edge, an experiment carried out recently, showcase the ability of 1C-NOCIS 2eOS to efficiently simulate NEXAS based on nonadiabatic molecular dynamics simulations.

Toward ultrafast soft x-ray spectroscopy of organic photovoltaic devices

Novel ultrafast x-ray sources based on high harmonic generation and at x-ray free electron lasers are opening up new opportunities to resolve complex ultrafast processes in condensed phase systems with exceptional temporal resolution and atomic site specificity. In this perspective, we present techniques for resolving charge localization, transfer, and separation processes in organic semiconductors and organic photovoltaic devices with time-resolved soft x-ray spectroscopy. We review recent results in ultrafast soft x-ray spectroscopy of these systems and discuss routes to overcome the technical challenges in performing time-resolved x-ray experiments on photosensitive materials with poor thermal conductivity and low pump intensity thresholds for nonlinear effects.

Electronic Structure of the Complete Series of Gas-Phase Manganese Acetylacetonates by X-ray Absorption Spectroscopy

Metal centers in transition metal-ligand complexes occur in a variety of oxidation states causing their redox activity and therefore making them relevant for applications in physics and chemistry. The electronic state of these complexes can be studied by X-ray absorption spectroscopy, which is, however, due to the complex spectral signature not always straightforward. Here, we study the electronic structure of gas-phase cationic manganese acetylacetonate complexes Mn(acac)1-3+ using X-ray absorption spectroscopy at the metal center and ligand constituents. The spectra are well reproduced by multiconfigurational wave function theory, time-dependent density functional theory as well as parameterized crystal field and charge transfer multiplet simulations. This enables us to get detailed insights into the electronic structure of ground-state Mn(acac)1-3+ and extract empirical parameters such as crystal field strength and exchange coupling from X-ray excitation at both the metal and ligand sites. By comparison to X-ray absorption spectra of neutral, solvated Mn(acac)2,3 complexes, we also show that the effect of coordination on the L3 excitation energy, routinely used to identify oxidation states, can contribute about 40-50% to the observed shift, which for the current study is 1.9 eV per oxidation state.

Acetylacetone Photolysis at 280 nm Studied by Velocity-Map Ion Imaging

The photolysis of acetylacetone (AcAc) has been studied using velocity-map ion imaging with pulsed nanosecond lasers. The enolone tautomer of AcAc (CH3C(O)CH═C(OH)CH3) was excited in the strong UV absorption band by UV pulses at 280 nm, preparing the S2(ππ*) state, and products were probed after a short time delay by single-photon VUV ionization at 118.2 nm. Two-color UV + VUV time-of-flight mass spectra show enhancement of fragments at m/z = 15, 42, 43, 58, and 85 at the lowest UV pulse energies and depletion of the parent ion at m/z = 100. Ion images of the five major fragments are all isotropic, indicating dissociation lifetimes that are long on the timescale of molecular rotation but shorter than the laser pulse duration (<6 ns). The m/z = 15 and 85 fragments have identical momentum distributions with moderate translational energy release, suggesting that they are formed as a neutral product pair and likely via a Norrish type I dissociation of the enolone to form CH3 + C(O)CH═C(OH)CH3 over a barrier on a triplet surface. The m/z = 43 fragment may be tentatively assigned to the alternative Norrish type I pathway that produces CH3CO + CH2C(O)CH3 on S0 following phototautomerization to the diketone, although alternative mechanisms involving dissociative ionization of a larger primary photoproduct cannot be conclusively ruled out. The m/z = 42 and 58 fragments are not momentum-matched and consequently are not formed as a neutral pair via a unimolecular dissociation pathway on S0. They also likely originate from the dissociative ionization of primary photofragments. RRKM calculations suggest that unimolecular dissociation pathways that lead to molecular products on S0 are generally slow, implying an upper-limit lifetime of <46 ns after excitation at 280 nm. Time-dependent measurements suggest that the observed photofragments likely do not arise from dissociative ionization of energized AcAc S0*.

Core-Projected Hybrids Fix Systematic Errors in Time-Dependent Density Functional Theory Predicted Core-Electron Excitations

Linear response time-dependent density functional theory (TDDFT) is widely applied to valence, Rydberg, and charge-transfer excitations but, in its current form, makes large errors for core-electron excitations. This work demonstrates that the admixture of nonlocal exact exchange in atomic core regions significantly improves TDDFT-predicted core excitations. Exact exchange admixture is accomplished using projected hybrid density functional theory [ J. Chem. Theory Comput. 2023, 19, 837-847]. Scalar relativistic TDDFT calculations using core-projected B3LYP accurately model core excitations of second-period elements C-F and third-period elements Si-Cl, without sacrificing performance for the relative shifts of core excitation energies. Predicted K-edge X-ray near absorption edge structure (XANES) of a series of sulfur standards highlight the value of this approach. Core-projected hybrids appear to be a practical solution to TDDFT's limitations for core excitations, in the way that long-range-corrected hybrids are a practical solution to TDDFT's limitations for Rydberg and charge-transfer excitations.

Tuning UV Pump X-ray Probe Spectroscopy on the Nitrogen K Edge Reveals the Radiationless Relaxation of Pyrazine: Ab Initio Simulations Using the Quasiclassical Doorway-Window Approximation

Transient absorption UV pump X-ray probe spectroscopy has been established as a versatile technique for the exploration of ultrafast photoinduced dynamics in valence-excited states. In this work, an ab initio theoretical framework for the simulation of time-resolved UV pump X-ray probe spectra is presented. The method is based on the description of the radiation-matter interaction in the classical doorway-window approximation and a surface-hopping algorithm for the nonadiabatic nuclear excited-state dynamics. Using the second-order algebraic-diagrammatic construction scheme for excited states, UV pump X-ray probe signals were simulated for the carbon and nitrogen K edges of pyrazine, assuming a duration of 5 fs of the UV pump and X-ray probe pulses. It is predicted that spectra measured at the nitrogen K edge carry much richer information about the ultrafast nonadiabatic dynamics in the valence-excited states of pyrazine than those measured at the carbon K edge.

A theoretical study of the time-resolved x-ray absorption spectrum of the photoionized BT-1T cation

The time-resolved x-ray absorption spectrum of the BT-1T cation (BT-1T+) is theoretically simulated in order to investigate the charge transfer reaction of the system. We employ both trajectory surface hopping and quantum dynamics to simulate the structural evolution over time and the changes in the state populations. To compute the static x-ray absorption spectra (XAS) of the ground and excited states, we apply both the time-dependent density functional theory and the coupled cluster singles and doubles method. The results obtained are in good agreement between the methods. It is, furthermore, found that the small structural changes that occur during the reaction have little effect on the static XAS. Hence, the tr-XAS can be computed based on the state populations determined from a nuclear dynamics simulation and one set of static XAS calculations, utilizing the ground state optimized geometry. This approach can save considerable computational resources, as the static spectra need not to be calculated for all geometries. As BT-1T is a relatively rigid molecule, the outlined approach should only be considered when investigating non-radiative decay processes in the vicinity of the Franck-Condon point.

Ultrafast X-ray Spectroscopy of Intersystem Crossing in Hexafluoroacetylacetone: Chromophore Photophysics and Spectral Changes in the Face of Electron-Withdrawing Groups

Intersystem crossings between singlet and triplet states represent a crucial relaxation pathway in photochemical processes. Herein, we probe the intersystem crossing in hexafluoro-acetylacetone with ultrafast X-ray transient absorption spectroscopy at the carbon K-edge. We observe the excited state dynamics following excitation with 266 nm UV light to the ¹ππ* (S₂) state with element and site-specificity using a broadband soft X-ray pulse produced by high harmonic generation. These results are compared to X-ray spectra computed from orbital optimized density functional theory methods. It is found that the electron-withdrawing fluorine atoms decongest the X-ray absorption spectrum by enhancing separation between features originating from different carbon atoms. This facilitates the elucidation of structural and electronic dynamics at the chromophore. The evolution of the core-to-valence resonances at the carbon K-edge reveals an ultrafast population transfer between the ¹nπ* (S₁) and ³ππ* (T₁) states on a 1.6 ± 0.4 ps time scale, which is similar to the 1.5 ps time scale earlier observed for acetylacetone [ J. Am. Chem. Soc. 2017, 139, 16576-16583, DOI: 10.1021/jacs.7b07532]. It therefore appears that terminal fluorination has little influence on the intersystem crossing rate of the acetylacetone chromophore. In addition, the significant role of hydrogen-bond opened and twisted rotational isomers is elucidated in the excited state dynamics by comparison of the experimental transient X-ray spectra with theory.

HHG at the Carbon K-Edge Directly Driven by SRS Red-Shifted Pulses from an Ytterbium Amplifier

In this work, we introduce a simplified approach to efficiently extend the high harmonic generation (HHG) cutoff in gases without the need for laser frequency conversion via parametric processes. Instead, we employ postcompression and red-shifting of a Yb:CaF2 laser via stimulated Raman scattering (SRS) in a nitrogen-filled stretched hollow core fiber. This driving scheme circumvents the low-efficiency window of parametric amplifiers in the 1100-1300 nm range. We demonstrate this approach being suitable for upscaling the power of a driver with an optimal wavelength for HHG in the highly desirable XUV range between 200 and 300 eV, up to the carbon K-edge. Due to the combination of power scalability of a low quantum defect ytterbium-based laser system with the high conversion efficiency of the SRS technique, we expect a significant increase in the generated photon flux in comparison with established platforms for HHG in the water window. We also compare HHG driven by the SRS scheme with the conventional self-phase modulation (SPM) scheme.

Effects of a redox-active diketone on the photochemical transformation of roxarsone: Mechanisms and environmental implications

Organoarsenical antibiotics pose a severe threat to the environment and human health. In aquatic environment, dissolved organic matter (DOM)-mediated photochemical transformation is one of the main processes in the fate of organoarsenics. Dicarbonyl is a typical redox-active moiety in DOM. However, the knowledge on the photoconversion of organoarsenics by DOM, especially the contributions of dicarbonyl moieties is still limited. Here, we systematically investigated the photochemical transformation of three organoarsenics with the simplest β-diketone, acetylacetone (AcAc), as a model dicarbonyl moiety of DOM. The presence of AcAc significantly enhanced the photochemical conversion of roxarsone (ROX), whereas only minor effects were observed for 3-amino-4-hydroxyphenylarsonic acid (HAPA) and arsanilic acid (ASA), because the latter two (with an amino (-NH₂) group) are more photoactive than ROX (with a nitro (-NO₂) group). The results demonstrate that AcAc was a potent photo-activator and the reduction of -NO₂ to -NH₂ might be a rate-limiting step in the phototransformation of ROX. At a 1:1 M ratio of AcAc to ROX, the photochemical transformation rate of ROX was increased by 7 folds. In O₂-rich environment, singlet oxygen, peroxide radicals, and ·OH were the main reactive species that led to the breakage of the C-As bond in ROX and the oxidation of the released arsono group to arsenate, whereas the triplet-excited state of AcAc (³AcAc*) and carbon-centered radicals from the photolysis of AcAc dominated in the reductive transformation of ROX. In anoxic environment, 3-amino-4-hydroxyphenylarsonic acid was one of the main reductive transformation intermediates of ROX, whose photolysis rate was about 35 times that of ROX. The knowledge obtained here is of great significance to better understand the fate of organoarsenics in natural environment.

Electronic State Population Dynamics upon Ultrafast Strong Field Ionization and Fragmentation of Molecular Nitrogen

Air lasing from single ionized N_{2}^{+} molecules induced by laser filamentation in air has been intensively investigated and the mechanisms responsible for lasing are currently highly debated. We use ultrafast nitrogen K-edge spectroscopy to follow the strong field ionization and fragmentation dynamics of N_{2} upon interaction with an ultrashort 800 nm laser pulse. Using probe pulses generated by extreme high-order harmonic generation, we observe transitions indicative of the formation of the electronic ground X^{2}Σ_{g}^{+}, first excited A^{2}Π_{u}, and second excited B^{2}Σ_{u}^{+} states of N_{2}^{+} on femtosecond timescales, from which we can quantitatively determine the time-dependent electronic state population distribution dynamics of N_{2}^{+}. Our results show a remarkably low population of the A^{2}Π_{u} state, and nearly equal populations of the X^{2}Σ_{g}^{+} and B^{2}Σ_{u}^{+} states. In addition, we observe fragmentation of N_{2}^{+} into N and N^{+} on a timescale of several tens of picoseconds that we assign to significant collisional dynamics in the plasma, resulting in dissociative excitation of N_{2}^{+}.

Nonadiabatic Excited State Dynamics of Organic Chromophores: Take-Home Messages

Nonadiabatic excited state dynamics are important in a variety of processes. Theoretical and experimental developments have allowed for a great progress in this area, while combining the two is often necessary and the best approach to obtain insight into the photophysical behavior of molecules. In this Feature Article we use examples of our recent work combining time-resolved photoelectron spectroscopy with theoretical nonadiabatic dynamics to highlight important lessons we learned. We compare the nonadiabatic excited state dynamics of three different organic molecules with the aim of elucidating connections between structure and dynamics. Calculations and measurements are compared for uracil, 1,3-cyclooctadiene, and 1,3-cyclohexadiene. The comparison highlights the role of rigidity in influencing the dynamics and the difficulty of capturing the dynamics accurately with calculations.

Direct observation of ultrafast exciton localization in an organic semiconductor with soft X-ray transient absorption spectroscopy

The localization dynamics of excitons in organic semiconductors influence the efficiency of charge transfer and separation in these materials. Here we apply time-resolved X-ray absorption spectroscopy to track photoinduced dynamics of a paradigmatic crystalline conjugated polymer: poly(3-hexylthiophene) (P3HT) commonly used in solar cell devices. The π→π^* transition, the first step of solar energy conversion, is pumped with a 15 fs optical pulse and the dynamics are probed by an attosecond soft X-ray pulse at the carbon K-edge. We observe X-ray spectroscopic signatures of the initially hot excitonic state, indicating that it is delocalized over multiple polymer chains. This undergoes a rapid evolution on a sub 50 fs timescale which can be directly associated with cooling and localization to form either a localized exciton or polaron pair.

A detailed understanding of ultrafast exciton dynamics is crucial for improving the efficiency of organic light-harvesting-devices. Here, the authors track exciton localization on a sub-50 fs timescale in an organic semiconductor using time resolved soft x-ray absorption spectroscopy.

Attosecond spectroscopy for the investigation of ultrafast dynamics in atomic, molecular and solid-state physics

Since the first demonstration of the generation of attosecond pulses (1 as = 10^-18s) in the extreme-ultraviolet spectral region, several measurement techniques have been introduced, at the beginning for the temporal characterization of the pulses, and immediately after for the investigation of electronic and nuclear ultrafast dynamics in atoms, molecules and solids with unprecedented temporal resolution. The attosecond spectroscopic tools established in the last two decades, together with the development of sophisticated theoretical methods for the interpretation of the experimental outcomes, allowed to unravel and investigate physical processes never observed before, such as the delay in photoemission from atoms and solids, the motion of electrons in molecules after prompt ionization which precede any notable nuclear motion, the temporal evolution of the tunneling process in dielectrics, and many others. This review focused on applications of attosecond techniques to the investigation of ultrafast processes in atoms, molecules and solids. Thanks to the introduction and ongoing developments of new spectroscopic techniques, the attosecond science is rapidly moving towards the investigation, understanding and control of coupled electron-nuclear dynamics in increasingly complex systems, with ever more accurate and complete investigation techniques. Here we will review the most common techniques presenting the latest results in atoms, molecules and solids.

Time-Resolved Optical Pump-Resonant X-ray Probe Spectroscopy of 4-Thiouracil: A Simulation Study

We theoretically monitor the photoinduced ππ* → nπ* internal conversion process in 4-thiouracil (4TU), triggered by an optical pump. The element-sensitive spectroscopic signatures are recorded by a resonant X-ray probe tuned to the sulfur, oxygen, or nitrogen K-edge. We employ high-level electronic structure methods optimized for core-excited electronic structure calculation combined with quantum nuclear wavepacket dynamics computed on two relevant nuclear modes, fully accounting for their quantum nature of nuclear motions. We critically discuss the capabilities and limitations of the resonant technique. For sulfur and nitrogen, we document a pre-edge spectral window free from ground-state background and rich with ππ* and nπ* absorption features. The lowest sulfur K-edge shows strong absorption for both ππ* and nπ*. In the lowest nitrogen K-edge window, we resolve a state-specific fingerprint of the ππ* and an approximate timing of the conical intersection via its depletion. A spectral signature of the nπ* transition, not accessible by UV-vis spectroscopy, is identified. The oxygen K-edge is not sensitive to molecular deformations and gives steady transient absorption features without spectral dynamics. The ππ*/nπ* coherence information is masked by more intense contributions from populations. Altogether, element-specific time-resolved resonant X-ray spectroscopy provides a detailed picture of the electronic excited-state dynamics and therefore a sensitive window into the photophysics of thiobases.

Relativistic Orbital-Optimized Density Functional Theory for Accurate Core-Level Spectroscopy

Core-level spectra of 1s electrons of elements heavier than Ne show significant relativistic effects. We combine advances in orbital-optimized density functional theory (OO-DFT) with the spin-free exact two-component (X2C) model for scalar relativistic effects to study K-edge spectra of third period elements. OO-DFT/X2C is found to be quite accurate at predicting energies, yielding a ∼0.5 eV root-mean-square error versus experiment with the modern SCAN (and related) functionals. This marks a significant improvement over the >50 eV deviations that are typical for the popular time-dependent DFT (TDDFT) approach. Consequently, experimental spectra are quite well reproduced by OO-DFT/X2C, sans empirical shifts for alignment. OO-DFT/X2C combines high accuracy with ground state DFT cost and is thus a promising route for computing core-level spectra of third period elements. We also explored K and L edges of 3d transition metals to identify limitations of the OO-DFT/X2C approach in modeling the spectra of heavier atoms.

Excited state absorption of DNA bases in the gas phase and in chloroform solution: a comparative quantum mechanical study

We study the excited state absorption (ESA) properties of the four DNA bases (thymine, cytosine, adenine, and guanine) by different single reference quantum mechanical methods, namely, equation of motion coupled cluster singles and doubles (EOM-CCSD), singles, doubles and perturbative triples (EOM-CC3), and time-dependent density functional theory (TD-DFT), with the long-range corrected CAM-B3LYP functional. Preliminary results at the Tamm-Dancoff (TDA) CAM-B3LYP level using the maximum overlap method (MOM) are reported for thymine. In the gas phase, the three methods predict similar One Photon Absorption (OPA) spectra, which are consistent with the experimental results and with the most accurate computational studies available in the literature. The ESA spectra are then computed for the ππ* states (one for pyrimidine, two for purines) associated with the lowest-energy absorption band, and for the close-lying nπ* state. The EOM-CC3, EOM-CCSD and CAM-B3LYP methods provide similar ESA spectral patterns, which are also in qualitative agreement with literature RASPT2 results. Once validated in the gas phase, TD-CAM-B3LYP has been used to compute the ESA in chloroform, including solvent effects by the polarizable continuum model (PCM). The predicted OPA and ESA spectra in chloroform are very similar to those in the gas phase, most of the bands shifting by less than 0.1 eV, with a small increase of the intensities and a moderate destabilization of the nπ* state. Finally, ESA spectra have been computed from the minima of the lowest energy ππ* state, and found in line with the available experimental transient absorption spectra of the nucleosides in solution, providing further validation of our computational approach.

Following excited-state chemical shifts in molecular ultrafast x-ray photoelectron spectroscopy

The conversion of photon energy into other energetic forms in molecules is accompanied by charge moving on ultrafast timescales. We directly observe the charge motion at a specific site in an electronically excited molecule using time-resolved x-ray photoelectron spectroscopy (TR-XPS). We extend the concept of static chemical shift from conventional XPS by the excited-state chemical shift (ESCS), which is connected to the charge in the framework of a potential model. This allows us to invert TR-XPS spectra to the dynamic charge at a specific atom. We demonstrate the power of TR-XPS by using sulphur 2p-core-electron-emission probing to study the UV-excited dynamics of 2-thiouracil. The method allows us to discover that a major part of the population relaxes to the molecular ground state within 220–250 fs. In addition, a 250-fs oscillation, visible in the kinetic energy of the TR-XPS, reveals a coherent exchange of population among electronic states.

Imaging the charge flow in photoexcited molecules would provide key information on photophysical and photochemical processes. Here the authors demonstrate tracking in real time after photoexcitation the change in charge density at a specific site of 2-thiouracil using time-resolved X-ray photoelectron spectroscopy.

Self-Consistent Field Methods for Excited States in Strong Magnetic Fields: a Comparison between Energy- and Variance-Based Approaches

Self-consistent field methods for excited states offer an attractive low-cost route to study not only excitation energies but also properties of excited states. Here, we present the generalization of two self-consistent field methods, the maximum overlap method (MOM) and the σ-SCF method, to calculate excited states in strong magnetic fields and investigate their stability and accuracy in this context. These methods use different strategies to overcome the well-known variational collapse of energy-based optimizations to the lowest solution of a given symmetry. The MOM tackles this problem in the definition of the orbital occupations to constrain the self-consistent field procedure to converge on excited states, while the σ-SCF method is based on the minimization of the variance instead of the energy. To overcome the high computational cost of the variance minimization, we present a new implementation of the σ-SCF method with the resolution of identity approximation, allowing the use of large basis sets, which is an important requirement for calculations in strong magnetic fields. The accuracy of these methods is assessed by comparison with the benchmark literature data for He, H₂, and CH⁺. The results reveal severe limitations of the variance-based scheme, which become more acute in large basis sets. In particular, many states are not accessible using variance optimization. Detailed analysis shows that this is a general feature of variance optimization approaches due to the masking of local minima in the optimization. In contrast, the MOM shows promising performance for computing excited states under these conditions, yielding results consistent with available benchmark data for a diverse range of electronic states.

Solvent Effects in the Ultraviolet and X-ray Absorption Spectra of Pyridazine in Aqueous Solution

Electrostatic interaction of the solvent with the solute and fluctuations of the solvent configurations may make excitation energies of the solute different from those in the gas phase. These effects may dominate photoinduced or chemical reaction dynamics in solution systems and can be observed as shifts or broadening of peaks in absorption spectra. In this work, the nitrogen K-edge X-ray absorption spectra were measured for pyridazine in the gas phase and in aqueous solution. The ultraviolet and X-ray absorption spectra of pyridazine in aqueous solution, as well as those in the gas phase, were then calculated with models based on the algebraic-diagrammatic construction through second order [ADC(2)] with the resolution-of-identity (RI) approximation and compared with the spectra obtained in experiments. For aqueous solution, explicit local solvation structures were extracted from an ab initio molecular dynamics (AIMD) trajectory of pyridazine in bulk water, and RI-ADC(2) was combined with the conductor-like screening model (COSMO). The experimental absorption spectra of pyridazine in aqueous solution were reproduced with good accuracy by theoretical treatment of an ensemble containing the explicit local solvation structures of pyridazine with relevant water molecules combined with the COSMO solvation model of water for long-range solvation.

X-ray transient absorption reveals the 1Au (nπ*) state of pyrazine in electronic relaxation

Electronic relaxation in organic chromophores often proceeds via states not directly accessible by photoexcitation. We report on the photoinduced dynamics of pyrazine that involves such states, excited by a 267 nm laser and probed with X-ray transient absorption spectroscopy in a table-top setup. In addition to the previously characterized ¹B_2u (ππ*) (S₂) and ¹B_3u (nπ*) (S₁) states, the participation of the optically dark ¹A_u (nπ*) state is assigned by a combination of experimental X-ray core-to-valence spectroscopy, electronic structure calculations, nonadiabatic dynamics simulations, and X-ray spectral computations. Despite ¹A_u (nπ*) and ¹B_3u (nπ*) states having similar energies at relaxed geometry, their X-ray absorption spectra differ largely in transition energy and oscillator strength. The ¹A_u (nπ*) state is populated in 200 ± 50 femtoseconds after electronic excitation and plays a key role in the relaxation of pyrazine to the ground state.

Mapping wave packet bifurcation at a conical intersection in CH3I by attosecond XUV transient absorption spectroscopy

Extreme ultraviolet (XUV) transient absorption spectroscopy has emerged as a sensitive tool for mapping the real-time structural and electronic evolution of molecules. Here, attosecond XUV transient absorption is used to track dynamics in the A-band of methyl iodide (CH₃I). Gaseous CH₃I molecules are excited to the A-band by a UV pump (277 nm, ∼20 fs) and probed by attosecond XUV pulses targeting iodine I(4d) core-to-valence transitions. Owing to the excellent temporal resolution of the technique, passage through a conical intersection is mapped through spectral signatures of nonadiabatic wave packet bifurcation observed to occur at 15 ± 4 fs following UV photoexcitation. The observed XUV signatures and time dynamics are in agreement with previous simulations [H. Wang, M. Odelius, and D. Prendergast, J. Chem. Phys. 151, 124106 (2019)]. Due to the short duration of the UV pump pulse, coherent vibrational motion in the CH₃I ground state along the C-I stretch mode (538 ± 7 cm^-1) launched by resonant impulsive stimulated Raman scattering and dynamics in multiphoton excited states of CH₃I are also detected.

Transient resonant Auger-Meitner spectra of photoexcited thymine

We present the first investigation of excited state dynamics by resonant Auger-Meitner spectroscopy (also known as resonant Auger spectroscopy) using the nucleobase thymine as an example. Thymine is photoexcited in the UV and probed with X-ray photon energies at and below the oxygen K-edge. After initial photoexcitation to a ππ* excited state, thymine is known to undergo internal conversion to an nπ* excited state with a strong resonance at the oxygen K-edge, red-shifted from the ground state π* resonances of thymine (see our previous study Wolf, et al., Nat. Commun., 2017, 8, 29). We resolve and compare the Auger-Meitner electron spectra associated both with the excited state and ground state resonances, and distinguish participator and spectator decay contributions. Furthermore, we observe simultaneously with the decay of the nπ* state signatures the appearance of additional resonant Auger-Meitner contributions at photon energies between the nπ* state and the ground state resonances. We assign these contributions to population transfer from the nπ* state to a ππ* triplet state via intersystem crossing on the picosecond timescale based on simulations of the X-ray absorption spectra in the vibrationally hot triplet state. Moreover, we identify signatures from the initially excited ππ* singlet state which we have not observed in our previous study.

Ultrafast Transfer and Transient Entrapment of Photoexcited Mg Electron in Mg@C_{60}

Electron relaxation is studied in endofullerene Mg@C_{60} after an initial localized photoexcitation in Mg by nonadiabatic molecular dynamics simulations. Two approaches to the electronic structure of the excited electronic states are used: (i) an independent particle approximation based on a density-functional theory description of molecular orbitals and (ii) a configuration-interaction description of the many-body effects. Both methods exhibit similar relaxation times, leading to an ultrafast decay and charge transfer from Mg to C_{60} within tens of femtoseconds. Method (i) further elicits a transient trap of the transferred electron that can delay the electron-hole recombination. Results shall motivate experiments to probe these ultrafast processes by two-photon transient absorption or photoelectron spectroscopy in gas phase, in solution, or as thin films.

Highly efficient soft x-ray spectrometer for transient absorption spectroscopy with broadband table-top high harmonic sources

We present a novel soft x-ray spectrometer for ultrafast absorption spectroscopy utilizing table-top femtosecond high-order harmonic sources. Where most commercially available spectrometers rely on spherical variable line space gratings with a typical efficiency on the order of 3% in the first diffractive order, this spectrometer, based on a Hettrick-Underwood design, includes a reflective zone plate as a dispersive element. An improved efficiency of 12% at the N K-edge is achieved, accompanied by a resolving power of 890. The high performance of the soft x-ray spectrometer is further demonstrated by comparing nitrogen K-edge absorption spectra from calcium nitrate in aqueous solution obtained with our high-order harmonic source to previous measurements performed at the electron storage ring facility BESSY II.

Ultraviolet photodissociation of gas-phase iron pentacarbonyl probed with ultrafast infrared spectroscopy

It is well known that ultraviolet photoexcitation of iron pentacarbonyl results in rapid loss of carbonyl ligands leading to the formation of coordinatively unsaturated iron carbonyl compounds. We employ ultrafast mid-infrared transient absorption spectroscopy to probe the photodissociation dynamics of gas-phase iron pentacarbonyl following ultraviolet excitation at 265 and 199 nm. After photoexcitation at 265 nm, our results show evidence for sequential dissociation of iron pentacarbonyl to form iron tricarbonyl via a short-lived iron tetracarbonyl intermediate. Photodissociation at 199 nm results in the prompt production of Fe(CO)₃ within 0.25 ps via several energetically accessible pathways. An additional 15 ps time constant extracted from the data is tentatively assigned to intersystem crossing to the triplet manifold of iron tricarbonyl or iron dicarbonyl. Mechanisms for formation of iron tetracarbonyl, iron tricarbonyl, and iron dicarbonyl are proposed and theoretically validated with one-dimensional cuts through the potential energy surface as well as bond dissociation energies. Ground state calculations are computed at the CCSD(T) level of theory and excited states are computed with EOM-EE-CCSD(dT).

XABOOM: An X-ray Absorption Benchmark of Organic Molecules Based on Carbon, Nitrogen, and Oxygen 1s → π* Transitions

The performance of several standard and popular approaches for calculating X-ray absorption spectra at the carbon, nitrogen, and oxygen K-edges of 40 primarily organic molecules up to the size of guanine has been evaluated, focusing on the low-energy and intense 1s → π* transitions. Using results obtained with CVS-ADC(2)-x and fc-CVS-EOM-CCSD as benchmark references, we investigate the performance of CC2, ADC(2), ADC(3/2), and commonly adopted density functional theory (DFT)-based approaches. Here, focus is on precision rather than on accuracy of transition energies and intensities-in other words, we target relative energies and intensities and the spread thereof, rather than absolute values. The use of exchange-correlation functionals tailored for time-dependent DFT calculations of core excitations leads to error spreads similar to those seen for more standard functionals, despite yielding superior absolute energies. Long-range corrected functionals are shown to perform particularly well compared to our reference data, showing error spreads in energy and intensity of 0.2-0.3 eV and ∼10%, respectively, as compared to 0.3-0.6 eV and ∼20% for a typical pure hybrid. In comparing intensities, state mixing can complicate matters, and techniques to avoid this issue are discussed. Furthermore, the influence of basis sets in high-level ab initio calculations is investigated, showing that reasonably accurate results are obtained with the use of 6-311++G**. We name this benchmark suite as XABOOM (X-ray absorption benchmark of organic molecules) and provide molecular structures and ground-state self-consistent field energies and spectroscopic data. We believe that it provides a good assessment of electronic structure theory methods for calculating X-ray absorption spectra and will become useful for future developments in this field.

An assessment of different electronic structure approaches for modeling time-resolved x-ray absorption spectroscopy

We assess the performance of different protocols for simulating excited-state x-ray absorption spectra. We consider three different protocols based on equation-of-motion coupled-cluster singles and doubles, two of them combined with the maximum overlap method. The three protocols differ in the choice of a reference configuration used to compute target states. Maximum-overlap-method time-dependent density functional theory is also considered. The performance of the different approaches is illustrated using uracil, thymine, and acetylacetone as benchmark systems. The results provide guidance for selecting an electronic structure method for modeling time-resolved x-ray absorption spectroscopy.

Improved insights in time-resolved photoelectron imaging

We review new light source developments and data analysis considerations relevant to the time-resolved photoelectron imaging technique. Case studies illustrate how these themes may enhance understanding in studies of excited state molecular dynamics.

Table-Top X-ray Spectroscopy of Benzene Radical Cation

Ultrafast table-top X-ray spectroscopy at the carbon K-edge is used to measure the X-ray spectral features of benzene radical cations (Bz⁺). The ground state of the cation is prepared selectively by two-photon ionization of neutral benzene, and the X-ray spectra are probed at early times after the ionization by transient absorption using X-rays produced by high harmonic generation (HHG). Bz⁺ is well-known to undergo Jahn-Teller distortion, leading to a lower symmetry and splitting of the π orbitals. Comparison of the X-ray absorption spectra of the neutral and the cation reveals a splitting of the two degenerate π* orbitals as well as an appearance of a new peak due to excitation to the partially occupied π-subshell. The π* orbital splitting of the cation, elucidated on the basis of high-level calculations in a companion theoretical paper [Vidal et al. J. Phys. Chem. A. http://dx.doi.org/10.1021/acs.jpca.0c08732], is discovered to be due to both the symmetry distortion and even more dominant spin coupling of the unpaired electron in the partially vacant π orbital (from ionization) with the unpaired electrons resulting from the transition from the 1s_C core orbital to the fully vacant π* orbitals.

Interplay of Open-Shell Spin-Coupling and Jahn-Teller Distortion in Benzene Radical Cation Probed by X-ray Spectroscopy

We report a theoretical investigation and elucidation of the X-ray absorption spectra of neutral benzene and of the benzene cation. The generation of the cation by multiphoton ultraviolet (UV) ionization and the measurement of the carbon K-edge spectra of both species using a table-top high-harmonic generation source are described in the companion experimental paper [Epshtein, M.; et al. J. Phys. Chem. A http://dx.doi.org/10.1021/acs.jpca.0c08736]. We show that the 1s_C → π transition serves as a sensitive signature of the transient cation formation, as it occurs outside of the spectral window of the parent neutral species. Moreover, the presence of the unpaired (spectator) electron in the π-subshell of the cation and the high symmetry of the system result in significant differences relative to neutral benzene in the spectral features associated with the 1s_C → π* transitions. High-level calculations using equation-of-motion coupled-cluster theory provide the interpretation of the experimental spectra and insight into the electronic structure of benzene and its cation. The prominent split structure of the 1s_C → π* band of the cation is attributed to the interplay between the coupling of the core → π* excitation with the unpaired electron in the π-subshell and the Jahn-Teller distortion. The calculations attribute most of the splitting (∼1-1.2 eV) to the spin coupling, which is visible already at the Franck-Condon structure, and we estimate the additional splitting due to structural relaxation to be around ∼0.1-0.2 eV. These results suggest that X-ray absorption with increased resolution might be able to disentangle electronic and structural aspects of the Jahn-Teller effect in the benzene cation.

Revealing electronic state-switching at conical intersections in alkyl iodides by ultrafast XUV transient absorption spectroscopy

Conical intersections between electronic states often dictate the chemistry of photoexcited molecules. Recently developed sources of ultrashort extreme ultraviolet (XUV) pulses tuned to element-specific transitions in molecules allow for the unambiguous detection of electronic state-switching at a conical intersection. Here, the fragmentation of photoexcited iso-propyl iodide and tert-butyl iodide molecules (i-C₃H₇I and t-C₄H₉I) through a conical intersection between ³Q₀/¹Q₁ spin–orbit states is revealed by ultrafast XUV transient absorption measuring iodine 4d core-to-valence transitions. The electronic state-sensitivity of the technique allows for a complete mapping of molecular dissociation from photoexcitation to photoproducts. In both molecules, the sub-100 fs transfer of a photoexcited wave packet from the ³Q₀ state into the ¹Q₁ state at the conical intersection is captured. The results show how differences in the electronic state-switching of the wave packet in i-C₃H₇I and t-C₄H₉I directly lead to differences in the photoproduct branching ratio of the two systems.

The reaction trajectories of photoexcited molecules may involve transitions through conical intersections, which are ubiquitous in nature but challenging to characterize. Here the authors provide a complete mapping of molecular dissociation of two model alkyl halides by ultrafast XUV transient absorption.

Probing competing relaxation pathways in malonaldehyde with transient X-ray absorption spectroscopy

Excited-state intramolecular hydrogen transfer (ESIHT) is a fundamental reaction relevant to chemistry and biology. Malonaldehyde is the simplest example of ESIHT, yet only little is known experimentally about its excited-state dynamics. Several competing relaxation pathways have been proposed, including internal conversion mediated by ESIHT and C[double bond, length as m-dash]C torsional motion as well as intersystem crossing. We perform an in silico transient X-ray absorption spectroscopy (TRXAS) experiment at the oxygen K-edge to investigate its potential to monitor the proposed ultrafast decay pathways in malonaldehyde upon photoexcitation to its bright S₂(ππ*) state. We employ both restricted active space perturbation theory and algebraic-diagrammatic construction for the polarization propagator along interpolated reaction coordinates as well as representative trajectories from ab initio multiple spawning simulations to compute the TRXAS signals from the lowest valence states. Our study suggests that oxygen K-edge TRXAS can distinctly fingerprint the passage through the H-transfer intersection and the concomitant population transfer to the S₁(nπ*) state. Potential intersystem crossing to T₁(ππ*) is detectable from reappearance of the double pre-edge signature and reversed intensities. Moreover, the torsional deactivation pathway induces transient charge redistribution from the enol side towards the central C-atom and manifests itself as substantial shifts of the pre-edge features. Given the continuous advances in X-ray light sources, our study proposes an experimental route to disentangle ultrafast excited-state decay channels in this prototypical ESIHT system and provides a pathway-specific mapping of the TRXAS signal to facilitate the interpretation of future experiments.

Ultrafast nonadiabatic dynamics probed by nitrogen K-edge absorption spectroscopy

Quantum dynamics simulations are used to simulate the ultrafast X-ray Absorption Near-Edge Structure (XANES) spectra of photoexcited pyrazine including two strongly coupled electronically excited states and four normal mode degrees of freedom.

Short-wavelength probes in time-resolved photoelectron spectroscopy: an extended view of the excited state dynamics in acetylacetone

Time-resolved photoelectron spectroscopy using a vacuum ultraviolet probe brings new insight to the excited state dynamics operating in acetylacetone.

Spin adapted implementation of EOM-CCSD for triplet excited states: Probing intersystem crossings of acetylacetone at the carbon and oxygen K-edges

We present an equation of motion coupled cluster singles and doubles approach for computing transient absorption spectra from a triplet excited state. The implementation determines the left and right excitation vectors by explicitly spin-adapting the triplet excitation space. As an illustrative application, we compute transient state X-ray absorption spectra at the carbon and oxygen K-edges for the acetylacetone molecule.