Femtosecond symmetry breaking and coherent relaxation of methane cations via x-ray spectroscopy

Non vertical ionization-dissociation model for strong IR induced dissociation dynamics of [Formula: see text]

Electron-nuclear coupling plays a crucial role in strong laser induced molecular dissociation dynamics. The interplay between electronic and nuclear degrees of freedom determines the pathways and outcomes of molecular fragmentation. However, a full quantum mechanical treatment of electron-nuclear dynamics is computationally intensive. In this work, we have developed a Strong Laser Induced non-adiabatic Multi-Ionic-Multi-Electric States (SLIMIMES) approach, which contains the electron-laser and electron-nuclear couplings. We validate our model using a showcase example: water dissociation under strong infrared (IR) laser pulses. Our investigation reveals the predominant role of a non-vertical dissociation pathway in the photo-ionization dissociation (PID) process of [Formula: see text]. This pathway originates from neutral [Formula: see text], which undergoes vertical multi-photon-single-ionization, reaching the intermediate dissociation states of [Formula: see text] within [Formula: see text]. Subsequently, [Formula: see text] dissociates into [Formula: see text], with both [Formula: see text] and [Formula: see text] fragments potentially ionizing an electron during interaction with the IR laser. This sequential PID pathway significantly contributes to the dissociation yields of water dication. Our calculations are consistent with recent experimental data, which focus on measuring the branching ratio of water dication dissociation. We aim for our model to provide a deeper understanding and a fresh perspective on the coupling between electron and nuclear dynamics induced by a strong IR laser field.

Vibronic Conical Intersection Trajectory Signatures in Wave Packet Coherences

Conical intersections are ubiquitous in the energy landscape of chemical systems, drive photochemical reactivity, and are extremely challenging to observe spectroscopically. Using two-dimensional electronic spectroscopy, we observe the nonadiabatic dynamics in Wurster's Blue after excitation to the lowest two vibronic excited states. The excited populations relax ballistically through a conical intersection in 55 fs to the electronic ground state potential energy surface as the molecule undergoes an intramolecular electron transfer. While the kinetics are identical on both vibronic energy surfaces, we observe different patterns of coherent oscillations after traversing the conical intersection indicating distinct nonadiabatic relaxation pathways through the conical energetic funnel. These coherences are not created directly by the excitation pulses but are the result of the dynamical trajectories projecting differently on the conical intersection vibrational space. Our spectroscopic data offers a fresh perspective into the complex conical intersection topology and dynamics that emphasizes the critical involvement of the intersection space in dictating the dynamics.

Jahn-Teller Effect on CF3I Photodissociation Dynamics

The Jahn-Teller (JT) effect, as a spontaneous symmetry-breaking mechanism arising from the coupling between electronic and nuclear degrees of freedom, is a widespread phenomenon in molecular and condensed matter systems. Here, we investigate the influence of the JT effect on the photodissociation dynamics of CF3I molecules. Based on ab initio calculation, we obtain the three-dimensional potential energy surfaces for 3Q0+ and 1Q1 states and establish a diabatic Hamiltonian model to study the wavepacket dynamics in the CF3I photodissociation process. Using the wave function of the final state after dissociation, we calculate the rotational density matrix of the CF3 fragment and analyze its rotational excitation under the JT effect, as well as its partial coherence property and selection rules. Our work paves the way to the experimental observation and quantification of the JT effect in molecular dissociation dynamics beyond the classical ball-and-stick model.

Few-femtosecond electron transfer dynamics in photoionized donor-π-acceptor molecules

The exposure of molecules to attosecond extreme-ultraviolet (XUV) pulses offers a unique opportunity to study the early stages of coupled electron-nuclear dynamics in which the role played by the different degrees of freedom is beyond standard chemical intuition. We investigate, both experimentally and theoretically, the first steps of charge-transfer processes initiated by prompt ionization in prototype donor-π-acceptor molecules, namely nitroanilines. Time-resolved measurement of this process is performed by combining attosecond XUV-pump/few-femtosecond infrared-probe spectroscopy with advanced many-body quantum chemistry calculations. We show that a concerted nuclear and electronic motion drives electron transfer from the donor group on a sub-10-fs timescale. This is followed by a sub-30-fs relaxation process due to the probing of the continuously spreading nuclear wave packet in the excited electronic states of the molecular cation. These findings shed light on the role played by electron-nuclear coupling in donor-π-acceptor systems in response to photoionization.

Dynamic Jahn-Teller effect in the strong spin-orbit coupling regime

Exotic quantum phases, arising from a complex interplay of charge, spin, lattice and orbital degrees of freedom, are of immense interest to a wide research community. A well-known example of such an entangled behavior is the Jahn-Teller effect, where the lifting of orbital degeneracy proceeds through lattice distortions. Here we demonstrate that a highly-symmetrical 5d1 double perovskite Ba2MgReO6, comprising a 3D array of isolated ReO6 octahedra, represents a rare example of a dynamic Jahn-Teller system in the strong spin-orbit coupling regime. Thermodynamic and resonant inelastic x-ray scattering experiments, supported by quantum chemistry calculations, undoubtedly show that the Jahn-Teller instability leads to a ground-state doublet, resolving a long-standing puzzle in this family of compounds. The dynamic state of ReO6 octahedra persists down to the lowest temperatures, where a multipolar order sets in, allowing for investigations of the interplay between a dynamic JT effect and strongly correlated electron behavior.

Measurement of coherent vibrational dynamics with X-ray Transient Absorption Spectroscopy simultaneously at the Carbon K- and Chlorine L2,3- edges

X-ray Transient Absorption Spectroscopy (XTAS) is a powerful probe for ultrafast molecular dynamics. The evolution of XTAS signal is controlled by the shapes of potential energy surfaces of the associated core-excited states, which are difficult to directly measure. Here, we study the vibrational dynamics of Raman activated CCl4 with XTAS targeting the C 1s and Cl 2p electrons. The totally symmetric stretching mode leads to concerted elongation or contraction in bond lengths, which in turn induce an experimentally measurable red or blue shift in the X-ray absorption energies associated with inner-shell electron excitations to the valence antibonding levels. The ratios between slopes of different core-excited potential energy surfaces (CEPESs) thereby extracted agree very well with Restricted Open-Shell Kohn-Sham calculations. The other, asymmetric, modes do not measurably contribute to the XTAS signal. The results highlight the ability of XTAS to reveal coherent nuclear dynamics involving  < 0.01 Å atomic displacements and also provide direct measurement of forces on CEPESs.

Benchmark Study of Core-Ionization Energies with the Generalized Active Space-Driven Similarity Renormalization Group

X-ray photoelectron spectroscopy (XPS) is a powerful experimental technique for probing the electronic structure of molecules and materials; however, interpreting XPS data requires accurate computational methods to model core-ionized states. This work proposes and benchmarks a new approach based on the generalized active space-driven similarity renormalization group (GAS-DSRG) for calculating core-ionization energies and treating correlation effects at the perturbative and nonperturbative levels. We tested the GAS-DSRG across three data sets. First, the vertical core-ionization energies of small molecules containing first-row elements are evaluated. GAS-DSRG achieves mean absolute errors below 0.3 eV, which is comparable to high-level coupled cluster methods. Next, the accuracy of GAS-DSRG is evaluated for larger organic molecules using the CORE65 data set, with the DSRG-MRPT3 level yielding a mean absolute error of only 0.34 eV for 65 core-ionization transitions. Insights are provided into the treatment of static and dynamic correlation, the importance of high-order perturbation theory, and notable differences from density functional theory in the predicted energy ordering of core-ionized states for specific molecules. Finally, vibrationally resolved XPS spectra of diatomic molecules (CO, N2, and O2) are simulated, showing excellent agreement with experimental data.

Prediction of photodynamics of 200 nm excited cyclobutanone with linear response electronic structure and ab initio multiple spawning

Simulations of photochemical reaction dynamics have been a challenge to the theoretical chemistry community for some time. In an effort to determine the predictive character of current approaches, we predict the results of an upcoming ultrafast diffraction experiment on the photodynamics of cyclobutanone after excitation to the lowest lying Rydberg state (S2). A picosecond of nonadiabatic dynamics is described with ab initio multiple spawning. We use both time dependent density functional theory (TDDFT) and equation-of-motion coupled cluster singles and doubles (EOM-CCSD) theory for the underlying electronic structure theory. We find that the lifetime of the S2 state is more than a picosecond (with both TDDFT and EOM-CCSD). The predicted ultrafast electron diffraction spectrum exhibits numerous structural features, but weak time dependence over the course of the simulations.

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.

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.

Predicting the X-ray Absorption Spectrum of Ozone with Single Configuration State Functions

X-ray absorption spectra (XAS) of biradicaloid species are often thought to represent a challenge to theoretical methods. This has led to the testing of recently developed multireference techniques on the XAS of ozone, but reproduction of the experimental spectral profile has proven difficult. We utilize a minimal model consisting of a single configuration state function (CSF) per excited state to model core-level excitations of ozone, with the orbitals of each CSF optimized using the restricted open-shell Kohn-Sham (ROKS) method. This protocol leads to semiquantitative agreement with experimental XAS. In fact, we find that low-lying core-hole excited states in biradicaloids can be approximated with individual CSFs, despite the presence of multireference character in the ground state. We also report that the 1s → π* and 1s → σ* transitions have quite distinct widths for O3. This reveals the importance of sampling over a representative range of geometries from the vibrational ground state for properly assessing the accuracy of electronic structure methods against experiments instead of the popular procedure of uniformly broadening stick spectra at the equilibrium geometry.

Few-femtosecond electronic and structural rearrangements of CH4+ driven by the Jahn-Teller effect

The Jahn-Teller effect (JTE) is central to the understanding of the physical and chemical properties of a broad variety of molecules and materials. Whereas the manifestations of the JTE in stationary properties of matter are relatively well studied, the study of JTE-induced dynamics is still in its infancy, largely owing to its ultrafast and non-adiabatic nature. For example, the time scales reported for the distortion of CH 4 + from the initial T d geometry to a nominal C 2 v relaxed structure range from 1.85 fs over 10 ± 2 fs to 20 ± 7 fs. Here, by combining element-specific attosecond transient-absorption spectroscopy and quantum-dynamics simulations, we show that the initial electronic relaxation occurs within 5 fs and that the subsequent nuclear dynamics are dominated by the Q2 scissoring and Q1 symmetric stretching modes, which dephase in 41 ± 10 fs and 13 ± 3 fs, respectively. Significant structural relaxation is found to take place only along the e-symmetry Q2 mode. These results demonstrate that CH 4 + created by ionization of CH 4 is best thought of as a highly fluxional species that possesses a long-time-averaged vibrational distribution centered around a D 2 d structure. The methods demonstrated in our work provide guidelines for the understanding of Jahn-Teller driven non-adiabatic dynamics in other more complex systems.

Ab Initio Investigation of the Auger Spectra of Methane, Ethane, Ethylene, and Acetylene

We present an ab initio computational study of the Auger spectra of methane, ethane, ethylene, and acetylene. Auger spectroscopy is an established technique to probe the electronic structure of molecules and exploits the Auger-Meitner effect that core-ionized states undergo. We compute partial decay widths using coupled-cluster theory with single and double substitutions (CCSD) and equation-of-motion CCSD theory combined with complex-scaled basis functions and Feshbach-Fano projection. We generate Auger spectra from these partial widths and draw conclusions about the strength of particular decay channels and trends among the four molecules. A connection to experimental results about fragmentation pathways of the electronic states produced by Auger decay is also made.

Nonadiabatic Photodissociation and Dehydrogenation Dynamics of n-Butyl Bromide Following p-Rydberg Excitation

Femtosecond time-resolved mass spectrometry, correlation mapping, and density functional theory calculations are employed to reveal the mechanism of C═C and C≡C formation (and related H₂ production) following excitation to the p-Rydberg states of n-butyl bromide. Ultrafast pump-probe mass spectrometry shows that nonadiabatic relaxation operates as a multistep process reaching an intermediate state within ∼500 fs followed by relaxation to a final state within 10 ps of photoexcitation. Absorption of three ultraviolet photons accesses the dense p-Rydberg state manifold, which is further excited by the probe beam for C─C bond dissociation and dehydrogenation reactions. Rapid internal conversion deactivates the dehydrogenation pathways, while activating carbon backbone dissociation pathways. Thus, unsaturated carbon fragments decay with the lifetime of p-Rydberg (∼500 fs), matching the growth recorded in saturated hydrocarbon fragments. The saturated hydrocarbon signals subsequently decay on the picosecond time scale as the molecule relaxes below the Rydberg states and into halogen release channels.