Robin Santra

We present a perturbative framework for computing the dynamics of an electronic wave packet launched by an attosecond x-ray pulse in a neutral molecule. The x-ray pulse excites the molecule via both x-ray absorption and impulsive stimulated x-ray Raman scattering (ISXRS), coherently populating both the core-excited states and valence-excited states. We describe the electronic structure within the equation-of-motion coupled-cluster framework, adopting a compact representation of the sum-over-states expression characterizing the second-order perturbative description of ISXRS. We study the coherent electronic dynamics in carbonyl sulfide and oxazole utilizing the time-dependent difference electron density, decomposing it in terms of its perturbative components. While the core-excited components dominate the initial stages of the dynamics, the valence-excited components dominate the electronic dynamics on a longer timescale. The pulse polarization controls the symmetry of the states included in the wave packet. We show how the spatial symmetry of the states plays a role in shaping the spatial properties of charge migration. The atomic specificity of the ISXRS process translates directly into the initial localization of the wave packet. This determines the starting point of charge migration, shaping its subsequent evolution.

We present an framework for computing inelastic electron scattering (IES) on isolated molecules and in condensed phases, focusing on liquid water as a prototypical target. Our method employs a pseudocontinuum treatment for electron-impact ionization. The scattered projectile is described by a plane wave, while the final states of the target, including the excited or ejected electron, are expanded in a square-integrable basis. This approach naturally describes the transition between the excitation and ionization regimes and enables molecular-hole-resolved IES cross sections to be extracted within a standard quantum-chemistry framework. For isolated water molecules, the computed IES cross sections agree with available experimental data. For liquid water, our calculations show that solvation has a small effect on the total IES cross section. We see, however, that the liquid environment leads to a substantial enhancement of electron-impact-induced ionization and a corresponding reduction of electron-impact-induced excitation. We discuss the total and hole-resolved single differential cross sections, offering a complete molecular picture of IES and paving the way toward a detailed simulation of radiation-damage formation in water.

The solvation mechanism of an excess electron in liquid water has been extensively investigated, yet significant controversies persist regarding the time scales, dynamics, and precise localization processes of this quantum species. Here, we employ ab initio molecular dynamics simulations to elucidate the impact of the degree of initial localization of an excess electron on the solvation process in liquid water. We demonstrate that the degree of initial delocalization of the excess electron critically governs its solvation dynamics. Furthermore, we reveal the role of excited-state dynamics in modulating solvation time scales and discuss the heating of the water structure due to the energy released via internal conversion to the electronic ground state. Our results reveal that the way how the electron is injected critically determines its solvation dynamics.

Ultrafast, high-power lasers operating in the near-infrared (NIR) region are key to accelerating ions to extremely high energies. By changing the laser wavelength from the NIR region to the hard X-ray range, the photon energy increases more than 10 000 times. The interaction mechanisms and, consequently, radiation attenuation lengths differ significantly between these two spectral ranges. Here we report the use of an X-ray free-electron laser (European XFEL, Germany) delivering 9.3 keV photons in 25 fs pulses and a total energy of 0.35 mJ on a solid target. Electrons and ions escaping from an irradiated 3 μm Cu foil into vacuum were investigated by a time-of-flight technique using windowless electron multipliers that enable the measurement of very weak currents. A model based on a shifted Maxwell–Boltzmann velocity distribution of species was used to analyze the detector signals. The method used made it possible to determine the temperatures of hot electrons and protons, their center-of-mass energy, the charge states of the isotopes 63Cu and 65Cu, and the magnitude of the voltage arising in the double layer that accelerated them, and to estimate the repetition frequency of their cascade emission from the plasma. Computer simulations revealed the evolution of the electron density and temperature, the ion charge state distribution, and the time scales of processes occurring in the bulk of irradiated matter. Good correlation of theoretical and experimental results over the range of high-energy-density states demonstrates the capability to provide critical data to develop plasma models in the warm dense matter regime.

Investigating the ultrafast dynamics of primary biological compounds is crucial for gaining insights into radiation damage. We computationally investigate the ionization-induced dynamics of glycine in an aqueous solution. By employing fewest-switches-surface hopping simulations, we specifically address ionization in different orbital levels of the glycine molecule as well as in water molecules in its solvation shell. Upon ionization, glycine undergoes rapid fragmentation on the Cα–C bond, resulting in the formation of CO2 and the methylamine (+H3NH2C⋅) radical. Our analysis shows that the solvation shell has little effect on the fragmentation dynamics. When ionized in water, or a deeper valence orbital of glycine, the system first relaxes to the ground state, involving the transfer of the valence hole between water and glycine. The associated redox reaction exemplifies the oxidizing power of H2O⋅+.

Ultraintense and ultrashort x-ray free-electron laser (XFEL) pulses promise single particle imaging (SPI) via enabling collection of diffraction data for nanocrystals, or nano-size nonperiodic objects before the destruction of the sample. Photoionization and subsequent processes lead to plasma generation within the sample in such experiments. The continuum energy levels of electrons for atoms and atomic ions are lowered due to plasma screening, also known as ionization potential depression (IPD). We theoretically investigate the plasma formation and the effect of IPD in the context of SPI with calculations performed on bulk glycine, mimicking the interior of irradiated biological macromolecules, e.g., proteins or viruses. To simulate the plasma formation dynamics, we employ a nonequilibrium, hybrid quantum-classical approach, combined with the evaluation of the transient IPD from first-principles electronic structure calculations considering the time-dependent microscopic environment, which in earlier work was applied to a solid-density plasma consisting of a single atomic species []. Here, this approach is extended to more than one atomic species for applications to biological macromolecules in SPI studies. Our work quantifies the effect and importance of IPD in XFEL-based imaging of biological systems and provides further guidance for simulations of electronic radiation damage dynamics toward successful SPI experiments.

We present an algorithm for finding chemical reaction pathways using a Monte Carlo transition state search (MCTSS) scheme. Our strategy is a bidirectional two-state approach that simultaneously drives two Monte Carlo trajectories from reactants to products, and vice versa, until the trajectories meet. The trajectories are driven in a Metropolis-like procedure with transition probabilities based on the real-space diffusion Monte Carlo algorithm. A computationally inexpensive structure preselection procedure is used to guide the two trajectories toward each other. We performed a proof-of-principle demonstration of the MCTSS algorithm for the model two-dimensional double-well potential and for the halogen anion SN2-substitution in halogenated methane. The MCTSS approach presented here is expected to be particularly useful when employing electronic structure methods that do not provide analytic gradients.

The extended X-ray absorption fine structure (EXAFS) spectroscopy is a powerful tool to determine the microscopic structure in the vicinity of a probe atom or molecule embedded in a host material. For absorbing atoms dissolved in a liquid, the disordered nature of the host poses challenges for the theoretical calculation of the EXAFS spectrum, especially when strong inelastic energy and momentum transfer between the photoelectron and the solvent occurs. We formulate here an effective quantum theory of EXAFS that is based on the use of an accurately parametrized complex dielectric function of the solvent, illustrated here for the case of water. We derive an effective complex self-energy within the GW approximation to determine the EXAFS signal within a single-scattering approach. To verify the approach, we recover the results for the inelastic mean free path of a photoelectron in water, as known in the literature. In addition, we apply this effective approach to the case of single bromide and chloride atoms dissolved in water and show that the theoretical results match available experimental data. Through advanced FEFF simulations, which include accurate multiple-scattering effects, we conclude that the contribution of the single-scattering processes is dominant. We show that a key role is played by the dielectric environment.

We report resonant inelastic x-ray scattering (RIXS) spectra of neon atoms interacting with intense x-ray pulses generated using an x-ray free-electron laser (XFEL). We find that an unexpected peak emerges near the K α line of Ne, which does not coincide with any physical resonances of neon ions. We perform theoretical calculations based on a quantum-state-resolved rate-equation approach with x-ray-induced processes including possible resonant excitations. Our dynamics simulations demonstrate that a sequence of multiple resonant photoabsorption events are involved and the interplay of those multiple resonances in combination with the relatively large spectral bandwidth of XFEL radiation leads to the emergent resonance-like structure at a position where no resonances exist. Our finding offers critical guidance for future applications of high-intensity RIXS at XFEL facilities.

Because of the Heisenberg uncertainty principle, the structure of a molecule fluctuates about its mean geometry, even in the ground state. Observing this fundamental quantum effect experimentally—particularly, revealing the collective nature of the structural quantum fluctuations—remains an unmet challenge for complex molecules. In this work, we achieved this for an 11-atom molecule by inducing its Coulomb explosion with an x-ray free-electron laser. We show that the structural fluctuations manifest themselves in correlated variations of ion momenta obtained through coincident detection of the atomic fragments from individual molecules. Our analysis scheme allows extracting these variations, despite our measurements covering only a fraction of the full 33-dimensional momentum space, thereby establishing a general approach for extracting information on high-dimensional structural dynamics using Coulomb explosion.