arXiv subjects

Robin Santra

Publications and source records attributed to Robin Santra.

At least 19 recordsLinked to original sources

arXiv2026

Preparation and control of electronic wave packets in neutral molecules via attosecond x-ray processes

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 OCS 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 time scale. 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 atom-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.

physics.chem-ph↗

arXiv2023

Theory of high-energy correlated multiphoton x-ray diffraction for synchrotron radiation sources

We present a theoretical formulation for the multiphoton diffraction phenomenology in the nonrelativistic limit, suitable for interpreting high-energy x-ray diffraction measurements using synchrotron radiation sources. A hierarchy of approximations and the systematic analysis of limiting cases are presented. A convolutional representation of the diffraction signal allows classification of the physical resources contributing to the correlation signatures. The formulation is intended for developing a theoretical description capable of describing plausible absence or presence of correlation signatures in elastic and inelastic diffractive scattering. Interpreting these correlation signatures in terms of the incoming field modulated many-body electronic density correlations provides a novel perspective for structural imaging studies. More essentially, it offers a framework necessary for theoretical developments of associated reconstruction algorithms.

quant-ph↗

arXiv2023

Multiple-core-hole resonance spectroscopy with ultraintense X-ray pulses

Understanding the interaction of intense, femtosecond X-ray pulses with heavy atoms is crucial for gaining insights into the structure and dynamics of matter. One key aspect of nonlinear light-matter interaction was, so far, not studied systematically at free-electron lasers -- its dependence on the photon energy. Using resonant ion spectroscopy, we map out the transient electronic structures occurring during the complex charge-up pathways. Massively hollow atoms featuring up to six simultaneous core holes determine the spectra at specific photon energies and charge states. We also illustrate how the influence of different X-ray pulse parameters that are usually intertwined can be partially disentangled. The extraction of resonance spectra is facilitated by the fact that the ion yields become independent of the peak fluence beyond a saturation point. Our study lays the groundwork for novel spectroscopies of transient atomic species in exotic, multiple-core-hole states that have not been explored previously.

physics.atom-ph↗

arXiv2022

Theoretical investigation of orbital alignment of x-ray-ionized atoms in exotic electronic configurations

We theoretically study orbital alignment in x-ray-ionized atoms and ions, based on improved electronic-structure calculations starting from the Hartree-Fock-Slater model. We employ first-order many-body perturbation theory to improve the Hartree-Fock-Slater calculations and show that the use of first-order-corrected energies yields significantly better transition energies than originally obtained. The improved electronic-structure calculations enable us also to compute individual state-to-state cross sections and transition rates and, thus, to investigate orbital alignment induced by linearly polarized x rays. To explore the orbital alignment of transiently formed ions after photoionization, we discuss alignment parameters and ratios of individual state-resolved photoionization cross sections for initially neutral argon and two exotic electronic configurations that may be formed during x-ray multiphoton ionization dynamics induced by x-ray free-electron lasers. We also present how the orbital alignment is affected by Auger-Meitner decay and demonstrate how it evolves during a sequence of one photoionization and one Auger-Meitner decay. Our present work establishes a step toward investigation of orbital alignment in atomic ionization driven by high-intensity x rays.

physics.atom-ph↗

arXiv2022

State-resolved ionization dynamics of neon atom induced by x-ray free-electron-laser pulses

We present a theoretical framework to describe state-resolved ionization dynamics of neon atoms driven by ultraintense x-ray free-electron-laser pulses. In general, x-ray multiphoton ionization dynamics of atoms have been described by time-dependent populations of the electronic configurations visited during the ionization dynamics, neglecting individual state-to-state transition rates and energies. Combining a state-resolved electronic-structure calculation, based on first-order many-body perturbation theory, with a Monte Carlo rate-equation method, enables us to study state-resolved dynamics based on time-dependent state populations. Our results demonstrate that configuration-based and state-resolved calculations provide similar charge-state distributions, but the differences are visible when resonant excitations are involved, which are also reflected in calculated time-integrated electron and photon spectra. In addition, time-resolved spectra of ions, electrons, and photons are analyzed for different pulse durations to explore how frustrated absorption manifests itself during the ionization dynamics of neon atoms.

physics.atom-ph↗

arXiv2021

Resonance-enhanced multiphoton ionization in the x-ray regime

Here, we report on the nonlinear ionization of argon atoms in the short wavelength regime using ultraintense x rays from the European XFEL. After sequential multiphoton ionization, high charge states are obtained. For photon energies that are insufficient to directly ionize a $1s$ electron, a different mechanism is required to obtain ionization to Ar$^{17+}$. We propose this occurs through a two-color process where the second harmonic of the FEL pulse resonantly excites the system via a $1s \rightarrow 2p$ transition followed by ionization by the fundamental FEL pulse, which is a type of x-ray resonance-enhanced multiphoton ionization (REMPI). This resonant phenomenon occurs not only for Ar$^{16+}$, but through multiple lower charge states, where multiple ionization competes with decay lifetimes, making x-ray REMPI distinctive from conventional REMPI. With the aid of state-of-the-art theoretical calculations, we explain the effects of x-ray REMPI on the relevant ion yields and spectral profile.

physics.atom-ph↗

arXiv2021

Transient ionization potential depression in nonthermal dense plasmas at high x-ray intensity

The advent of x-ray free-electron lasers (XFELs), which provide intense ultrashort x-ray pulses, has brought a new way of creating and analyzing hot and warm dense plasmas in the laboratory. Because of the ultrashort pulse duration, the XFEL-produced plasma will be out of equilibrium at the beginning and even the electronic subsystem may not reach thermal equilibrium while interacting with a femtosecond time-scale pulse. In the dense plasma, the ionization potential depression (IPD) induced by the plasma environment plays a crucial role for understanding and modeling microscopic dynamical processes. However, all theoretical approaches for IPD have been based on local thermal equilibrium (LTE) and it has been controversial to use LTE IPD models for the nonthermal situation. In this work, we propose a non-LTE (NLTE) approach to calculate the IPD effect by combining a quantum-mechanical electronic-structure calculation and a classical molecular dynamics simulation. This hybrid approach enables us to investigate the time evolution of ionization potentials and IPDs during and after the interaction with XFEL pulses, without the limitation of the LTE assumption. In our NLTE approach, the transient IPD values are presented as distributions evolving with time, which cannot be captured by conventional LTE-based models. The time-integrated ionization potential values are in good agreement with benchmark experimental data on solid-density aluminum plasma and other theoretical predictions based on LTE. The present work is promising to provide critical insights into nonequilibrium dynamics of dense plasma formation and thermalization induced by XFEL pulses.

physics.plasm-ph↗

arXiv2020

Atomic-scale imaging of laser-driven electron dynamics in solids using subcycle-resolved x-ray-optical wave mixing

We investigate laser-driven electron dynamics in solids on the atomic scale and in real space within Floquet formalism, and develop a method based on subcycle-resolved x-ray-optical wave mixing to reconstruct those dynamics. We analyze how time-reversal and inversion symmetries influence properties of optically-induced charge distributions and microscopic electron currents. Several examples for the $μ$th-order microscopic optical response of band-gap crystals are shown and compared for cases when there is either a considerable or a vanishing $μ$th-order macroscopic response. We then analyze the consequence of crystal symmetries on subcycle-resolved x-ray-optical wave mixing, a process in which an x-ray pulse of a duration shorter than the optical cycle of the driving pulse interacts with a laser-dressed crystal. Based on this analysis, we develop a method to reconstruct amplitudes and phases of Fourier components of optically-induced charge distributions from momentum and delay dependence of x-ray-optical wave-mixing spectra. Subcycle-resolved x-ray-optical wave mixing also reveals phases of temporal oscillations of microscopic optical response and some properties of microscopic laser-driven electron currents.

physics.optics↗

arXiv2020

Microscopic electron dynamics in nonlinear optical response of solids

We investigate the microscopic properties of the nonlinear optical response of crystalline solids within Floquet theory, and demonstrate that optically-induced microscopic charge distributions display complex spatial structure and nontrivial properties. Their spatial symmetry and temporal behavior are governed by crystal symmetries. We find that even when a macroscopic optical response of a crystal is forbidden, the microscopic optical response can, in fact, be nonzero. In such a case, the optically-induced charge redistribution can be considerable, even though the corresponding Fourier component of the time-dependent dipole moment per unit cell vanishes. We develop a method that makes it possible to completely reconstruct the microscopic optically-induced charge distributions by means of subcycle-resolved x-ray-optical wave mixing. We also show how, within this framework, the direction of the instantaneous microscopic optically-induced electron current flow can be revealed.

quant-ph↗

arXiv2020

Nonlinear resonant X-ray Raman scattering

We report the observation of a novel nonlinear effect in the hard x-ray range. Upon illuminating Fe and Cu metal foils with intense x-ray pulses tuned near their respective K edges, photons at nearly twice the incoming photon energy are emitted. The signal rises quadratically with the incoming intensity, consistent with two-photon excitation. The spectrum of emitted high-energy photons comprises multiple Raman lines that disperse with the incident photon energy. Upon reaching the double K-shell ionization threshold, the signal strength undergoes a marked rise. Above this threshold, the lines cease dispersing, turning into orescence lines with energies much greater than obtainable by single electron transitions, and additional Raman lines appear. We attribute these processes to electron-correlation mediated multielectron transitions involving double-core hole excitation and various two-electron de-excitation processes to a final state involving one or more L and M core-holes.

physics.atom-ph↗

arXiv2019

Femtosecond laser produced periodic plasma in a colloidal crystal probed by XFEL radiation

With the rapid development of short-pulse intense laser sources, studies of matter under extreme irradiation conditions enter further unexplored regimes. In addition, an application of X-ray Free- Electron Lasers (XFELs), delivering intense femtosecond X-ray pulses allows to investigate sample evolution in IR pump - X-ray probe experiments with an unprecedented time resolution. Here we present the detailed study of periodic plasma created from the colloidal crystal. Both experimental data and theory modeling show that the periodicity in the sample survives to a large extent the extreme excitation and shock wave propagation inside the colloidal crystal. This feature enables probing the excited crystal, using the powerful Bragg peak analysis, in contrast to the conventional studies of dense plasma created from bulk samples for which probing with Bragg diffraction technique is not possible. X-ray diffraction measurements of excited colloidal crystals may then lead towards a better understanding of matter phase transitions under extreme irradiation conditions.

cond-mat.soft↗

arXiv2019

Detecting coherent core-hole wave-packet dynamics in N2 by time- and angle-resolved inner-shell photoelectron spectroscopy

We propose an imaging technique to follow core-hole wave-packet oscillations in the nitrogen molecule. In this scheme, an attosecond x-ray pulse core-ionizes the nitrogen molecule and a subsequent attosecond x-ray pulse probes the evolution of the electron dynamics. We can image the oscillation of the core-hole between the two atomic sites by measuring the angular correlation between photoelectrons. Analytical relations for the angular correlation are derived based on the plane-wave approximation for the photoelectron wave function. We validate these results with a scattering calculation for the photoelectron wave function. The feasibility of the experimental realization of this scheme is discussed in light of current and future capabilities of x-ray free-electron lasers.

physics.chem-ph↗

arXiv2019

Theoretical evidence for the sensitivity of charge-rearrangement-enhanced x-ray ionization to molecular size

It was recently discovered that molecular ionization at high x-ray intensity is enhanced, in comparison with that of isolated atoms, through a phenomenon called CREXIM (charge-rearrangement-enhanced x-ray ionization of molecules). X-ray absorption selectively ionizes heavy atoms within molecules, triggering electron transfer from neighboring atoms to the heavy atom sites and enabling further ionization there. The present theoretical study demonstrates that the CREXIM effect increases with the size of the molecule, as a consequence of increased intramolecular electron transfer from the larger molecular constituents attached to the heavy atoms. We compare x-ray multiphoton ionization dynamics of xenon, iodomethane, and iodobenzene after interacting with an intense x-ray pulse. Although their photoionization cross sections are similar, iodomethane and iodobenzene molecules are more ionized than xenon atoms. Moreover, we predict that the average total charge of iodobenzene is much larger than that of iodomethane, because of the large number of electrons in the benzene ring. The positive charges transferred from the iodine site to the benzene ring are redistributed such that the higher carbon charges are formed at the far end from the iodine site. Our first-principles calculations provide fundamental insights into the interaction of molecules with x-ray free-electron laser (XFEL) pulses. These insights need to be taken into account for interpreting and designing future XFEL experiments.

physics.chem-ph↗

arXiv2019

Scientific Opportunities with an X-ray Free-Electron Laser Oscillator

An X-ray free-electron laser oscillator (XFELO) is a new type of hard X-ray source that would produce fully coherent pulses with meV bandwidth and stable intensity. The XFELO complements existing sources based on self-amplified spontaneous emission (SASE) from high-gain X-ray free-electron lasers (XFEL) that produce ultra-short pulses with broad-band chaotic spectra. This report is based on discussions of scientific opportunities enabled by an XFELO during a workshop held at SLAC on June 29 - July 1, 2016

physics.ins-det↗

arXiv2019

Simulated XUV Photoelectron Spectra of THz-pumped Liquid Water

Highly intense, sub-picosecond terahertz (THz) pulses can be used to induce ultrafast temperature jumps (T-jumps) in liquid water. A supercritical state of gas-like water with liquid density is established, and the accompanying structural changes are expected to give rise to time-dependent chemical shifts. We investigate the possibility of using extreme ultraviolet (XUV) photoelectron spectroscopy as a probe for ultrafast dynamics induced by sub-picosecond THz pulses of varying intensities and frequencies. To this end, we use ab initio methods to calculate photoionization cross sections and photoelectron energies of (H2O)$_{20}$ clusters embedded in an aqueous environment represented by point charges. The cluster geometries are sampled from ab initio molecular dynamics simulations modeling the THz-water interactions. We find that the peaks in the valence photoelectron spectrum are shifted by up to 0.4 eV after the pump pulse, and that they are broadened with respect to unheated water. The shifts can be connected to structural changes caused by the heating, but due to saturation effects they are not sensitive enough to serve as a thermometer for T-jumped water.

physics.chem-ph↗

arXiv2018

Theory of x-ray scattering from laser-driven electronic systems

We describe, within the framework of quantum electrodynamics, an interaction between a non-resonant hard x-ray pulse and an electronic system in the presence of a temporally periodic laser field driving electron dynamics in this system. We apply Floquet theory to describe the laser-driven electronic system, and then obtain the scattering probability of an arbitrary nonresonant x-ray pulse from such a system employing the density-matrix formalism. We show that the scattering probability can be connected to the time-dependent electron density of the driven electronic system only under certain conditions, in particular, if the bandwidth of the probe x-ray pulse is sufficiently narrow to spectroscopically resolve transitions to different final states. A special focus is laid on application of the theory to laser-driven crystals in a strongly nonperturbative regime. We show how the time-dependent electron density of a crystal can be reconstructed from energy-resolved scattering patterns. This is illustrated by a calculation of a diffraction signal from a driven MgO crystal.

physics.optics↗

arXiv2018

Towards attochemistry: Control of nuclear motion through conical intersections and electronic coherences

The effect of nuclear dynamics and conical intersections on electronic coherences is investigated employing a two-state, two-mode linear vibronic coupling model. Exact quantum dynamical calculations are performed using the multi-configuration time-dependent Hartree method (MCTDH). It is found that the presence of a non-adiabatic coupling close to the Franck-Condon point can preserve electronic coherence to some extent. Additionally, the possibility of steering the nuclear wavepackets by imprinting a relative phase between the electronic states during the photoionization process is discussed. It is found that the steering of nuclear wavepackets is possible given that a coherent electronic wavepacket embodying the phase difference passes through a conical intersection. A conical intersection close to the Franck-Condon point is thus a necessary prerequisite for control, providing a clear path towards attochemistry.

physics.chem-ph↗

arXiv2018

XCALIB: a focal spot calibrator for intense X-ray free-electron laser pulses based on the charge state distributions of light atoms

We develop the XCALIB toolkit to calibrate the beam profile of an X-ray free-electron laser (XFEL) at the focal spot based on the experimental charge state distributions (CSDs) of light atoms. Accurate characterization of the fluence distribution at the focal spot is essential to perform the volume integrations of physical quantities for a quantitative comparison between theoretical and experimental results, especially for fluence dependent quantities. The use of the CSDs of light atoms is advantageous because CSDs directly reflect experimental conditions at the focal spot, and the properties of light atoms have been well established in both theory and experiment. To obtain theoretical CSDs, we use XATOM, a toolkit to calculate atomic electronic structure and to simulate ionization dynamics of atoms exposed to intense XFEL pulses, which involves highly excited multiple core hole states. Employing a simple function with a few parameters, the spatial profile of an XFEL beam is determined by minimizing the difference between theoretical and experimental results. We have implemented an optimization procedure employing the reinforcement learning technique. The technique can automatize and organize calibration procedures which, before, had been performed manually. XCALIB has high flexibility, simultaneously combining different optimization methods, sets of charge states, and a wide range of parameter space. Hence, in combination with XATOM, XCALIB serves as a comprehensive tool to calibrate the fluence profile of a tightly focused XFEL beam in the interaction region.

physics.app-ph↗