Compton spectra of atoms at high x-ray intensity

Compton scattering is the nonresonant inelastic scattering of an x-ray photon by an electron and has been used to probe the electron momentum distribution in gas-phase and condensed-matter samples. In the low x-ray intensity regime, Compton scattering from atoms dominantly comes from bound electrons in neutral atoms, neglecting contributions from bound electrons in ions and free (ionized) electrons. In contrast, in the high x-ray intensity regime, the sample experiences severe ionization via x-ray multiphoton multiple ionization dynamics. Thus, it becomes necessary to take into account all the contributions to the Compton scattering signal when atoms are exposed to high-intensity x-ray pulses provided by x-ray free-electron lasers (XFELs). In this paper, we investigate the Compton spectra of atoms at high x-ray intensity, using an extension of the integrated x-ray atomic physics toolkit, \textsc{xatom}. As the x-ray fluence increases, there is a significant contribution from ionized electrons to the Compton spectra, which gives rise to strong deviations from the Compton spectra of neutral atoms. The present study provides not only understanding of the fundamental XFEL--matter interaction but also crucial information for single-particle imaging experiments, where Compton scattering is no longer negligible.Comment: 24 pages, 10 figures. This is an author-created, un-copyedited version of an article accepted for publication in the special issue of "Emerging Leaders" in J. Phys. B: At. Mol. Opt. Phys. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from i

Interplay between relativistic energy corrections and resonant excitations in x-ray multiphoton ionization dynamics of Xe atoms

In this paper, we theoretically study x-ray multiphoton ionization dynamics of heavy atoms taking into account relativistic and resonance effects. When an atom is exposed to an intense x-ray pulse generated by an x-ray free-electron laser (XFEL), it is ionized to a highly charged ion via a sequence of single-photon ionization and accompanying relaxation processes, and its final charge state is limited by the last ionic state that can be ionized by a single-photon ionization. If x-ray multiphoton ionization involves deep inner-shell electrons in heavy atoms, energy shifts by relativistic effects play an important role in ionization dynamics, as pointed out in [Phys.\ Rev.\ Lett.\ \textbf{110}, 173005 (2013)]. On the other hand, if the x-ray beam has a broad energy bandwidth, the high-intensity x-ray pulse can drive resonant photo-excitations for a broad range of ionic states and ionize even beyond the direct one-photon ionization limit, as first proposed in [Nature\ Photon.\ \textbf{6}, 858 (2012)]. To investigate both relativistic and resonance effects, we extend the \textsc{xatom} toolkit to incorporate relativistic energy corrections and resonant excitations in x-ray multiphoton ionization dynamics calculations. Charge-state distributions are calculated for Xe atoms interacting with intense XFEL pulses at a photon energy of 1.5~keV and 5.5~keV, respectively. For both photon energies, we demonstrate that the role of resonant excitations in ionization dynamics is altered due to significant shifts of orbital energy levels by relativistic effects. Therefore it is necessary to take into account both effects to accurately simulate multiphoton multiple ionization dynamics at high x-ray intensity

New Development of Theoretical and Computational Methods for Probing Strong-Field Multiphoton Processes

The study of the strong-field multiphoton processes is a subject of much current significance in physics and chemistry. Recent progress of laser technology has triggered a burst of attosecond science where the electron dynamics plays a vital role in underlying physics. The nonlinear strong-field phenomena, such as multiphoton ionization, multiphoton resonance, high-order harmonic generation, etc, are beyond the perturbative regime and demand novel theoretical approaches for better understanding. This dissertation aims at developing new theoretical and computational methods with innovative spatial and temporal treatments, and delivering comprehensive studies of strong-field multiphoton processes explored by the proposed methods. The time-dependent Voronoi-cell finite difference method is a new grid-based method for electronic structure and dynamics calculations of polyatomic molecules. The spatial part is accurately treated by the Voronoi-cell finite difference method on multicenter molecular grids, featuring high adaptivity and simplicity. The temporal part is solved by the split-operator time propagation technique, allowing accurate and efficient non-perturbative treatment of electronic dynamics in strong fields. The method is applied to self-interaction-free time-dependent density-functional calculations to probe multiphoton processes of polyatomic molecules in intense ultrashort laser fields with arbitrary field-molecule orientation, highlighting the importance of multielectron effects. The generalized Floquet theory is extended for the investigations of an atom in intense frequency-comb laser fields and a qubit system driven by intense oscillating fields. For the frequency-comb laser generated by a temporal train of pulses, the many-mode Floquet theory is extended to treat the interaction of an atom and a series of comb frequencies, demonstrating coherent control of simultaneous multiphoton resonance processes. For the strongly driven qubit, the Floquet theory is extended and its analytic solution is derived to explore multiphoton quantum interference in the superconducting flux qubit

Many-mode Floquet theoretical approach for coherent control of multiphoton dynamics driven by intense frequency-comb laser fields

This is the published version, also available here: http://dx.doi.org/10.1103/PhysRevA.77.063406.We extend the many-mode Floquet theorem (MMFT) for the investigation of multiphoton resonance dynamics driven by intense frequency-comb laser fields. The frequency comb structure generated by a train of short laser pulses can be exactly represented by a combination of the main frequency and the repetition frequency. MMFT allows non-perturbative and exact treatment of the interaction of a quantum system with the frequency-comb laser fields. We observe simultaneous multiphoton resonance processes between a two-level system and frequency-comb laser. The multiphoton processes can be coherently controlled by tuning the laser parameters such as the carrier-envelope phase (CEP) shift. In particular, high-order harmonic generation shows immense enhancement by tuning the CEP shift, due to simultaneous multiphoton resonances

Multielectron effects on the orientation dependence and photoelectron angular distribution of multiphoton ionization of CO2 in strong laser fields

This is the publisher's version, also available electronically from http://journals.aps.org/pra/abstract/10.1103/PhysRevA.80.011403.We perform an ab initio study of multiphoton ionization (MPI) of carbon dioxide in intense linearly polarized laser pulses with arbitrary molecular orientation by means of a time-dependent density-functional theory (TDDFT) with proper long-range potential. We develop a time-dependent Voronoi-cell finite difference method with highly adaptive molecular grids for accurate solution of the TDDFT equations. Our results demonstrate that the orientation dependence of MPI is determined by multiple orbital contributions and that the electron correlation effects are significant. The maximum peak of MPI is predicted to be at 40° in good agreement with recent experimental data. Photoelectron angular distribution reveals the delicate relation between the orientation dependence and the molecular orbital symmetry

A molecular-dynamics approach for studying the non-equilibrium behavior of x-ray-heated solid-density matter

When matter is exposed to a high-intensity x-ray free-electron-laser pulse, the x rays excite inner-shell electrons leading to the ionization of the electrons through various atomic processes and creating high-energy-density plasma, i.e., warm or hot dense matter. The resulting system consists of atoms in various electronic configurations, thermalizing on sub-picosecond to picosecond timescales after photoexcitation. We present a simulation study of x-ray-heated solid-density matter. For this we use XMDYN, a Monte-Carlo molecular-dynamics-based code with periodic boundary conditions, which allows one to investigate non-equilibrium dynamics. XMDYN is capable of treating systems containing light and heavy atomic species with full electronic configuration space and 3D spatial inhomogeneity. For the validation of our approach we compare for a model system the electron temperatures and the ion charge-state distribution from XMDYN to results for the thermalized system based on the average-atom model implemented in XATOM, an ab-initio x-ray atomic physics toolkit extended to include a plasma environment. Further, we also compare the average charge evolution of diamond with the predictions of a Boltzmann continuum approach. We demonstrate that XMDYN results are in good quantitative agreement with the above mentioned approaches, suggesting that the current implementation of XMDYN is a viable approach to simulate the dynamics of x-ray-driven non-equilibrium dynamics in solids. In order to illustrate the potential of XMDYN for treating complex systems we present calculations on the triiodo benzene derivative 5-amino-2,4,6-triiodoisophthalic acid (I3C), a compound of relevance of biomolecular imaging, consisting of heavy and light atomic species

Efficient electronic structure calculation for molecular ionization dynamics at high x-ray intensity

We present the implementation of an electronic-structure approach dedicated to ionization dynamics of molecules interacting with x-ray free-electron laser (XFEL) pulses. In our scheme, molecular orbitals for molecular core-hole states are represented by linear combination of numerical atomic orbitals that are solutions of corresponding atomic core-hole states. We demonstrate that our scheme efficiently calculates all possible multiple-hole configurations of molecules formed during XFEL pulses. The present method is suitable to investigate x-ray multiphoton multiple ionization dynamics and accompanying nuclear dynamics, providing essential information on the chemical dynamics relevant for high-intensity x-ray imaging.Comment: 28 pages, 6 figure

Quantum-mechanical calculation of ionization potential lowering in dense plasmas

The charged environment within a dense plasma leads to the phenomenon of ionization potential depression (IPD) for ions embedded in the plasma. Accurate predictions of the IPD effect are of crucial importance for modeling atomic processes occurring within dense plasmas. Several theoretical models have been developed to describe the IPD effect, with frequently discrepant predictions. Only recently, first experiments on IPD in Al plasma have been performed with an x-ray free-electron laser (XFEL), where their results were found to be in disagreement with the widely-used IPD model by Stewart and Pyatt. Another experiment on Al, at the Orion laser, showed disagreement with the model by Ecker and Kr\"oll. This controversy shows a strong need for a rigorous and consistent theoretical approach to calculate the IPD effect. Here we propose such an approach: a two-step Hartree-Fock-Slater model. With this parameter-free model we can accurately and efficiently describe the experimental Al data and validate the accuracy of standard IPD models. Our model can be a useful tool for calculating atomic properties within dense plasmas with wide-ranging applications to studies on warm dense matter, shock experiments, planetary science, inertial confinement fusion and studies of non-equilibrium plasmas created with XFELs.Comment: 13 pages, 9 figures, to be published in Phys. Rev. X; added references [46,47

Incoherent x-ray scattering in single molecule imaging

Imaging of the structure of single proteins or other biomolecules with atomic resolution would be enormously beneficial to structural biology. X-ray free-electron lasers generate highly intense and ultrashort x-ray pulses, providing a route towards imaging of single molecules with atomic resolution. The information on molecular structure is encoded in the coherent x-ray scattering signal. In contrast to crystallography there are no Bragg reflections in single molecule imaging, which means the coherent scattering is not enhanced. Consequently, a background signal from incoherent scattering deteriorates the quality of the coherent scattering signal. This background signal cannot be easily eliminated because the spectrum of incoherently scattered photons cannot be resolved by usual scattering detectors. We present an ab initio study of incoherent x-ray scattering from individual carbon atoms, including the electronic radiation damage caused by a highly intense x-ray pulse. We find that the coherent scattering pattern suffers from a significant incoherent background signal at high resolution. For high x-ray fluence the background signal becomes even dominating. Finally, based on the atomic scattering patterns, we present an estimation for the average photon count in single molecule imaging at high resolution. By varying the photon energy from 3.5 keV to 15 keV, we find that imaging at higher photon energies may improve the coherent scattering signal quality

Floquet formulation for the investigation of multiphoton quantum interference in a superconducting qubit driven by a strong ac field

This is the published version, also available here: http://dx.doi.org/10.1103/PhysRevA.79.032301.We present a Floquet treatment of multiphoton quantum interference in a strongly driven superconducting flux qubit. The periodically time-dependent Schrödinger equation can be reduced to an equivalent time-independent infinite-dimensional Floquet matrix eigenvalue problem. For resonant or nearly resonant multiphoton transitions, we extend the generalized Van Vleck (GVV) nearly degenerate high-order perturbation theory for the treatment of the Floquet Hamiltonian, allowing the reduction of the infinite-dimensional Floquet matrix to an N×N effective Hamiltonian, where N is the number of eigenstates under consideration. The GVV approach allows accurate treatment of ac Stark shift, power broadening, time-dependent and time-averaged transition probability, etc., well beyond the rotating wave approximation. We extend the Floquet and GVV approaches for numerical and analytical studies of the multiphoton resonance processes and quantum interference phenomena for the superconducting flux qubit system (N=2) driven by intense ac fields