109 research outputs found
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
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