18 research outputs found
Spatio-temporal interference of photo electron wave packets and time scale of non-adiabatic transition in high-frequency regime
The method of the envelope Hamiltonian [K. Toyota, U. Saalmann, and J. M.
Rost, New J. Phys. {\bf 17}, 073005~(2015)] is applied to further study a
detachment dynamics of a model negative ion in one-dimension in high-frequency
regime. This method is based on the Floquet approach, but the time-dependency
of an envelope function is explicitly kept for arbitrary pulse durations.
Therefore, it is capable of describing not only a photo absorption/emission but
also a non-adiabatic transition which is induced by the time-varying envelope
of the pulse. It was shown that the envelope Hamiltonian accurately retrieves
the results obtained by the time-dependent Schr\"odinger equation, and
underlying physics were well understood by the adiabatic approximation based on
the envelope Hamiltonian. In this paper, we further explore two more aspects of
the detachment dynamics, which were not done in our previous work. First, we
find out features of both a {\it spatial} and {\it temporal} interference of
photo electron wave packets in a photo absorption process. We conclude that
both the interference mechanisms are universal in ionization dynamics in
high-frequency regime. To our knowledge, it is first time that both the
interference mechanisms in high-frequency regime are extracted from the first
principle. Second, we extract a pulse duration which maximize a yield of the
non-adiabatic transition as a function of a pulse duration. It is shown that it
becomes maximum when the pulse duration is comparable to a time-scale of an
electron
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
Siegert pseudostate perturbation theory: one- and two-threshold cases
Perturbation theory for the Siegert pseudostates (SPS) [Phys.Rev.A 58, 2077
(1998) and Phys.Rev.A 67, 032714 (2003)] is studied for the case of two
energetically separated thresholds. The perturbation formulas for the
one-threshold case are derived as a limiting case whereby we reconstruct More's
theory for the decaying states [Phys.Rev.A 3,1217(1971)] and amend an error.
The perturbation formulas for the two-threshold case have additional terms due
to the non-standard orthogonality relationship of the Siegert Pseudostates. We
apply the theory to a 2-channel model problem, and find the rate of convergence
of the perturbation expansion should be examined with the aide of the variance
instead of the real and imaginary parts of
the perturbation energy individually
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.Comment: 28 pages, 7 figure
Siegert状態展開法による一電子原子系のstabilization領域における光電子スペクトル
本論文ではstabilization領域における一電子原子系の光電子スペクトルをSiegert状態展開法を用いて高精度に計算し、結果を考察した。stabilizationとは、光子エネルギーが電子の結合エネルギーよりも十分高い”高周波レーザー”中において、レーザー強度ある閾値を越えるとイオン化レートが減る現象である。本論文では二種類の新奇な物理現象を見出した。(ⅰ)多光子吸収ピーク中に振動構造が現れた。高周波Floquet理論〔1〕による解析によって、これが光電子波束対の干渉縞であることを示した〔2〕。また、干渉縞の形成にはstabilizationが重要な役割を果たすことを明らかにした。(ⅱ)超低速電子を示すピークが現れた。その生成メカニズムは多光子吸収、トンネル効果といったよく知られたイオン化メカニズムでは説明できない。本論文ではその起源が、電子の有効ポテンシャルが時間の関数としてゆっくり変形している状況における非断熱遷移であることを示した〔3〕。〔1〕M. Gavrila and J.Z. Kaminski, Phys. Rev.Lett. 52, 613 (1984)〔2〕K. Toyota、O. I. Tolstikhin, T.Morishita, and S. Watanabe, Phys. Rev. A76.043418 (2007),Phys. Rev. A 78, 033432 (2008)〔3〕K. Toyota、O. I. Tolstikhin, T.Morishita, and S. Watanabe, Phys. Rev.Lett.103、153003 (2009)電気通信大学200