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 $D= ||E-\sum_{n}\lambda^n E^{(n)}||$ instead of the real and imaginary parts of the perturbation energy individually

Siegert擬状態の構築と応用

電気通信大学200

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