Fluorescence Time Delay in Multistep Auger Decay as an Internal Clock

S. Kosugi, F. Koike, M. Iizawa, M. Oura, T. Gejo, K. Tamasaku, J. R. Harries, R. Guillemin, M. N. Piancastelli, M. Simon, and Y. Azuma

Phys. Rev. Lett. 124, 183001 – Published 8 May 2020

Abstract

Differences in postcollision interaction (PCI) effects on Kr $L_{3} M_{4, 5} M_{4, 5}$ Auger electron spectra were observed, depending on whether the initial photoionization occurred slightly above the $K$ threshold or slightly above the $L_{3}$ threshold. For the former, $K L$ fluorescence emission most likely happens and then Auger processes due to the $L_{3}$ hole follow. The time delay due to fluorescence causes a reduced shift of the Auger peak and tailing toward lower energy, since the Auger overtaking of the photoelectron happens later in time and at a location farther away from the ionic core, compared to the case for the simple one-step $L_{3} M_{4, 5} M_{4, 5}$ Auger decay after $L$ -shell photoionization. Time-dependent theory for PCI in multistep processes agrees well with experiment, illustrating the effect as an internal clock for the time-sequence of the dynamical process.

Received 13 November 2019
Accepted 14 April 2020

DOI:https://doi.org/10.1103/PhysRevLett.124.183001

Physics Subject Headings (PhySH)

Atomic & molecular structure Atomic orbital Atomic spectra Autoionization & Auger processes Electronic excitation & ionization Electronic structure of atoms & molecules Electronic transitions Photoemission

Atomic, Molecular & Optical

Authors & Affiliations

S. Kosugi^1,2, F. Koike¹, M. Iizawa ^1,2,†, M. Oura ², T. Gejo^2,3, K. Tamasaku², J. R. Harries⁴, R. Guillemin⁵, M. N. Piancastelli ^2,5,6, M. Simon ^2,5, and Y. Azuma ^1,2,*

¹Department of Materials and Life Sciences, Sophia University, Tokyo 102-8554, Japan
²RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan
³Graduate School of Materials Science, University of Hyogo, Kamigori-cho, Ako-gun 678-1297, Japan
⁴National Institutes for Quantum and Radiological Science and Technology, SPring-8, 1-1-1 Kouto, Sayo, Hyogo 679-5148, Japan
⁵Sorbonne Université, CNRS, UMR 7614, Laboratoire de Chimie Physique-Matière et Rayonnement, F-75005 Paris, France
⁶Department of Physics and Astronomy, Uppsala University, Box 516, SE-75120 Uppsala, Sweden

^*Corresponding author. y-azuma@sophia.ac.jp
^†Present address: Department of Physics, Rikkyo University, Tokyo 171-8501, Japan.

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Vol. 124, Iss. 18 — 8 May 2020

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Images

Figure 1
Kr $L_{3} M_{4, 5} M_{4, 5}$ Auger electron spectra near the $K$ threshold for several excess energies $E_{exc}$ . Assignments are from $2 p$ photoionization studies [26]. The vertical dotted line at 1460.1 eV shows the nominal $L_{3} M_{4, 5} M_{4, 5}$ ( ${{}^{1}G}_{4}$ ) Auger energy [26]. The solid curves are superpositions of Eq. (3) for two-step cascades with $Γ_{2 p} = 1.17$ and $Γ_{1 s} = 3.92 eV$ [27], and spectral heights normalized to experiment at the ${{}^{1}G}_{4}$ peaks.
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Figure 2
The Kr $L_{3} M_{4, 5} M_{4, 5}$ ( ${{}^{1}G}_{4}$ ) Auger line for $E_{exc (1 s)} = 95.6$ , 5.6, and 0.6 eV. Open (blue) circles: present experiment. Solid (crimson) curves: two-step decay model [Eq. (3)] with $Γ_{2 p} = 1.17$ and $Γ_{1 s} = 3.92 eV$ [27]. Dotted (orange) curve: one-step decay model with $Γ_{2 p} = 1.17 eV$ . Broken (light green) curve: one-step effective decay width model with $Γ_{eff} = 0.90 eV$ . Theoretical curves are convoluted with $Γ_{3 d}^{2} = 0.16 eV$ [28] and the experimental resolution. The vertical dotted line at 1460.1 eV shows the nominal $L_{3} M_{4, 5} M_{4, 5}$ ( ${{}^{1}G}_{4}$ ) Auger energy [26].
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Figure 3
(a) The Kr $L_{3} M_{4, 5} M_{4, 5}$ ( ${{}^{1}G}_{4}$ ) Auger line for $E_{exc} = 0.6 eV$ . Open circles (blue): experiment above the $1 s$ threshold. Open triangles (light blue): experiment above the $2 p$ threshold. Solid curve (crimson): theory for two-step decay [Eq. (3)] with $Γ_{2 p} = 1.17$ , $Γ_{1 s} = 3.92 eV$ [27]. Dotted curve (orange): theory for one-step decay [Eq. (1)] with $Γ_{2 p} = 1.17 eV$ . (b) Simplified fluorescence time delay model [Eq. (4)], for delay times $T = τ_{1 s} \times 0$ , 1, 2, 3, 4, 1000, where $τ_{1 s} = 168$ as is the $K$ -shell lifetime. $T = τ_{1 s} \times 0$ corresponds to no fluorescence time delay, and $T = τ_{1 s} \times 1000$ to no PCI.
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Figure 4
Schematic of the fluorescence time delay effect on the PCI. The solid (black) curve shows the photoelectron radial distance $r$ against time $t$ . The broken (green) and dotted (red) curves are for Auger electrons without and with fluorescence delay time $T$ , respectively. The Auger electrons overtake the photoelectron at points $A$ and $B$ . The $r$ at $B$ is larger than at $A$ , so the PCI effect is weaker.
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Figure 5
The PCI shifts of Kr $L_{3} M_{4, 5} M_{4, 5}$ ${{}^{1}G}_{4}$ Auger line. Open (red) circles: $K$ -shell photoionization. Open (green) squares: $L$ -shell photoionization. Solid (orange) curve: two-step model [Eq. (3)]. Dot-dashed (purple) curve: one-step model with $Γ_{2 p} = 1.17 eV$ [Eq. (1)]. Dotted (light blue) curve: one-step model with $Γ_{eff} = 0.90 eV$ . Dot-dashed (crimson) curve: semiclassical model [9] for $Γ_{2 p} = 1.17 eV$ . Broken (blue) curve: same for $Γ_{eff} = 0.90 eV$ .
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