Probing Resonant Photoionization Time Delay by Self-Referenced Molecular Attoclock

Jihong Tong, Xiwang Liu, Wenhui Dong, Wenyu Jiang, Ming Zhu, Yidan Xu, Zitan Zuo, Peifen Lu, Xiaochun Gong, Xiaohong Song, Weifeng Yang, and Jian Wu

Phys. Rev. Lett. 129, 173201 – Published 18 October 2022

Abstract

Attosecond time-resolved electron tunneling dynamics have been investigated by using attosecond angular streaking spectroscopy, where a clock reference to the laser field vector is required in atomic strong-field ionization and the situation becomes complicated in molecules. Here we reveal a resonant ionization process via a transient state by developing an electron-tunneling-site-resolved molecular attoclock in $Ar − {Kr}^{+}$ . Two distinct deflection angles are observed in the photoelectron angular distribution in the molecular frame, corresponding to the direct and resonant ionization pathways. We find the electron is temporally trapped in the Coulomb potential wells of the $Ar − {Kr}^{+}$ before finally releasing into the continuum when the electron tunnels through the internal barrier. By utilizing the direct tunneling ionization as a self-referenced arm of the attoclock, the time delay of the electron trapped in the resonant state is revealed to be $3.50 \pm 0.04 fs$ . Our results give an impetus to exploring the ultrafast electron dynamics in complex systems and also endow a semiclassical presentation of the electron trapping dynamics in a quantum resonant state.

Received 1 November 2021
Revised 28 March 2022
Accepted 23 September 2022

DOI:https://doi.org/10.1103/PhysRevLett.129.173201

Physics Subject Headings (PhySH)

Electronic excitation & ionization Interatomic & molecular potentials Ultrafast phenomena

Atomic, Molecular & Optical

Authors & Affiliations

Jihong Tong ^1,*, Xiwang Liu^2,*, Wenhui Dong³, Wenyu Jiang¹, Ming Zhu², Yidan Xu¹, Zitan Zuo¹, Peifen Lu¹, Xiaochun Gong ^1,5,†, Xiaohong Song², Weifeng Yang ^2,‡, and Jian Wu^1,4,5,6,§

¹State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China
²School of Science and Center for Theoretical Physics, Hainan University, Haikou 570288, China
³Department of Physics, College of Science, Shantou University, Shantou, Guangdong 515063, China
⁴Chongqing Key Laboratory of Precision Optics, Chongqing Institute of East China Normal University, Chongqing 401121, China
⁵Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi 030006, China
⁶CAS Center for Excellence in Ultra-intense Laser Science, Shanghai 201800, China

^*These authors contributed equally to this work.
^†xcgong@lps.ecnu.edu.cn
^‡wfyang@hainanu.edu.cn
^§jwu@phy.ecnu.edu.cn

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Vol. 129, Iss. 17 — 21 October 2022

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Images

Figure 1
(a) Schematic diagram of the direct tunneling ionization pathway, where the electron is released from the argon atom side. The red arrow with $T_{dir}$ indicates the tunneling process, and the green curve is the typical electron trajectory. (b) The stopwatch of the direct electron tunneling process, where $e_{i}^{d}$ and $e_{f}^{d}$ indicate the initial and final momentum directions, $θ_{f}^{d}$ indicates the electron tunneling deflection angle, and $θ_{offset}^{d}$ is the streaking angle from the laser field. Schematic diagram of (c) Coulomb trapping and (d) resonant ionization process, where the red arrow labeled with $T_{ex}$ indicates the nonadiabatic excitation process, the purple and yellow curves are the electron trajectories, and the blue dot is the electron tunneling exit. (e) The stopwatch of the resonant ionization pathway. Arrows are labeled the same as (b) but for the resonant ionization pathway.
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Figure 2
(a) Experimentally measured electron momentum distribution recorded by the pump pulse alone. The parallel areas of white dashed lines indicate the selected electron range used in the $Ar − {Kr}^{+}$ ion source selection routine [59, 60, 61]. The color map is normalized to its peak value. (b) Measured electron momentum distribution of the doubly ionized Ar-Kr, where the white dashed sectors indicate the electron released by the elliptically polarized probe laser pulse. (c) The measured probe pulse liberated electron momentum distribution with the $Ar − {Kr}^{+}$ oriented as sketched in the inset. The first electron is selected from the pump pulse liberated with a ion momentum gating of $| ϕ_{ion}^{rel} | < 30 °$ and $p z_{sum}^{ion} > 0.3 a . u .$ , where $ϕ_{ion}^{rel} = \arctan (p z_{ion}^{rel} / p y_{ion}^{rel})$ , and $p_{ion}^{rel} = p_{ion}^{Ar +} - p_{ion}^{Kr +}$ . (d) The same as (c) but for theoretically calculated results. (e) The measured and simulated PADs in the polarization plane for $p z_{e} < 0$ , where exp.1 and exp.2 correspond to the electron kinetic energy ranges in 6–12 and 5–16 eV, respectively. The blue solid lines are the fitting curves. The local maxima are labeled with dashed lines in (c) and (d). The green and orange areas show the direct and resonant ionization-pathway-resolved PADs.
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Figure 3
(a) Theoretically calculated electron total energy distribution by summing over kinetic and potential energies. The blue curve shows the initial total energy distribution of the released electron $E_{ini}^{tot}$ at the tunneling exit, and the red curve shows the final total energy distribution of electrons with negative energy after the interaction with the laser field. (b) Direct and (c) resonant ionization-pathway-resolved electron momentum distributions, corresponding to the green and orange areas in (a), and the molecular orientation is the same as in Fig. 2.
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Figure 4
(a) and (b) Time-dependent total energy evolution of the electron trajectories from (a) the excitation path and (b) the resonant tunneling ionization path. The electron trapping time corresponds to the platform with total energy around $- 0.40 a . u .$ , and the colors show the probability of each trajectory. (c) The same as (a),(b) but for the typical electron trajectories presented in Figs. 1, 1, and 1. The gray lines in (a)–(c) show the electric field along the major axis of the elliptically polarized probe laser field. The inset in (c) shows the calculated electron wave functions of the ground state and resonant excited state of the $Ar − {Kr}^{+}$ .
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