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Physical Review Letters

Attosecond Molecular Angular Streaking with All-Ionic Fragments Detection

Wei Quan, Vladislav Viktorovich Serov, MingZheng Wei, Meng Zhao, Yu Zhou, YanLan Wang, XuanYang Lai, Anatoli S. Kheifets, Xiaojun Liu

Published Nov 26, 2019• 52 citations
Publisher

Abstract

Attosecond angular streaking (or "attoclock") is an insightful technique for probing the ultrafast electron dynamics in strong laser fields. Up until recently, this technique relied solely on an accurate measurement of the photoelectron momentum distribution and has remained restricted to atomic targets. Here, we propose a novel attosecond angular streaking scheme applicable to molecules, for which the ionic fragments of dissociative ionization are detected in the polarization plane of a close-to-circular polarized laser light. Our ionic attoclock measurements are consistent with theoretical results from a numerical solution of the time-dependent Schrödinger equation and an upper bound of 10 as on the tunneling time from the attoclock readings in the H_{2} molecule has been given, which is significantly smaller than any definitions of tunneling time available in the literatures.

DOI
10.1103/physrevlett.123.223204

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Attosecond angular streaking (or "attoclock") is an insightful technique for probing the ultrafast electron dynamics in strong laser fields. Up until recently, this technique relied solely on an accurate measurement of the photoelectron momentum distribution and has remained restricted to atomic targets. Here, we propose a novel attosecond angular streaking scheme applicable to molecules, for which the ionic fragments of dissociative ionization are detected in the polarization plane of a close-to-circular polarized laser light. Our ionic attoclock measurements are consistent with theoretical results from a numerical solution of the time-dependent Schrödinger equation and an upper bound of 10 as on the tunneling time from the attoclock readings in the H_{2} molecule has been given, which is significantly smaller than any definitions of tunneling time available in the literatures.
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±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.
Theoretical models often fail to describe the dynamics due to numerical limitations, and we need to develop better approximate models for the ultrafast dynamics on an atomic scale, where quantum mechanics becomes essential. Novel time-resolved attosecond streaking techniques such as energy streaking and the attoclock (i.e. angular streaking) are currently being applied in an attempt to answer a very fundamental question in quantum mechanics: how fast can light remove a bound electron from an atom or a solid?
Attoclock is a promising chronoscopy of the ultrafast dynamics of atoms and molecules in intense laser fields. The attoclock procedure is established based on the one-to-one correspondence between the photoelectron emission angle and the tunneling instant at each photoelectron kinetic energy for ionization of atoms and molecules subject to elliptically polarized strong laser fields. In this work, our joint theoretical and experimental study demonstrates that this correspondence could be broken down for photoelectrons emitted in a direction close to the minimum yield. Two trajectories with different tunneling instants and different initial velocities are found to correspond to a specific final momentum of the photoelectron in this direction, and a multi-peak structure appears in the photoelectron kinetic energy spectrum that can be attributed to interference between these two trajectories. Our work is essential for a deeper understanding and further development of the attoclock scheme.
Attosecond chronoscopy enables the exploration of correlated electron dynamics in real time. One key observable of attosecond physics is the determination of "time zero" of photoionization, the time delay with which the wave packet of the ionized electron departs from the ionic core. This observable has become accessible by experimental advances in attosecond streaking and reconstruction of attosecond beating by interference of two-photon transitions (RABBIT) techniques. In this Letter, we explore photoionization time delays by strong extreme ultraviolet fields beyond the linear-response limit. We identify novel signatures in time delays signifying strong coupling between atoms and light fields and the light-field dressing of the ion. As a prototypical case, we study the interelectronic coherence and entanglement in helium driven by a strong extreme ultraviolet field. By the numerical solution of the time-dependent Schrödinger equation in its full dimensionality, we show that the time delay of the photoionized electron allows one to monitor the ultrafast variations of coherence dynamics and entanglement in real time.
Strong-field tunneling ionization is the initial step for various ultrafast dynamics in intense laser-atom and/or molecule interactions. Time-resolving tunneling ionization is of crucial importance for accurately understanding these ultrafast processes. In our previous work [Phys. Rev. Lett. 121, 253203 (2018).], we proposed an attosecond photoelectron interferometer based on strong-field photoelectron holography to resolve the tunneling ionization. Here, we show more details about how the complex ionization time of tunneling is retrieved with our scheme. We demonstrate that the Coulomb interaction, which is a tedious and complex problem in dealing with photoelectron interference in strong-field ionization, can be safely canceled in our scheme. This is further confirmed by performing our scheme for both the model atoms with short-range and Coulomb potentials. Additionally, the validity and accuracy of our scheme are confirmed by its application in different targets and different laser parameters. Our scheme provides a reliable and versatile method to resolve the time information of strong-field tunneling ionization.
The classical Keldysh-Rutherford (KR) model of the attoclock [A. W. Bray et al., Phys. Rev. Lett. 121, 123201 (2018)] was introduced to interpret attosecond angular streaking on atomic hydrogen in the low laser intensity regime. In this model, the photoelectron acquires its final velocity immediately after exiting the tunnel and then scatters elastically on the Coulomb field of the residual ion. In the present work, we make a dynamic extension of the KR model and consider a gradual acceleration of the photoelectron by the driving laser field. By doing so, we observe and explain a significant displacement of the peaks of the angular distribution of the mean photoelectron momentum and the angular resolved ionization probability. These peaks should be close within the original KR model.