Ionization of multi-electron atomic and molecular systems driven by intense and ultrashort laser pulses

Lead Research Organisation: University College London
Department Name: Physics and Astronomy

Abstract

Attoseconds (10^(-18) sec) are the natural time-scale for multi-electron effects during complete ionization and break-up of multi-electron atoms and molecules. The recent advances in generating ultrashort laser pulses raise the possibility to investigate atomic, molecular, and nuclear physics at this new time-scale, bringing a revolution in our microscopic knowledge and understanding of matter. Two fascinating and complementary challenges of Attoscience are to identify the physical mechanisms underlying the correlated multi-electron dynamics--of fundamental interest to, for instance, molecular imaging--in atomic and molecular systems and to devise schemes to probe/control these mechanisms. This is the overall aim of the proposed research. Steering the electronic motion for manipulating small molecules will pave the way for modifying the structure of complex biomolecules, thus impacting such diverse fields as physics, chemistry, biology and material science. The problem consists of exploring the interaction of complex atoms and molecules driven by intense and ultrashort laser pulses. Given the state of the art in computational capabilities, solving this problem with three-dimensional (3-d) first-principle techniques, namely, quantum mechanical ones, is an immense task. Thus, classical/semiclassical techniques, which are much faster than quantum mechanical ones, will be instrumental in exploring the correlated electron dynamics in driven complex atomic and molecular systems. I recently developed, in the context of the driven double ionization of Helium, a 3-d classical method that addresses the full fragmentation of driven systems. The advantage of this technique is that it is much faster than quantum mechanical treatments and it accounts for the Coulomb singularity--the infinitely strong force an electron experiences when it is close to the atomic center. It is thus a step forward compared to previous classical studies which ignore the Coulomb singularity altogether. I propose to generalize this quasiclassical technique, and develop an efficient and sophisticated numerical tool for the treatmentof the full fragmentation of complex driven atomic and molecular systems.Using this 3-d quasiclassical technique, I will first address multi-electron effects in three electron atoms driven by strong laser pulses--a problem vastly unexplored. One of the main goals is to probe (time-resolve) the main mechanisms/paths the three electrons follow to escape during the fragmentation process when the atom is interacting with a very weak field (single photon absorption). I will do so using a circularly polarized infrared ultrashort laser pulse as an attosecond clock to map the information obtained from the observed spectra of the final fragments to the attosecond correlated electron dynamics. I will then proceed to explore the correlated electron dynamics in the double ionization of two- active or two-electron diatomic molecules with moving nuclei when driven by intense ultrashort laser pulses. This problem is at the forefront of Attoscience and is far from being theoretically well understood. Using pulses of different intensity I will be able to explore different ionization regimes and for each regime explore the different mechanisms that govern the two electron escape, the effect of the two atomic centers on the double ionization, and the interplay of processes that result in different final products. The vision is to generalize these studies to tackle driven triatomic molecules with moving nuclei--an unexplored problem--and study the break-up geometries and their dependence on the initial molecular state. Finally, combining my expertise on probing single photon processes and on multi-electron effects of strongly driven molecules, I will address time-resolving and controlling the electronic motion during the break-up of driven multi-center molecules using combinations of ultrashort laser pulses.

Publications

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Emmanouilidou A (2013) Direct three-photon triple ionization of Li and double ionization of Li + in Journal of Physics B: Atomic, Molecular and Optical Physics

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Emmanouilidou A (2012) Multiple electron trapping in the fragmentation of strongly driven molecules in New Journal of Physics

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Kübel M (2019) Streaking strong-field double ionization in Physical Review A

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Price H (2014) Four-electron break-up geometries in beryllium in Chaos, Solitons & Fractals

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Price H (2011) Energy sharing in the two-electron attosecond streak camera in New Journal of Physics

 
Description We have developed 3-d semiclassical tools to address two-electron atoms driven by ultrafast and ultra intense laser pulses. In particular, we have developed with Prof. Paul Corkum and Dr. Andre Staudte the concept of the two-electron streak camera. To do so, we generalized the one electron attosecond streaking camera to time-resolve the correlated two-electron escape dynamics during a collision process involving a deep core electron. The collision process is triggered by an XUV attosecond pulse and probed by a weak infrared field. The principle of our two-electron streak camera is that by placing the maximum of the vector potential of the probing field at the time of collision we get the maximum splitting of the inter-electronic angle of escape. We thus identified the time of collision. Moreover, we have described the main double ionization mechanisms in non-sequential double ionization in the context of Helium as a function of intensity of the infrared laser pulse that is triggering the processes. Our results for single differential probabilities of double ionization as a function of the sum of the electrons energies was found to be in very good agreement with the ab-initio results of Prof. Ken Taylor. Furthermore, we have developed 3-d semiclassical techniques to address ionization during the fragmentation of two-electron diatomic molecules driven by near-infrared laser pulses. In addition, we have developed the theoretical tools to address the processes triggered in atoms by free-electron lasers. We have developed the quantum mechanical techniques to address single photon ionization processes and Auger decays and computed the yields as a function of intensity for the formation of high charged ions.
Exploitation Route The techniques we have developed as a result of this proposal have been used as a stepping stone to address more complex strongly-driven processes.

Namely, we have used these tools to develop techniques that address strongly-driven two-electron triatomic molecules. Our findings on Rydberg formation in strongly-driven molecules have already motivated a large number of theoretical and experimental studies regarding the mechanisms of Rydberg formation in strongly-driven atoms and molecules. Moreover, we have very recently developed a state-of-the-art theoretical technique and computational tool that allows us to account for non-dipole effects in strongly driven systems. In the next few months we will perform ground-breaking studies of the magnetic field effects on the formation of Rydberg states and non-sequential double ionization in strongly-driven molecules. Currently, there are no theoretical studies of magnetic field effects in double ionization of strongly-driven multi-centre molecules due to the computational and theoretical complexity of these processes.
Sectors Digital/Communication/Information Technologies (including Software)

Education

Energy

 
Description Our techniques and results have been used by us and other AMO scientists as a stepping stone to explore more complex systems. Since 2015, I have been communicating the results of my research to junior high and high school students with the aim to educate young students about ultrafast phenomena and laser technologies and inspire them to follow a career path in science.
Sector Digital/Communication/Information Technologies (including Software),Education
 
Description Semi-classical models for ultra-fast multi-electron phenomena in intense electro-magnetic laser fields
Amount £336,665 (GBP)
Funding ID EP/N031326/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 09/2016 
End 09/2021
 
Description Theoretical Atomic and Molecular Physics (TAMOP) of the National Science Foundation (NSF) in the US
Amount $200,000 (USD)
Organisation National Science Foundation (NSF) 
Sector Public
Country United States
Start 08/2009 
End 09/2013
 
Title Two-electron dynamics in ultra-short and intense electromagnetic fields 
Description The last few months we have developed in my group a new state-of-the-art theoretical and computational tool to describe the two-electron dynamics in intense and ultra-short laser pulses. The innovative aspect and strength of this theoretical technique and computational tool is that it allows one to account for non-dipole effects, i.e. the effect of the magnetic field, in strongly-driven molecules. The vast majority of strong-field studies use the dipole approximation to describe strongly-driven molecules and in most studies the nuclei are fixed. This powerful computational tool will allow our group to perform the first studies of magnetic field effects during the break-up of strongly driven diatomic and triatomic molecules. In the next few months we will investigate the effect of the magnetic field in the different mechanisms of Rydberg formation, we have introduced the latter a few years ago, as well as the magnetic effects on non-sequential double ionization. This tool along with the magnetic field effects will be published in the next few months. 
Type Of Material Improvements to research infrastructure 
Year Produced 2020 
Provided To Others? No  
Impact This tool will have significant impact on studies of magnetic field effects during the break-up of strongly-driven multi-centre molecules. It will also allow to perform studies of magnetic field effects on non-sequential double ionization, which have never been performed before. 
 
Title Qunatum-mechanical tool for FEL-driven atoms 
Description We have developed a set of quantum mechanical tools to describe the interaction of Free-Electron Lasers with complex atoms. 
Type Of Material Computer model/algorithm 
Provided To Others? No  
Impact papers and invitations to conferences 
 
Title Semiclassical Toolkit for strongly-driven molecules 
Description We developed a semiclassical set of tools to describe in three-dimensions the break-up of molecules driven by intense and infrared laser pulses 
Type Of Material Computer model/algorithm 
Year Produced 2014 
Provided To Others? Yes  
Impact papers and invitations to conferences 
 
Title Semiclassical tool and method to account for non-dipole effects in strongly-driven molecules 
Description The last few months we have developed a new theoretical and computational tool to account for magnetic field effects in molecules driven by intense laser fields. This new tool will have big impact in the attoscience and strong field community since almost all studies up to now do not include magnetic field effects. 
Type Of Material Computer model/algorithm 
Year Produced 2019 
Provided To Others? No  
Impact This theoretical and computational tool we have developed in 2020, we will soon be published in an appropriate journal to effectively disseminate the results. We will use this technique to perform ground-breaking studies of magnetic field effects during the break-up of strongly-driven diatomic and triatomic molecules. 
 
Description Ionization in multi-electron atoms and molecules by intense laser pulses 
Organisation National Research Council of Canada
Country Canada 
Sector Public 
PI Contribution My collaboration with experimentalists Prof. P.B. Corkum and A.Staudte has resulted in four publications
Collaborator Contribution Provide experimental feedback on strongly-driven processes in atoms and molecules
Impact 4 co-authored papers
Start Year 2009
 
Description LMU Munich 
Organisation Ludwig Maximilian University of Munich (LMU Munich)
Country Germany 
Sector Academic/University 
PI Contribution We provided the theory for non-sequential double ionization of Ar for a range of intensities triggered by near-single cycle pulses. This collaboration resulted in 1 publication. In addition, we have collaborated on identifying a new mechanism of non-sequential double ionization for low intensities. Mainly we have identified the slingshot non-sequential double ionization, where two electrons escape opposite to each other. The main reason for the anti-correlation pattern is that one electron undergoes a slingshot motion around the nucleus as is the case in the gravitational slingshot motion of a spacecraft. The nucleus assists the electron in escaping in conjunction to the laser field. That is the change in momentum is in the same direction as the force from the field thus resulting in the electron absorbing the energy needed to escape.
Collaborator Contribution The group of Prof. M. F. Kling provided the experimental results for the above project. Moreover, in the second project Prof. M. Kling and B. Bergues contributed to formulating the new mechanism of non-sequential double ionization.
Impact This collaboration resulted in a very accurate theoretical description of the ultra-fast phenomena underlying non-sequential double ionization in atoms triggered by ultra-short laser pulses and the theory we provided explained very well the experimental results of Prof. M. F. Kling. Moreover, this collaboration resulted in identifying and formulating a new mechanism in strong field physics, namely, slingshot non-sequential double ionization which is the analog of the gravitational slingshot motion in strong field physics.
Start Year 2017
 
Description National Research Council in Ottawa and University of Ottawa 
Organisation National Research Council - Ottawa
Country Canada 
Sector Public 
PI Contribution My team was the main contributor in a collaboration with experimentalist Prof. P. B Corkum that resulted to the identification of a new non-dipole effect in non-sequential double ionization of strongly-driven He, namely, to non-dipole re-collision gated ionization In 2018, we collaborated with the group of Prof. Paul Corkum and Dr. Andre Staudte to control double ionization of Ar. To do so, we employed two-colour laser fields, one at 800 nm and the other at 2400 nm.
Collaborator Contribution Prof. P. B. Corkum discussed with me the signatures of non-dipole effects in single ionization of atoms which his attoscience experimental group has identified in 2011, and then we were then able to investigate new effects in double ionization of atoms. In 2019, my experimental collaborators have provided the experiments for strongly-driven Ar by two-colour laser fields, while my group has provided the theory. The work from this latter project has resulted in a publication where we identify how to streak the ionization times of two electrons in double ionization when Ar is strongly driven by a two-colour laser field.
Impact This collaboration has resulted in the identification of a new non-dipole effect and the publication of two journal papers. The collaboration is multi-disciplinary in the sense that Prof. Corkum is an experimentalist in the field of attoscience and my group is a theoretical and computational group in attosecond and strong-field science. Our joint work in 2018,. will result in an experimental scheme of identifying the ionization time of electrons in non-sequential double ionization of atoms driven by intense fields.
Start Year 2008
 
Description Seminar Series : Frontiers of Ultafast Science UK 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Other audiences
Results and Impact Starting in 2017, I have initiated a seminar series "Frontiers of Ultafast Science UK". This seminar series takes place in different Institutions and Universities across the UK. Up to now I have organised 5 such seminars up to now. The goal is to bring together scientists working in different ultrafast-science fields such as, atomic and molecular physics, condensed matter physics, quantum optics, ultrafast chemistry and so on.
Year(s) Of Engagement Activity 2017,2018,2019,2020