At intensities below-the-recollision threshold, we show that re-collision-induced excitation with one electron escaping fast after re-collision and the other electron escaping with a time delay via a Coulomb slingshot motion is one of the most important mechanisms of non-sequential double ionization, for strongly-driven He at 400 nm. Slingshot-NSDI is a general mechanism present for a wide range of low intensities and pulse durations. Anti-correlated two-electron escape is its striking hallmark. This mechanism offers an alternative explanation of anti-correlated two-electron escape obtained in previous studies.
Multi-electron dynamics in atoms and molecules very often occur on sub- to few-femtosecond time scales. The available intensities of extreme-ultraviolet (XUV) attosecond pulses have previously allowed the time-resolved investigation of two-photon, two-electron interactions. Here we study double and triple ionization of argon atoms involving the absorption of up to five XUV photons using a pair of intense attosecond pulse trains (APTs). By varying the time delay between the two APTs with attosecond precision and the spatial overlap with nanometer precision, we obtain information on complex nonlinear multi-photon ionization pathways. Our experimental and numerical results show that A r 2 + is predominantly formed by a sequential two-photon process, whereas the delay dependence of the A r 3 + ion yield exhibits clear signatures of the involvement of a simultaneous two-photon absorption process. Our experiment suggests that it is possible to investigate multi-electron dynamics using attosecond pulses for both pumping and probing the dynamics.
We study the interaction of a heteronuclear diatomic molecule, carbon monoxide, with a free-electron laser (FEL) pulse. We compute the ion yields and the intermediate states by which the ion yields are populated. We do so using rate equations, computing all relevant molecular and atomic photo-ionisation cross-sections and Auger rates. We find that the charge distribution of the carbon and oxygen ion yields differ. By varying the photon energy, we demonstrate how to control higher-charged states being populated mostly by carbon or oxygen. Moreover, we identify the differences in the resulting ion yields and pathways populating these yields between a homonuclear molecule, molecular nitrogen, and a heteronuclear molecule, carbon monoxide, interacting with an FEL pulse. These two molecules have similar electronic structure. We also identify the proportion of each ion yield which accesses a two-site double-core-hole state and tailor pulse parameters to maximise this proportion.
We study the interaction of xenon with an 850 eV photon energy FEL pulse. We compute singlephoton ionisation cross sections and Auger rates by adopting to atoms a formalism we previously developed for diatomic molecules. In this formulation, a bound orbital is expressed as a sum of wave functions each corresponding to a different l quantum number. In contrast, in previous formulations only one l quantum number is associated with a bound orbital. As a result, in the non-relativistic regime, the description of the bound states is more accurate in our computations. Employing a Monte-Carlo technique, we find that our results for the ion state yields of xenon compare well with experimental results. Moreover, we find that when xenon is driven by two FEL pulses of the same energy but different pulse duration, higher-charged states are produced in the case of the longer duration and less intense laser pulse. An analysis of the ionization pathways reveals that less single-photon absorptions underlie the formation of each higher-charged state for the longer duration pulse compared to the shorter one. We find that the reason for the formation of higher-charged states for the longer duration pulse is the prevalence of Auger cascades.
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