The potential energy curves for molecular ions up to N-2(4+) are calculated in an ab initio manner using the multi configurational self-consistent field method. Specifically, we implement in an automatic way a previously used double loop optimisation scheme within the multi configurational self-consisted field method. We obtain the potential energy curves up to N-2(4+) ions with any combination of core, inner valence, and outer valence holes. Finally, we provide the code used to generate these potential energy curves.

The potential energy curves for molecular ions up to $N_2^{4+}$ are calculated in an ab initio manner using the multi configurational self-consistent field method. Specifically, we implement in an automatic way a previously used double loop optimisation scheme within the multi configurational self-consisted field method. We obtain the potential energy curves up to $N_2^{4+}$ ions with any combination of core, inner valence, and outer valence holes. Finally, we provide the code used to generate these potential energy curves.

Multi-electron dynamics in atoms and molecules very often occur on sub- to few-femtosecond timescales. The available intensities of extreme-ultraviolet (XUV) attosecond pulses have previously only allowed the time-resolved investigation of two-photon, two-electron interactions. Here we demonstrate attosecond control over double and triple ionization of argon atoms involving the absorption of up to five XUV photons. In an XUV-pump XUV-probe measurement using a pair of attosecond pulse trains (APTs), the Ar$^{2+}$ ion yield exhibits a weak delay dependence, showing that its generation predominantly results from the sequential emission of two electrons by photoabsorption from the two APTs. In contrast, the Ar$^{3+}$ ion yield exhibits strong modulations as a function of the delay, which is a clear signature of the simultaneous absorption of at least two XUV photons. The experimental results are well reproduced by numerical calculations that provide detailed insights into the ionization dynamics. Our results open up new opportunities for the investigation and control of multi-electron dynamics and complex electron correlation mechanisms on extremely short timescales.

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 multiple ionization of Ar using a pair of intense attosecond pulse trains. By controlling the delay with attosecond precision and the spatial overlap with nanometer precision, complex multiphoton ionization pathways can be identified.

We demonstrate attosecond control of the multi-photon multiple ionization of argon. While a weakly oscillating Ar2+ is found in an autocorrelation measurement, the Ar3+ ion yield strongly oscillates due to direct multi-photon absorption.

We investigate the interaction of Xe with isolated attosecond XUV pulses. Specifically, we calculate the ion yields and determine the pathways leading to the formation of ionic charged states up to Xe$^{5+}$. To do so, in our formulation we account for single-photon absorption, sequential multi-photon absorption, direct two-photon absorption, single and double Auger decays, and shake-off. We compare our results for the ion yields and for ion yield ratios with recent experimental results obtained for 93 eV and 115 eV attosecond XUV pulses. In particular, we investigate the role that a sequence of two single-photon ionization processes plays in the formation of Xe$^{4+}$. We find that each one of these two processes ionizes a core electron and thus leads to the formation of a double core-hole state. Remarkably, we find that the formation of Xe$^{5+}$ involves a direct two-photon absorption process and the absorption of a total of three photons.

We study the interaction of xenon with an 850 eV photon energy FEL pulse. We compute single-photon 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.

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 photoionisation 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 do so by employing a Monte-Carlo technique. We compute the single-photon ionisation cross sections and Auger rates, used in the Monte-Carlo technique, by adopting to atoms a formalism we previously developed for diatomic molecules. We determine the yields of the ion states of driven xenon and compare with previously obtained experimental results. To better understand the yields obtained, we identify the prevalent pathways leading to the formation of each final ion state of xenon. We gain further insight into the high yields of highly-charged ion states by comparing the yields and dominant pathways of these ion states of xenon when driven by different FEL pulses that have the same energy. We show that higher-charged ion states have higher yields when xenon is driven by longer-duration pulses due to Auger cascades taking place between subsequent single-photon ionisations.

At intensities below the recollision threshold, we show that recollision-induced excitation with one electron escaping fast after recollision and the other electron escaping with a time delay via a Coulomb slingshot motion is one of the most important mechanisms of nonsequential double ionization (NSDI), for strongly driven He at 400 nm. Slingshot NSDI is a general mechanism present for a wide range of low intensities and pulse durations. Anticorrelated two-electron escape is its striking hallmark. This mechanism offers an alternative explanation of anticorrelated two-electron escape obtained in previous studies.

G. P. Katsoulis,1 A. Hadjipittas,1 B. Bergues,2, 3 M. F. Kling,2, 3 and A. Emmanouilidou1 Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom Department of Physics, Ludwig-Maximilians-Universität Munich, Am Coulombwall 1, D-85748 Garching, Germany Max Planck Institute of Quantum Optics, Hans-Kopfermann-Str. 1, D-85748 Garching, Germany (Dated: November 6, 2018)

At intensities below-the-recollision threshold, we show that re-collision-induced excitation with subsequent nucleus-assisted ionization, RESNI, is one of the most important mechanisms of non-sequential double ionization, for strongly-driven He at 400 nm. In RESNI the electron that is still bound after re-collision is assisted by the nucleus and the laser field to ionize mostly past even extrema of the laser field after re-collision. RESNI is a general mechanism present for a wide range of low intensities and pulse durations. We find that anti-correlated two-electron escape is the striking hallmark of RESNI. This mechanism offers an alternative explanation to multiple re-collisions, which was put forth to explain anti-correlated two-electron escape obtained in previous studies.