High harmonic spectra show that laser-induced strong field ionization of water has a significant contribution from an inner-valence orbital. Our experiment uses the ratio of H2O and D2O high harmonic yields to isolate the characteristic nuclear motion of the molecular ionic states. The nuclear motion initiated via ionization of the highest occupied molecular orbital (HOMO) is small and is expected to lead to similar harmonic yields for the two isotopes. In contrast, ionization of the second least bound orbital (HOMO-1) exhibits itself via a strong bending motion which creates a significant isotope effect. We elaborate on this interpretation by simulating strong field ionization and high harmonic generation from the water isotopes using the time-dependent Schrödinger equation. We expect that this isotope marking scheme for probing excited ionic states in strong field processes can be generalized to other molecules.

In a recent PACER (Probing Attosecond dynamics with Chirp-Encoded Recollisions) experiment on ammonia that comprises a comparison of the high-harmonic spectra of the isotopes NH3 and ND3, the nuclear dynamics of the created ammonia cation is traced with a time resolution of about 100 attoseconds. For modelling the experiment the autocorrelation functions between the neutral initial state and the ionic wave packet are extracted from experimental photoelectron spectra incorporating a correction for the geometry-dependent strong-field ionisation probability. Good agreement is found between model and experiment, but in addition an unexpected maximum in the autocorrelation ratio is predicted by the model, however occurring at 5 fs and thus outside the experimentally covered time interval. In this work the autocorrelation functions are calculated explicitly using a one-dimensional model for describing the inversion motion of ammonia and its cation, adopting a position-dependent mass for considering the coupling with the stretching mode of the hydrogen atoms in neutral ammonia. This results in a clear physical picture explaining the occurrence of the previously predicted maximum in the ratio of the autocorrelation functions. Furthermore, different initial states and two different ways of incorporating strong-field corrections to the Franck-Condon approximation are briefly discussed.

We represent low dimensional quantum mechanical Hamiltonians by moderately sized finite matrices that reproduce the lowest O(10) bound-state energies and wave functions to machine precision. The method extends also to Hamiltonians that are neither Hermitian nor PT symmetric and thus allows one to investigate whether or not the spectra in such cases are still real. Furthermore, the approach is especially useful for problems in which a position-dependent mass is adopted, for example in effective-mass models in solid-state physics or in the approximate treatment of coupled nuclear motion in molecular physics or quantum chemistry. The performance of the algorithm is demonstrated by considering the inversion motion of different isotopes of ammonia molecules within a position-dependent mass model and some other examples of one- and two-dimensional Hamiltonians that allow for the comparison to analytical or numerical results in the literature.

A quantum simulator based on ultracold optically trapped atoms for simulating the physics of atoms and molecules in ultrashort intense laser fields is introduced. The slowing down by about 13 orders of magnitude allows to watch in slow motion the tunneling and recollision processes that form the heart of attosecond science. The extreme flexibility of the simulator promises a deeper understanding of strong-field physics, especially for many-body systems beyond the reach of classical computers. The quantum simulator can experimentally straightforwardly be realized and is shown to recover the ionization characteristics of atoms in the different regimes of laser-matter interaction.In his renowned lecture, "Simulating physics with computers" [1] Richard P. Feynman suggested the use of quantum simulators, i.e. precisely controllable quantum systems, to simulate other quantum systems that cannot be described theoretically due to their exponentially growing Hilbert space. For instance, the Mott-insulator to superfluid phase transition in condensed-matter systems [2] was predicted [3] to be observable with ultracold atoms in an optical lattice and then successfully demonstrated [4,5]. Also the Higgs mechanism [6], high temperature superconductivity [7], or Zitterbewegung [8] (to name just a few) were successfully investigated by quantum simulation. Moreover, the quantum simulation of electrons in crystalline solids exposed to laser fields [9] has been proposed. Strong-field physics has contributed considerably to the understanding of the light-matter interaction. The progress leading to pulses on the attosecond timescale [11] has even raised visions of real-time imaging of molecular processes [12] and orbital tomography [13]. Yet, attosecond many-body physics is challenging. An exact investigation on classical computers beyond the single-active-electron approximation becomes prohibitively complex for many-electron systems. In fact, the numerical treatment of two-electron systems like He or H 2 is today still state of the art [14][15][16][17]. Thus, simplified models are widely used for interpreting modern experiments. These models are controversial and their validation is difficult for several reasons. First, the used light pulses are bound to the specifications of the laser. The wavelength range of lasers is limited, mostly Ti:sapphire lasers are used. The pulse shapes are restricted and can often only be reproduced and determined up to a considerable uncertainty. The intensity and timescale of laser pulses are already pushed to a limit where further improvements require major technical or even principle developments with new limitations, like free-electron lasers. Second, atoms, ions, and molecules are complicated many-body systems. Their internal structure cannot be simply manipulated. For example, a variation of the number of electrons or protons underlies constraints due to electroneutrality. Third, although the correlation of electronic and nuclear motion is known to influence the ionization behavior [18,19], in mo...

The alignment-and internuclear-distance-dependent ionization of H 2 exposed to intense, ultrashort laser fields is studied by solving the time-dependent two-electron Schrödinger equation. In the regime of perturbative few-photon ionization, a strong dependence of the ionization yield on the internuclear distance is found. While this finding confirms a previously reported breakdown of the fixed-nuclei approximation for parallel alignment, a simpler explanation is provided and it is demonstrated that this breakdown is not due to vibrational dynamics during the laser pulse. The persistence of this effect even for randomly aligned molecules is demonstrated. Furthermore, the transition from the multiphoton to the quasistatic (tunneling) regime is investigated considering intense 800 nm laser pulses. While the obtained ionization yields differ significantly from the prediction of Ammosov-Delone-Krainov rates, we find a surprisingly good quantitative agreement after introducing a simple frequency-dependent correction to the standard tunneling formula.

The geometry-dependent ionization behavior of the ammonia molecule is investigated. Different theoretical approaches for obtaining the ionization yield are compared, all of them showing a strong dependence of the ionization yield on the inversion coordinate at long wavelengths (≥ 800 nm). It is shown how this effect can be exploited to create and probe nuclear wave packets in neutral ammonia using Lochfraß. Furthermore, imaging of a wave packet tunneling through the barrier of a double-well potential in real time is discussed.

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