Study of molecules and clusters in intense laser fields using the quantum trajectory-based Coulomb-corrected strong-field approximation
Final Report Abstract
The main aim of the project was the further development of the trajectory-based Coulomb corrected strong-field approximation (TCSFA). Unlike in the plain strong-field approximation (SFA), in TCSFA, one takes the Coulomb interaction between the outgoing electron and the parent ion into account, either perturbatively, iteratively (i.e., given the final photoelectron momentum at the detector, the initial momentum is sought) or via a shooting method, where the full, classical equation of motion including Coulomb interaction from the tunnel exit on is considered. This is important because it has been shown in the past that many of the features observed in photoelectron spectra from experiment or ab initio numerical solutions are due to Coulomb effects. Only if the origin of these spectral features is understood they may be used for structural imaging or other purposes. We therefore proposed to extend the TCSFA towards (i) diatomic molecules, (ii) C60 and clusters, and (iii) more complex field conficurations, e.g., elliptical polarization or beyond the dipole approximation. In the course of the project it became clear that a much more fundamental and interesting question has to be addressed before the TCSFA is applied to complex systems, namely, how to treat recollisions of the complex quantum orbits that are properly matched to the ground state wavefunction. The Coulomb-correction leads to branch cuts that form gates in the complex-time plane whenever a recollision occurs. In order for the quantum orbits to be analytic, these branch cuts have to be circumnavigated when calculating the transition matrix element. As a result, the coventionally chosen quantum pathway from the complex ionization time straight down to the real-time axis and then towards infinity to the detector might not be possible. As the imaginary part of the time governs the ionization probability, this means that the ionization probability is not yet determined “at the tunnel exit”, and the conventional separation into a tunneling step and subsequent, quasi-free electron motion is impossible. This is similar to the double-slit experiment, where also the path behind the slits matters for the final outcome. In photoelectron spectra, the fact that the quantum path “after” the tunnel exit still matters for the ionization probability manifests itself as an enhanced yield around photoelectron energies of twice the ponderomotive potential, as investigated in detail. The TCSFA had so far been applied to single-active-electron problems. It has been extended for the treatment of metal clusters where the collective field due to the oscillating electron cloud about the ionic jellium background may generate much more energetic electrons than known from laser-atom interaction via surface-plasmon assisted rescattering in clusters (SPARC). Another highlight of the project was the semi-analytical treatment of Coulomb-corrected strong-field quantum trajectories beyond the dipole approximation. Non-dipole effects in strong-field photoelectron momentum spectra have been revealed experimentally for surprisingly low laser intensities. Even more interesting, for certain photoelectron momenta the spectra were found to be shifted against the laser propagation direction, i.e., opposite to the direction in which the light pressure acts. Only the interplay between Lorentz and Coulomb force may give rise to such counterintuitive dynamics. We tackled the problem using an extended TCSFA with magnetic non-dipole effects taken into account. The full TCSFA with numerically simulated quantum orbits revealed the counter-intuitive shift against the laser propagation. We were able to reproduce this result (almost) analytically by approximating the interaction of the magnetically corrected quantum orbit with the Coulomb potential, giving maximum insight into the underlying quantum dynamics.
Publications
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Collective-field-corrected strong field approximation for laser-irradiated metal clusters. J. Phys. B: At. Mol. Opt. Phys. 47, 124029 (2014)
Th. Keil, D. Bauer
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Laser-driven recollisions under the Coulomb barrier. Phys. Rev. Lett. 117, 243003 (2016)
Th. Keil, S.V. Popruzhenko, D. Bauer
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Coulomb-corrected strong-field quantum trajectories beyond dipole approximation. J. Phys. B: At. Mol. Opt. Phys. 50, 194002 (2017)
Th. Keil, D. Bauer
