Atomic Physics

The atomic physics section centers on the question: How and when are ionized electrons driven back to the ionic core by an ultrastrong, ultrashort laser pulse? The answer to this question holds the key to future breakthroughs like the real-time imaging of biomolecules with bright short-wave light sources. In a strong field most ionized electrons drift far from the core and cannot contribute to energy transfer processes. However, a few –very few– find their way back to the core, and bring with them the energy they absorbed from the laser, thereby becoming the drivers of the all-important phenomena of High Harmonic Generation (HHG) and multiple ionization. That is why predicting which electrons return matters greatly. The recollision scenario states that after ionization (presumably by tunnelling ionization through the potential barrier created by the combination of the oscillating electric field and the Coulomb potential), the ionized electron is potentially hurled back to the atomic (or molecular) core by the laser field alone. Upon collision with the core, the kinetic energy gained from the laser field is transferred to other electrons of the core (potentially leading to nonsequential multiple ionization) or emitted as high-frequency electromagnetic radiation (potentially leading to HHG) when the pre-ionized electron recombines with the core.

In order to gain insight into these phenonema, we consider the classical mechanical treatment of the underlying processes. The main advantage of classical mechanics is the power-law scaling of its representation with system size, as compared with the exponential increase of complexity of quantum mechanics. Another advantage of the classical treatment compared to its quantum counterpart is the notion of a trajectory, which allows for an in-depth analysis of the electronic dynamics in phase space on its natural spatial and temporal timescales. In our recent work on recollision physics in linearly polarized laser fields, we found that classical-dynamical structures, originating from resonances of the atom-field system, determine the initial conditions that lead to a recollision. To picture the workings of these structures, it is helpful to imagine the initial conditions for electrons in the continuum as hypervolumes in multidimensional position-momentum (or “phase”) space. The manifolds in question act like barriers – or “scissors”– which partition this space of initial conditions into smaller volumes which lead to differing outcomes. The same phase space partitioning features prominently in the Transition State Theory of chemical reactions where we have identified the multidimensional manifolds which separate reactive volumes from nonreactive ones for realistic chemical reactions. Sophisticated methods are gradually becoming available to compute these manifolds, some of them in TraX, and we take full advantage of these tools.