Skip to main content

Transition States for Multielectron Ionization Phenomena

Final Report Summary - TRANS-MI (Transition States for Multielectron Ionization Phenomena)

Subjecting atoms or molecules to external perturbations, originating from the surrounding environment or from external electromagnetic fields, represents one of the main avenues to a better understanding of laser-matter interactions, of the intrinsic motion of atoms or molecules (on their own spatial and temporal scales) and their interaction with the environment. In the long run, it contributes to understanding the dynamical mechanisms responsible for chemical reactions, which are transitions in these multi-electron systems mediated by selected exchanges of energy inside these complex systems. The challenge is two-fold: These systems usually involve a large number of degrees of freedom, and the interactions between these degrees of freedom usually display a very complex -chaotic- dynamics which is very difficult to comprehend. The FP7 Marie Curie IRSES TranS-MI tackled these fundamental issues by using a multidisciplinary combination of tools from atomic physics, chemical physics, nonlinear dynamics and applied mathematics. The objective was to go beyond intensive numerical calculations of trajectories by identifying the relevant mechanisms at play at the microscopic level, using for instance, dynamically invariant objects in phase space or transition state theory. This approach has the tremendous advantage of reducing the dimensionality and the complexity of the analysis drastically, and focusing on the relevant part of phase space responsible for the transitions undergone by the atom or the molecule.

The work performed during TranS-MI is mostly fundamental research but it is also the necessary driver of technological applications. For instance, understanding the dynamics of atoms or molecules driven by ultrashort and near-optical lasers remains a tremendous challenge while it exerts a significant impact on science and industry by enabling a series of cutting-edge techniques: the production of lasers with ever shorter wavelengths by high harmonic generation, analysis by laser-induced electron diffraction, and orbital tomography, to quote just a few applications. For the study of molecules interacting with their environment, understanding how external factors influence reaction rates and controlling chemical reactivity opens the way for selective chemistry, e.g. avoiding the presence of undesirable products and contributing to reducing environmental contamination.

For atoms or molecules subjected to intense laser pulses, we have determined the dynamical mechanisms responsible for "recollisions", which are the way the energy exchange between the laser and the target (the atom or the molecule) takes place. Specifically, we have, for the first time, demonstrated that phase-space structures (periodic orbits and their manifolds) drive recollision processes (as they do in chemical reactions). This discovery has already allowed us to use the laser parameters to control the laser-matter interaction. This discovery enhances our understanding of the recollision process. We are currently exploring the connections of these results with research in applied mathematics and celestial mechanics.

We have also studied chemical reactions which are driven by a laser field, or by other interactions with their changing environments such as what arises from a nonstatic solvent. In many cases, a reaction occurs when the reacting atoms overcome an energetic barrier: This can be likened to a hiker getting across a mountain range by traversing a mountain pass rather than the highest mountain peak. In these cases, we found that there is a special trajectory that remains near the top of the mountain pass for all time even in the presence of driving which would result in the mountains moving up and down. It can be understood as marking the point where the chemical reaction (or transition across the mountain range) occurs. We have shown that a reaction rate can often be obtained from the stability properties of this special trajectory. This approach allows us to calculate reaction rates efficiently, without having to compute millions of reactive trajectories, as most other computational schemes require.

The FP7 IRSES TranS-MI network was composed of three European beneficiaries, CNRS (France), Loughborough University (UK), Universidad Politecnica de Madrid (Spain) and one partner, Georgia Institute of Technology (USA). The secondments between these institutions have strengthened and developed new collaborative links between them. In the four years of the project, four workshops have been organized: the kick-off meeting in Atlanta in August 2012, the Illuminyating workshop in Loughborough in May 2013, the TranS-MI symposium in Atlanta in August 2014, the Illuminyating workshop in Madrid in May 2015. These network-wide meetings were the opportunities to exchange ideas between the members of the network, and also to showcase our results to a broader and multi-disciplinary scientific audience. They have been the catalyzers for new collaborations which went beyond our expectations established from the original IRSES project.