Studies of Ultrafast Molecular processes by Multiparticle Imaging Techniques – SUMMIT

How are molecular orbitals formed from the atomic orbitals of the constituencies when a chemical bond is formed or inversely how do the atomic orbitals emerge from the molecular ones if a bond breaks? The current project aims to make a major contribution towards answering this question. We will use synchrotron radiation to core excite a molecule to a steeply repulsive dissociating potential energy curve and use the Auger electrons which are emitted during the dissociation as a probe providing the most detailed information of the evolution of the internuclear distances and the shape of the transient orbitals at the instant of their decay. Energy and direction of the Auger electrons and fragment ions will be imaged in coincidence. With this approach we will gain an unprecedented combination of time (core hole clock), spectral (narrow band width synchrotron) and spatial information (Auger electron diffraction).

This becomes possible by combining the world leading expertise of both teams in multiparticle coincidence imaging. While all previous experimental studies could detect only some of the fragments (electrons and ions) from the decay or focused on only a small region of the quantum mechanical phase space we aim at imaging in coincidence all electrons and all ions with 4p detection solid angle, opening a completely new perspective:

We will obtain the first electron diffraction images of the dissociation as it occurs. By detuning the excitation energy we will gain time resolution. Thanks to the coincidence detection we will obtain these diffraction images in the body-fixed frame of the molecule. Since the dissociation is much faster than rotation, the measured direction of the ionic fragments yields the orientation of the molecule at the instant of photo excitations. The electron which is emitted during the decay is diffracted in the molecular potential. The internuclear distances at the time of the decay as well as the electronic density distribution is encoded in the angular distribution of the electron in the molecular frame. The molecule illuminates itself during the decay and the coincident detection of electrons and ions allows measuring this “self-image” of the dissociating molecule.

The French partner of the consortium has pioneering contributions to these ultrafast processes, the German partner has pioneered multi particle imaging techniques (COLTRIMS). We will team up in this project to use the most advanced imaging techniques to read out all the time and spatial information encoded in the fragments and their relative angles and energies at once.
Our experiments will be performed at two complementary synchrotron radiation sources BESSY in Berlin (low energy) and SOLEIL, Saclay (high energy). We have performed a joint preparatory test experiment on the L shell ionization of HCl at BESSY which clearly demonstrates the power of our approach. HCl as a bench mark system will be further studied on the HCl K-shell. Compared to the L shell here more ways the electron cloud can relax and rearrange are open making the electronic and nuclear dynamics even richer. Building on the knowledge gained for a simple diatomic, we will then explore ultrafast dissociation of H2S at both the S 2p and the S 1s edges.
In a third line of experiments we will study ultrafast dissociation of O2. Here we will monitor the ultrafast motion by observing the Doppler shift of the Auger electrons. This work builds on non coincident studies of the Doppler effects by the French partner and a joint exploratory coincidence experiment performed at BESSY. We will excite a K electron to a repulsive orbital. This will trigger the dissociation. During the dissociation first a fast Auger electron will be emitted and subsequently a very low energy (0.5 eV) atomic Auger electron. We will measure both electrons and the molecular axis in coincidence. The Doppler shift of the slow electron will unveil how and when the symmetry is broken during the bond rupture.