Capturing electron emission dynamics

The Universe is made of a fixed number of elements that form all kinds of molecules. These molecules are not static and unchanging — from activity of electrons and nuclei to complex 3D conformational changes, molecules are in motion. Molecular dynamics govern the properties of materials and the functions of biological systems, and superexcited states provide a window on quantum mechanical mechanisms.

Such states cannot be initiated from neutral ground-state molecules using conventional femtosecond lasers whose capabilities are limited by their possible wavelengths. Scientists overpassed the hurdle by initiating such states from fast beams of negative ions or metastable neutral species that are energetically closer to superexcited states. EU funding of the project 'Time resolved superexcited state dynamics' (EXTREME DYNAMICS) gave them the opportunity.

A custom-made laser was equipped with a pulse shaper to control and optimise the spectral phase of ultrafast pulses. A fast ion beam set-up was also constructed to create cold molecular and cluster anions (negatively charged species produced by addition of electrons to neutral ones).

The team then integrated a system for photofragment spectroscopy. It relies on a very-high–resolution imaging system and a time-to-digital converter. The equipment measures the time that fragments hit a micro-channel plate detector and the 2D positions of the fragments on the detector. A dedicated spectrometer enables separation of products.

Application of the set-up revealed a novel multiple detachment scenario. In some cases, two or more electrons are ejected from the parent anion (negatively charged) molecule. Losing the excess electrons that gave the molecule a negative charge plus one or more additional ones produces cationic (positively charged) products. Exploiting their newly constructed instrumentation, researchers characterised a new, highly efficient non-sequential mechanism and showed that it is different from a well-established double-ionisation mechanism of neutral systems.

Progress achieved within the scope of the EXTREME DYNAMICS project led to numerous publications and development of a new course at the host institution, as well as workshops and seminars at other institutions. The results will aid in modelling, predicting and eventually controlling the outcome of chemical reactions.