Full resolution of the electronic and nuclear dynamics in
elementary chemical reactions by photoelectron spectroscopy
with 100-as to 10-fs pulses

We will study ultrafast chemistry in the extreme, focusing on the fastest elementary reactions. Direct and barrierless processes in molecules occur after the optical excitation in areas of the potential energy surfaces that cannot be reached by probe radiation from the ground state (“dark states”). The progress of the ultrafast reaction can therefore at best be followed by transitions to poorly known higher states and very little is known about the mechanisms. Time resolved photoelectron spectroscopy is not hampered by this limitation. It has, however, not yet been implemented with the intrinsic speed of the electronic and nuclear dynamics, typically 20 fs for prototypical organic molecules.

We plan to combine 5 to 10 fs excitation pulses tunable from 200 to 400 nm and with adjustable bandwidths covering several octaves (A.1.2 and existing expertise on achromatically phase matched SHG) and even shorter wavelengths obtained via spectrally selected higher harmonic radiation (from 5 to 50 eV; A.2.2) in a photoelectron spectrometer. This spectrometer will be equipped with angular resolution and/or mass selected ion detection to enable coincidence measurements. The deliberate selection of the EUV pulse length between 100 as and 10 fs permits optimisation between the necessary temporal resolution and the energetic resolution needed to resolve the electronic states. The tunability of the pump and probe light will help to differentiate between various species. The ultrabroad bandwidth of the pump could open up the opportunity to create electron nuclear wavepackets for selected molecules, similarly to C.1.4. The entanglement between electronic and nuclear dynamics and the related energy flow from electronic excitation towards nuclei could then be studied.

The range of molecules investigated will span from diatomics, where we plan to investigate the nonadiabatic dynamics connected to electronic nuclear wavepackets generated by the coherent excitation of several electronic levels at once, up to biomolecules where we hope to decipher the intramolecular energy transport via electron/hole (exciton) mobility. MAP’s unprecedented temporal resolution will allow for the first time to fully resolve the primary steps in these elementary chemical reactions. This project is supported by theoretical efforts (C.2.6), and technically related to C.1.2 and C.1.4. It is also a building block towards coherent control envisioned in C.2.5. It draws on source development of A.1.2, A.1.3 and A.1.4.