In the years 2018-2020 the electronic structure of hydrated proton complexes in acetonitrile has been studied. Crucial was the development of a stable acetonitrile flatjet, used in the July and November 2018 beamtime measurements at the Berlin BESSYII user facility. Strong orbital interactions and electric field effects by the positively charged proton have been probed with oxygen K-edge spectroscopy. In the May/June 2019 BESSYII beamtime charge distribution changes of a photoacid molecule, 8-aminopyrene-1,3,6-trisulfonate (APTS) along the four stages of its Förster cycle, have been explored with picosecond N K-edge spectroscopy, with additional characterization of APTS with UV/IR pump-probe measurements. For both projects publications have been published in 2022 in the renowned journal Angewandte Chemie International Edition. The publication on the O K-edge spectroscopy of the hydrated proton complex has led to a News item in Chemistry World (by Kira Welter, dated 31.10.2022).
A collaboration with Yale University on ultrafast X-ray absorption spectroscopy probing the vibronic wavepacket dynamics of photodissociation of ICN and of the photoinduced enol* --> keto* tautomerization reaction of 2-(2′-hydroxyphenyl)benzothiazole (HBT) has resulted in publications in Angewandte Chemie International Edition and The Journal of Physical Chemistry Letters. Our findings on HBT indicate that ultraviolet/X-ray pump-probe spectroscopy provides a unique way to probe ultrafast electronic structure rearrangements in photoinduced chemical reactions essential to understanding the mechanism of PCET.
The COVID-19 pandemic has slowed down the XRayProton activities at large scale facilities. To prevent a severe standstill, activities focussed more on experiments using the laser laboratories at the Max Born Institute. These include the determination of proton transport dynamics of 7HQ interacting with added bases using femtosecond UV-pump-IR-probe spectroscopy. This enables a benchmarking of photoacid-base reaction pairs with much detail, as reported in two publications, one on the 7HQ – formate and the other on the 7HQ – imidazole system (published in The Journal of Physical Chemistry Letters).
Meanwhile research activities at large scale facilities have resumed operation in the course of 2021. Experiments on photoacid – base reaction dynamics have been studied in 2021 – 2022 during four beamtimes at BESSYII and one remote access beamtime at LCLS in December 2021. A more refined analysis exploiting the jitter between UV pump and X-ray probe has provided insight into the interplay of hydrogen bond rearrangements and excess vibrational energy redistribution on the proton transfer dynamics of photoacid - imidazole reaction paits. A steady-state nitrogen K-edge spectroscopic study has been pursued as a benchmark for the role of proton delocalisation in a ow-barrier double-well potential for the proton transfer coordinate in the protonated imidazole dimer (this study has been disseminated in The Journal of Physical Chemistry Letters).
A second alternate route to guarantee a continuation of the XRayProton project is the further development of ultrafast UV-pump-soft-X-ray-probe spectroscopy using table-top laser systems exploiting extreme high-order harmonic generation (HHG). In previous years this has resulted in a successful demonstration of steady-state C and N K-edge spectroscopy of small molecules and ions in aqueous solution. In further pursuing this methodology in terms of improvement in HHG efficiency, stability and delivery at the flatjet sample target, and improving the photon flux at the CCD detector, a new spectrometer has been designed and implemented with optimized x-ray optics using reflective zone plates, as described in a publication in Structural Dynamics. First femtosecond pump-probe spectroscopic experiments using extreme HHG pulses with energies extending into the N K-edge spectral region have been accomplished. Strong field ionization of molecular nitrogen N2 and the photodissocation of nitrogen dioxide, has led to publications in Physical Review Lettes and The Journal of Physical Chemistry Letters.
