Electronic structure and dynamics of solvated chromophores: femtosecond photoelectron spectroscopy from a liquid water jet
Final Report Abstract
Within this project, we have successfully established time-resolved photoelectron spectroscopy as an ultrafast spectroscopic tool to study the excited state dynamics of molecules in solution. The experimental setup used in this project makes use of UV or VUV laser pulses to probe the excited state of a solute or solvent molecule in aqueous solution. It is worth mentioning, that the photon energy is not suciently high to ionize ground state molecules. Therefore, this new technique is essentially background-free and allows for highly sensitive measurements even of very dilute samples. The focus of this project was on the hydrated electron and the excited state dynamics of DNA bases and nucleosides. Although the hydrated electron is one of the most important reagents in radiation chemistry, many of its properties are yet unknown. Among those there is also the important fundamental (and seemingly simple) quantity binding energy. We generated hydrated electrons by photodetachment of iodide anions both at the surface and in the bulk as well as by photoexcitation of liquid water in different excitation schemes. We determined the vertical detachment energy, i. e. the binding energy, of fully hydrated electrons to be 3.4 eV. In addition to the previously known recombination channel with the iodine radical, we found yet another decay channel with a sub-ps decay rate, which may point towards a surface-specic decay channel. Our work solved a long-standing discussion on the probe depth of liquid jet photoelectron spectroscopy at low kinetic energies and proved a distinct surface sensitivity. Our conservative estimates show a probe depth of about 5 nm. When hydrated electrons are generated by photoexcitation of liquid water, we have observed structures that are different from the fully hydrated electron and were previously unknown. For example, a hydrated electron cation complex at the interface is formed after excitation of a solvent molecule with 7.75 eV photons. The binding energy of this complex is 2.45 eV in H2O, which is signicantly smaller than the binding energy of the fully hydrated electron. Also, most of these complexes decay on a sub-ps timescale. The cation stabilizes the excess charge and is essential for this structure. In addition to the hydrated electron, we investigated the excited state dynamics of DNA bases and nucleosides. Our studies allow to follow the evolution of the excited state, both in time and energy and complements all-optical studies. Especially for guanosine, we have derived a different interpretation of the excited state dynamics.
Publications
- Role of alkali cations for the excited state dynamics of liquid water near the surface, J. Chem. Phys. 137, 024503 (2012)
F. Buchner, H. H. Ritze, M. Beutler, T. Schultz, I. V. Hertel, A. Lübcke
(See online at https://doi.org/10.1063/1.4732582) - Solvated electrons at the water-air interface: surface versus bulk signal in low kinetic energy photoelectron spectroscopy, Phys. Chem. Chem. Phys. 14, 5837 (2012)
F. Buchner, T. Schultz, A. Lübcke
(See online at https://doi.org/10.1039/c2cp23305c) - Electrokinetic charging and evidence for charge evaporation in liquid microjets of aqueous solution, J. Phys. Chem. B 117, 2422 (2013)
N. Preissler, F. Buchner, T. Schultz, A. Lübcke
(See online at https://doi.org/10.1021/jp304773n) - Time-resolved photoelectron spectroscopy of adenine and adenosine in aqueous solution, Phys. Chem. Chem. Phys. 15, 11402 (2013)
F. Buchner, H. H. Ritze, J. Lahl, A. Lübcke
(See online at https://doi.org/10.1039/c3cp51057c)
