The technique of liquid-microjet photoelectron (PE) spectroscopy has become an emerging field to study electronic structure of chemical, biological, or atmospheric relevant aqueous solutions, and this includes chemical-reaction products as well as high-energy radiation induced damage via secondary ionization and free electrons. The latest results and ongoing research topics of X-ray PE spectroscopy from liquid water and aqueous solution will be presented.
After a short overview of the current directions in the field, the first topic will cover the recent study on non-local relaxation processes after core-level ionization. Relaxation dynamics is revealed from resonant photoelectron spectroscopy measurements. For neat liquid water we find a unique de-excitation channel, involving core-hole delocalization, which is a signature of energy and charge transfer to a neighbor water molecule. In particular, the role of nuclear motion during decay, depending on the bonding structure, will be discussed. By comparing of results from H2O(aq) and D2O(aq) the recently described proton-transfer mediated (PTM) change separation process[1] is introduced, and then expanded to other molecules with the ability to form hydrogen bonds in solution.
The second part of the talk concentrates on photoelectron angular distribution (PAD) and electron scattering, which is closely related to probing depth. PE spectroscopy is well suited for determining solute density profiles across the liquid interface. However, correlation between measured signal and solute abundance is complicated, since important parameters, like ionization cross sections and PADs for ions and molecules but also accurate electron scattering cross sections in solution, are not known. First core-level PADs from liquid water and aqueous solution at a wide range of photon energies are reported. Although emission anisotropy is reduced compared to gas phase, PADs are found to not become fully isotropic, which in turn can be utilized to extract electron scattering ratios in the liquid[2]. Future applications and implications for interfacial depth-profile studies are discussed.
[1] S. Thurmer, M. On?ak, N. Ottosson, R. Seidel, U. Hergenhahn, S. E. Bradforth, P. Slavi?ek, B. Winter; Nat. Chem. 5: 590-596 (2013)
[2] S. Thurmer, R. Seidel, M. Faubel, W. Eberhardt, J. C. Hemminger, S. E. Bradforth, B. Winter; Phys. Rev. Lett. 111, 173005 (2013)
