Speaker: Stephen E. Bradforth, USC
Program Description:
In the liquid phase, intermolecular interactions often control electronic structure, and therefore chemical reactivity, in particular excited state and ionization dynamics.
Photoelectron spectroscopy applied to liquid microjets has emerged as powerful way to access both solvent and solute electronic structure directly. Using liquid phase XPS, we have investigated the peculiar property liquid ammonia possesses in being able to stably support excess electrons, transitioning from an electrolyte to a metal. At low concentrations, electrons exist as localized solvated electrons and, with modest concentration increase, spin pair to form diamagnetic solutions, the basis for Birch’s reagent in organic synthesis. Further concentration increase yields a shiny golden liquid metal. Our experiments map out how the electronic band structure changes on approach to the metallic threshold. Complementary experiments have been conducted to explore high concentrations of excess electrons in water.
Recent time-resolved XAS and electron diffraction experiments conducted at LCLS have revived interest in the first few steps of direct ionization of liquid water – wherein an excess electron and an excess hole are produced. Our group has over a number of years, studied the first few steps of water ionization after well-defined amounts of energy are deposited with time resolution better than 50 fs, and has combined this with ab initio MD simulations. These studies will be reviewed to help define productive avenues for new studies with LCLS-II to better understand the primary steps in ionization events in water.
Background references:
1. Buttersack, T. et al. Photoelectron spectra of alkali metal–ammonia microjets: From blue electrolyte to bronze metal. Science 368, 1086–1091 (2020).
2. Mason, P. E. et al. Spectroscopic evidence for a gold-coloured metallic water solution. Nature 595, 673–676 (2021).
3. Elles, C. G., Rivera, C. A., Zhang, Y., Pieniazek, P. A. & Bradforth, S. E. Electronic structure of liquid water from polarization-dependent two-photon absorption spectroscopy. J Chem Phys 130, 084501 (2009).
