Ultrafast Roaming Mechanisms in Ethanol Probed by Intense Extreme Ultraviolet Free-Electron Laser Radiation: Electron Transfer versus Proton Transfer
Enliang Wang, Nora G. Kling, Aaron LaForge, Razib Obaid, Shashank Pathak, Surjendu Bhattacharyya, Severin Meister, Florian Trost, Hannes Lindenblatt, Patrizia Schoch, M. Kübel, Thomas Pfeifer, Artem Rudenko, Sergio Díaz‐Tendero, Fernando Martı́n, R. Moshammer, Daniel Rolles, N. Berrah
Abstract
Ultrafast H 2 + and H 3 + formation from ethanol is studied using pump-probe spectroscopy with an extreme ultraviolet (XUV) free-electron laser. The first pulse creates a dication, triggering H 2 roaming that leads to H 2 + and H 3 + formation, which is disruptively probed by a second pulse. At photon energies of 28 and 32 eV, the ratio of H 2 + to H 3 + increases with time delay, while it is flat at a photon energy of 70 eV. The delay-dependent effect is ascribed to a competition between electron and proton transfer. High-level quantum chemistry calculations show a flat potential energy surface for H 2 formation, indicating that the intermediate state may have a long lifetime. The ab initio molecular dynamics simulation confirms that, in addition to the direct emission, a small portion of H 2 undergoes a roaming mechanism that leads to two competing pathways: electron transfer from H 2 to C 2 H 4 O 2+ and proton transfer from C 2 H 4 O 2+ to H 2 .