Ultrafast Roaming Mechanisms in Ethanol Probed by Intense Extreme Ultraviolet Free-Electron Laser Radiation: Electron Transfer versus Proton Transfer.

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 .