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 .