Role of the Environment in Quenching the Production of H3+ from Dicationic Clusters of Methanol

Ionization and subsequent isomerization of organic molecules has been suggested as an important source of trihydrogen ${\mathrm{H}}_{3}^{+}$ cations in outer space. The high interest in such reactions has initiated many experimental and theoretical studies for various molecules. Here, we report measurements as well as ab initio molecular dynamics simulations on the fragmentation of dicationic methanol monomers and clusters ionized by low-energy (90 eV) electrons. Experimentally, for dicationic monomers, a fragmentation channel for the formation of ${\mathrm{H}}_{3}^{+}$ in coincidence with a ${\mathrm{COH}}^{+}$ cation is observed. The simulations show that an intermediate neutral ${\mathrm{H}}_{2}$ is formed in the first step, and its roaming around the dication ends in the formation of ${\mathrm{H}}_{3}^{+}$. The entire reaction takes about 100--500 fs. The calculated kinetic energy release for the ${\mathrm{H}}_{3}^{+}+{\mathrm{COH}}^{+}$ ion pair is in excellent agreement with the experimental result. In contrast, for the dicationic clusters, due to the possibility of distributing the two charges onto different molecules, several fast dissociation channels occur and suppress the roaming of ${\mathrm{H}}_{2}$ and formation of ${\mathrm{H}}_{3}^{+}$. The present Letter suggests that the quenching of ${\mathrm{H}}_{3}^{+}$ formation by the chemical environment is a general phenomenon in dicationic clusters of organic molecules.