Fast Heavy-Ion-Induced Anion-Molecule Reactions on the Methanol Droplet Surface

To gain insight into complex ion-molecule reactions induced by MeV-energy heavy ion irradiation of condensed matter, we performed a mass spectrometric study of secondary ions emitted from methanol microdroplet surfaces by 2.0 MeV C²⁺. We observed positive and negative secondary ions, including fragments, clusters, and reaction products. We found that a wider variety of negative ions than positive ions (such as C₂H^-, C₂HO^-, C₂H₅O^-, and C₂H₃O₂^-) were formed. We performed measurements for deuterated methanol (CH₃OD) droplets to identify the hydrogen elimination site of the intermediates involved in the reactions and to reveal the mechanism that generates various negative reaction product ions. Comparing the results of CH₃OD with CH₃OH droplets, we propose that the primary formation mechanism is association reactions of anion and neutral fragments, such as CH₃O^- + CO → C₂H₃O₂^-. Quantum chemical calculations confirmed that the reactions can proceed with no barrier. This study provides new insights into the importance of rapid anion-molecule reactions among fragments as the mechanism that generates complex molecular species in fast heavy-ion-induced reactions.