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 C 2+ . 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 C 2 H - , C 2 HO - , C 2 H 5 O - , and C 2 H 3 O 2 - ) were formed. We performed measurements for deuterated methanol (CH 3 OD) 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 CH 3 OD with CH 3 OH droplets, we propose that the primary formation mechanism is association reactions of anion and neutral fragments, such as CH 3 O - + CO → C 2 H 3 O 2 - . 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.