Reactions of size-selected protonated water clusters H~+(H_2O)_n (n = 2-6) with an acetone molecule in a guided ion beam apparatus

摘要

The reactions of size-selected protonated water clusters, H~+(H_2O)_n(n=2-6), with an acetone molecule were studied in a guided ion beam (GIB) apparatus. The collision energy dependence of the reaction cross-sections was investigated over the range of 0.05-2.0 eV in the center-of-mass frame. Water cluster beams with very narrow kinetic energy spreads, 0.02-0.04 eV, were generated to allow accurate measurement of the reaction cross-sections. We observed reaction products of incorporation and dissociation: the incorporation products are cluster ions having acetone whereas the dissociation products are protonated water clusters resulting from the evaporation of water molecule(s) from the parent cluster ions. Our results demonstrate that the incorporation process is dominant at low collision energies and that the proportion of the dissociation process increases as the collision energy increases. The incorporation cross-section at n = 5 is found to be the largest among the cluster sizes examined at the collision energy as low as 0.1 eV, which may be due to the water molecule in the second shell of the cluster. We also find that when the collision energy is larger than 0.5 eV, the dissociation cross-section has a maximum at n = 5, which is also probably due to the water molecule in the second shell of the cluster, and a minimum at n = 3, which may occur because the cluster ion core of the trimer, H_3O~+, has one H-site that is not occupied by water molecules.