Collision-induced dissociation of protonated water clusters

摘要

Collision-induced dissociation (CID) has been studied for protonated water clusters H~+(H_2O)_n, with n = 2-8, colliding with argon atoms at a laboratory energy of 8 keV. The experimental data have been taken with an apparatus (Device for Irradiation of Molecular Clusters, 'Dispositif d'Irradiation d'Agrégats Moléculaire, DIAM) that has been recently constructed at the Institut de Physique Nucléaire de Lyon. It includes an event-by-event mass spectrometry detection technique, COINTOF (correlated ion and neutral fragment time of flight). The latter device allows, for each collision event, to detect and identify in a correlated manner all produced neutral and charged fragments. For all the studied cluster ions, it has allowed us to identify branching ratios for the loss of i = 1 to i = n water molecules, leading to fragment ions ranging from H~+(H_2O)_(i=n-1) all the way down to the production of protons. Using a corresponding calibration technique we determine total charged fragment production cross sections for incident protonated water clusters H~+(H_2O)_n, with n = 2-7. Observed trends for branching ratios and cross sections, and a comparison with earlier data on measured attenuation cross sections for water clusters colliding with other noble gases (He and Xe), give insight into the underlying dissociation mechanisms.