THE IMPORTANCE OF HIGH-IMPACT PARAMETER INTERACTIONS IN THE COLLISION-INDUCED DISSOCIATION OF PROTONATED WATER CLUSTERS BY ARGON USING A WIEN VELOCITY FILTER

High impact parameter, i.e., glancing collisions, are of particular in terest in ion beam experiments because ions experiencing such collisio ns remain available in the ion beam for further experiments. The colli sional activation and dissociation processes for glancing collisions o f protonated water clusters and argon have been studied with a new and simple, single-stage technique to detect fragment ions using a Wien v elocity filter. The technique is specific with regard to the mass of a fragment ion and the mass of the parent from which it originates. A r elation is derived and experimentally verified which governs the opera tion of the device. Absolute values of the attenuation cross section w ith argon of 11 (6), 25 (8), 36 (11), 47 (17), and 66(10) angstrom hav e been determined for H3O+, H5O2+, H7O3+, H9O4+, and H11O5+, respectiv ely, at beam energies in the range of 300-1000 eV. Absolute values of the glancing collision induced dissociation cross section of 0.62(4), 4.3(1), and 9.1(6) angstrom2 have been determined for the detectable f ragment ions of H5O2+, H7O3+, and H9O4+, respectively. Branching ratio s upon activation by glancing collisions of 0.91-0.09:0 and 0.78:0.18: 0.04:0 have been determined for successive loss of waters by H7O3+ and H9O4+, respectively. The one water loss channel predominates. A multi ple collision analysis was performed which characterizes the fraction of parent ions which suffer a glancing collision without dissociating or being knocked out of the beam. Our results suggest that the ion bea m which emerges from a collision cell can harbor a surprisingly large fraction of parent ions that have obtained a large amount of internal excitation, perhaps approximately 0.7 eV per ion suffering a glancing collision.