MEASUREMENTS OF KINETIC-ENERGY RELEASE FOLLOWING THE UNIMOLECULAR AND COLLISION-INDUCED DISSOCIATION OF ARGON CLUSTER IONS, AR-N(+), FOR N IN THE RANGE 2-60

A double-focusing mass spectrometer in conjunction with a cluster beam source has been used to measure the average kinetic energy released following the unimolecular and collision-induced fragmentation (CID) of argon cluster ions. Measurements on unimolecular decay have been made for clusters in the range Ar5+-Ar60+, and for the CID studies the range was Ar2+-Ar30+. Within the observation time window, the kinetic energy release results for the loss of a single argon atom via unimolecular decay are consistent with internal energy being partitioned statistically. Three separate CID routes are identified: (i) loss of one Ar atom; (ii) rapid (< 10(-7)s) loss of two Ar atoms within the confines of a collision cell; (iii) sequential loss of two Ar atoms on a time scale > 10(-7) s. It is proposed that the CID of small cluster ions proceeds via electronic excitation; but that as the clusters increase in size (n > 4) vibrational excitation predominates. A simple spectator model of collisional excitation accounts for the experimental CID results in cluster ions beyond Ar15+.