INVESTIGATIONS ON PROCESSES INDUCED BY SINGLE-SCATTERING AND MULTIPLE-SCATTERING OF ELECTRONS IN CLUSTERS INVOLVING EXCIMER-INDUCED FRAGMENTATION AND A NEW METASTABLE FRAGMENTATION CHANNEL OBSERVED FOR ARNO+ASTERISK CLUSTER IONS

A new metastable decay reaction of Ar(n)O+ cluster ions produced by th e electron impact ionization of argon-oxygen clusters has been discove red, leading to the evaporation of more than two monomers in the metas table time regime. It is estimated that an energy of approx. 1 eV has to be released from the intramolecular to intermolecular modes prior t o this metastable decay. In contrast to previously observed excimer-in duced fragmentation of pure argon cluster ions, it has been concluded that the localized metastable state of an ionic rather than a neutral species is involved in the fragmentation. A reaction mechanism involvi ng an electronic transition from the a2PI ArO+ (O(3P)-Ar+(2P)) excite d state to the X4SIGMA- ArO+ (O+(4S)-Ar(1S)) ground state of the ArOcomplex ion within the cluster and subsequent release of considerable amount of vibrational energy to cluster modes is suggested to account for the observed fragmentation. Based on these measurements, a lifetim e of approx. 2 mus is proposed for the excited state a2PI ArO+.