EXCITATION AND IONIZATION OF HE-4 CLUSTERS BY ELECTRONS

Clusters are produced by expanding high pressure (P0 less-than-or-equal-to 20 bar), low temperature (T0 greater-than-or-equal-to 5 K) helium gas through a 5-mu-m nozzle into a vacuum. The neutral beam time-of-flight distribution has three peaks which we associate with distinct groups of large and small clusters, and atoms. The beam is ionized by electron impact and the resulting time resolved charged fragment mass distribution reveals in addition to previously observed anomalies ("magic numbers") a new strong He-4+ signal at high source pressures and low temperatures. The dependence of the various charged and neutral metastable fragment currents on the bombarding electron energy reveals that each has a unique appearance potential. A comparison with the calculated energy required for an electron to create various electronic excitations in the interior of a large cluster indicates that the production and dynamical evolution of metastable 3S1 atomic and a 3-SIGMA-u+ molecular excitations plays a significant role in the formation of charged fragments from large clusters, but that the production of detectable metastable cluster fragments apparently proceeds via decay of high lying excitonic states. The strong He-4+ signal does not appear until the incident electron has about enough energy to create two metastable 3S1 excitations in a cluster. Thus we propose that this ion signal results from the recombination of a pair of a 3-SIGMA-u+ molecular excitons in or on a large cluster, or possibly from the dynamical evolution of a metastable spin quartet bound hole-exciton pair.