Metastable dissociation and kinetic energy release of helium clusters upon electron impact ionization

Neutral helium clusters, formed in a supersonic beam, are subjected to electron impact ionization. We report resolved size distributions of cations of size n ≤ 300. The distributions show the well established anomalies at n = 10 and 14, but none for larger sizes. MIKE scans of the cluster ions confirm the anomalies at n = 10 and 14, and another one at n = 7. Surprisingly broad distributions of fragment ions are observed for parent sizes 15 ≤ n ≤ 30. These findings are attributed to radiative heating. The kinetic energy released upon loss of one helium atom (KER) has been measured. The values drop below 1 meV for n > 20. Neither the metastable fragment distribution nor the KER data provide evidence for the release of large, quantized energies that one would expect for relaxation of any long-lived vibronic states of the ionic core.