Observation of sequential decay series in metastable Ar clusters: ${\mathrm{Ar}}_{\mathit{n}}^{+\mathrm{*}}$→${\mathrm{Ar}}_{\mathit{n}\mathrm{-}1}^{+\mathrm{*}}$→${\mathrm{Ar}}_{\mathit{n}\mathrm{-}2}^{\mathrm{*}}$

The first experimental observation of sequential decay of metastable cluster ions is reported herein. Ar cluster ions are produced by electron-impact ionization of a supersonic Ar cluster beam. Some of these ions are observed to decay via sequential decay series in the microsecond time scale, i.e., ${\mathrm{Ar}}_{\mathit{n}}^{+\mathrm{*}}$→${\mathrm{Ar}}_{\mathit{n}\mathrm{-}1}^{+\mathrm{*}}$→${\mathrm{Ar}}_{\mathit{n}\mathrm{-}2}^{+\mathrm{*}}$ (with n≥7), evaporating a single monomer in each of these successive decays. Conversely, the dominant metastable decay of ${\mathrm{Ar}}_4$ ${\mathrm{}}^{+\mathrm{*}}$→${\mathrm{Ar}}_2$ ${\mathrm{}}^{+}$ proceeds predominantly via a single-step fissioning process leading to the simultaneous evaporation of two neutral Ar monomers.