Metastable decay of oxygen cluster cations and anions

Abstract

Stoichiometric and non-stoichiometric, positive and negative oxygen cluster ions (up to n≈ 70) have been produced in a crossed neutral cluster/electron beam ion source. The abundance and stability of these ions have been studied with a double focusing sector field mass spectrometer (reversed geometry) at two different time windows after ion formation (in the metastable time region). Positive and negative mass spectra exhibit distinct abundance anomalies, however, at different cluster sizes. Abundance maxima and minima correlate in both cases with corresponding small and large metastable fractions (for the loss of O₂) determined in the first field-free region, respectively. In accordance with the predictions of Klots, the probability of losing one monomer from (O₂)⁺_n exhibits a non-exponential dependence on time since ion formation. Metastable fractions determined for (O₂)^–_n ions produced with ca. 0 eV electrons are generally larger than those for ions produced with ca. 7 eV electrons. This observation is due to a two-step dissociative production process of the latter ions via the ²π_u state leading to less energetic ions than in case of direct attachment of zero-energy electrons. Furthermore, non-stoichiometric ions (O₂)_nO⁺ and (O₂)_nO^– not only decay by O₂ evaporation but also by O evaporation (and higher homologues in case of the positive ions). Branching ratios for the different decay channels observed may be rationalized in terms of energy considerations. Finally, strong autodetachment has been observed in case of small O₂ cluster anions. Autodetachment lifetimes appear to increase with increasing cluster size.