Electron attachment to oxygen clusters at low electron energies. Formation and stability of (O 2) n− and (O 2) nO −

Abstract

Electron attachment to O 2 clusters formed by nozzle expansion was investigated in a molecular-beam electron-impact mass spectrometer system. Two homologous series, (O 2) n − and (O 2) n O − (with n up to 15), were observed. Their relative attachment cross-section functions show significant differences. (O 2) n O − ions have a similar overall cross-section shape to O − produced by dissociative attachment to O 2 (i.e. a single peak at ∼ 7 eV), whereas (O 2) n − ions have an additional peak at ∼ 0 eV electron energy. The latter result explains the large thermal electron attachment rates observed in high-pressure and low-temperature swarm experiments. Moreover, measured stagnation pressure characteristics for different ions at different electron energies show that (O 2) n − formed with electrons close to zero energy are probably produced by a less dissociative production mechanism than other ions. Electron ionization of O 2 clusters was also studied. Measured mass spectra at 70 eV show a characteristic odd/even alternation for (O) n + ions.