Vacuum ultraviolet photoelectron spectroscopy of transient species. Part 2.—The use of phase-sensitive detection for investigating the electronic states of O +2

Abstract

The vacuum ultra-violet photoelectron spectrum of O₂(³Σ^–_g) and O₂(¹Δ_g) obtained using He I radiation, is presented. A phase-sensitive detection technique, which enhances the relative signals from transient species present in a discharge system, is described and discussed. The two additional new states of O⁺₂ which are present in the O₂(¹Δ_g) spectrum compared with O₂(³Σ^–_g) are identified as ²Δ_g and ²Φ_u with the aid of restricted open shell INDO-SCF calculations including the effect of configuration interaction. The energies of the ²Φ_u and ²Δ_g states are ∼6.59 and 7.72 eV respectively above that of the X²Π_g ionic state. The bond lengths of these two states and of the ²Σ^–_g state have been obtained by comparison of observed band intensities with those determined by a Franck–Condon calculation, using a procedure which is described. These bond lengths are ∼1.40, 1.33 and 1.30 Å for the ²Φ_u, ²Δ_g and ²Σ^–_g states respectively. The He II spectrum showed a further new state of O⁺₂ which is tentatively identified as ²Δ_u. Higher states of O⁺₂ which have been observed in ESCA studies are also assigned.