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(1Δg) 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(1Δg) spectrum compared with O2(3Σ–g) are identified as 2Δg and 2Φu with the aid of restricted open shell INDO-SCF calculations including the effect of configuration interaction. The energies of the 2Φu and 2Δg states are ∼6.59 and 7.72 eV respectively above that of the X2Πg 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 2Φu, 2Δg and 2Σ–g states respectively. The He II spectrum showed a further new state of O+2 which is tentatively identified as 2Δu. Higher states of O+2 which have been observed in ESCA studies are also assigned.