Photoelectron spectroscopy of the oxygen dimer and clusters

The He I photoelectron spectra of gas‐phase oxygen dimer and oxygen clusters have been measured in a pulsed beam. The spectrum of the dimer (O2)2 is characterized by broad bands which show 1:1 correspondence with those of the monomer O2. The vertical ionization energies are 0.2 ± 0.1 eV lower than those of the monomer. The photoelectron bands observed for mixtures of small clusters are identical to the dimer bands except for further shifts of up to ∼0.3 eV to lower ionization energies. The clusters spectra show bandwidths and bandshapes which are effectively the same as those measured for thin films of condensed O2, indicating that the oxygen dimer (O2)2 is the ionization chromophore in each case. This supports the dimer‐ion hypothesis proposed for the ionization of van der Waals clusters of closed‐shell molecules. Ab initio calculations on (O2)2+ show good agreement with the experimental data, and suggest that the ground dimer‐ion electronic state is of rectangular rather than trans‐planar geometry.