Ozone is a C 2v molecule (following Mulliken's convention , the molecule is placed in the yz-plane and the molecular symmetry axis aligns with the z-direction) with three normal modes: symmetric stretch, symmetric bend, and asymmetric stretch (Fig. ). The asymmetric stretch is of b 2 symmetry. The two lowest electronic states of the ozone cation, 2 A 1 and 2 B 2 , are very close in energy-separated by the vertical gap (at the neutral's geometry) of mere 0.123 eV (or 990 cm -1 , see Table )-and can be coupled by the b 2 vibration, giving rise to significant vibronic effects in the photoelectron spectrum. The vertical gap corresponds to the difference in the energies of electronic states at the same geometry (here, the structure of the neutral). While vertical energy gap is easy to compute, they are not observable experimentally. The experimentally accessible value is the adiabatic gap T 00 -the energy difference between the vibronic levels of the two state.