Although the 248-nm KrF laser photon energy is less than the O2 dissociation energy by about 0.12 eV, O3 is still produced in the laser irradiation of O2 at this wavelength. The O3 is produced by two processes: an initiation process and an autocatalytic process. In the present work, these two O3 formation possesses have been studied in pure O2 and in O2+N2 and O2+Ar mixtures at pressures between 200 and 1600 torr and at temperatures between 298 and 370 ¿K. Evidence is presented that the initiation process produces O3 through chemical reactions between ground state O2 and excited O2 in the Herzberg states (A 3&Sgr;u+, A' 3Δu and c 1&Sgr;u-), which are produced by the photoabsorption of O2 at 248-nm. For the autocatalytic process, the results are consistent with the proposal that the O3 formation is accelerated by photodissociating vibrationally excited O2(&ngr;), produced in the photolysis of O3 following its initial formation. Both the O2 Herzberg states and O2(&ngr;) may play important roles in the odd oxygen (Ox) chemistry in the middle atmosphere. It is estimated in the present work that the O2(A 3&Sgr;u+)+O2 reaction may yield up to ~6% of the total odd oxygen production rate near 50 km; this odd oxygen source is not included in current photochemical models. ¿ American Geophysical Union 1992