Angle Resolved Photoelectron Spectroscopy of O2(a1Dg) with Synchrotron Radiation Alberto De Fanis Chemistry Department, University of Southampton, SOuthampton, UK Relative cross section for the O2+(X2Pg) v+=0,1,2,3 <-- O2(a1Dg)+hn ionization process has been studied with synchrotron radiation in the photon energy range 11.0-17.0eV, and it shows resonant structure arising from autoionisation from a neutral Rydberg state. The structure is also reflected in the asymmetry parametr for angular distribution.