NEW RESONANCES IN HIGH-RESOLUTION EELS OF ADSORBED MOLECULES - ELECTRONIC EXCITATION OF PHYSISORBED O-2

We present an electron energy loss study of the X3SIGMA(g)- --> a 1DEL TA(g) electronic excitation of O2 monolayers physisorbed on Pt(111) an d graphite. We find negative ion resonances at incident electron energ ies of approximately 6 and 13 eV. The resonance at 13 eV has not previ ously been observed in either gas phase or adsorbed O2. The angular va riation of the a 1DELTA(g) intensity indicates that the 13 eV resonanc e most likely corresponds to the 2DELTA(u) negative ion state. This as signment contrasts with the assignment of the 13 eV Resonance reported in electron stimulated desorption studies of condensed O2.