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 1DELTA(g) electronic excitation of O2 monolayers physisorbed on Pt(111) and graphite. We find negative ion resonances at incident electron energies of approximately 6 and 13 eV. The resonance at 13 eV has not previously been observed in either gas phase or adsorbed O2. The angular variation of the a 1DELTA(g) intensity indicates that the 13 eV resonance most likely corresponds to the 2DELTA(u) negative ion state. This assignment contrasts with the assignment of the 13 eV Resonance reported in electron stimulated desorption studies of condensed O2.