O-2 adsorption on Ag(001) at 100 K has been investigated by HREELS. Co
ntrary to previous reports we could resolve 4 oxygen related peaks, 3
of which are due to the internal stretching vibration as confirmed by
isotope labelling. The lowest frequency mode at 63 meV is assigned to
molecular oxygen chemisorbed at defect sites. The further two features
are very close to each other in frequency, at 79 and 84 meV. We assig
n them to molecules chemisorbed in a peroxidic state in two different
adsorption sites. Initially both sites are filled simultaneously while
at higher coverage adsorption in the higher frequency site is favoure
d.