Vibrational excitations and structure of oxygen on Fe(110)

Vibrational spectra of oxygen adsorbed on a clean Fe(110) surface at 300 K have been measured by high resolution electron energy loss spectroscopy (EELS). In the exposure range up to 6 L a single loss around 500 cm^-1 is observed which is interpreted to be due to the stretching vibration of atomic oxygen adsorbed at a 2-fold long-bridge site. Above exposures of 6 L a second loss around 400 cm^-1 appears which is attributed to the formation of a disordered oxide layer. Subsequent heating of the sample leads to the observation of a (5×12) LEED pattern which is explained by a mixed oxygen-iron surface structure which is nearly identical to the (111) face of bulk FeO. A weak loss around 910 cm^-1 appears after oxygen exposures at elevated sample temperatures. This loss is attributed to the formation of bulk oxide.