THE LOW-ENERGY THERMAL EXCITATION SPECTRUM OF NITROGEN MOLECULES ADSORBED ON NI(110) - IMPLICATIONS FOR MOLECULAR ADSORPTION SITES

The adsorption of N-2, molecules on Ni(110) has been investigated by h igh-resolution He-atom energy-loss spectroscopy. Two external vibratio ns in the subthermal (< 30 meV) energy regime at 5.75 and 4.5 meV have been observed and are shown to imply the existence of two different a dsorption sites, which are assigned to an on-top and a bridge-site. Th is finding is at Variance with the interpretation of data from electro n energy loss spectrescopy and InfraRed (IR) spectroscopy, where only one mode of the internal stretch vibration upsilon(1) at frequencies i s observed for the chemisorbed nitrogen molecules. A reanalysis of pre viously published high-resolution IR data reveals that the presence of the second unexpected adsorbate species correlates with a feature in the IR spectra, which previously has been assigned to N-2 molecules ad sorbed at defect sites. These findings reveal that in this case the ex ternal vibrations are significantly more sensitive to different adsorp tion sites than the internal vibration, the latter exhibiting a differ ence between on-top and bridge sites of less than 6 cm(-1).