N 2 chemisorbed on Fe(lll) is identified by photoelectron spectroscopy and highresolution electron energy-loss spectroscopy as a n-bonded surface complex in which both nitrogen atoms interact with the metal. This species forms the immediate precursor to dissociation.PACS numbers: 82.65.Jv, 31.20.Tz, 82.80.Di, 82.80.Pv Molecular nitrogen adsorption on transition metals has been studied in detail on various substrates by photoemission and vibrational spectroscopies, 1 and in all cases it has so far been concluded that the molecule bonds mainly via the 3o-^ and 2cr g molecular orbitals in an "end-on" configuration to the surface, i.e., M-N-N. Atomic nitrogen adsorption has also been studied widely, 2 but no experimental data have been reported for the actual surface intermediate in which the breaking of the dinitrogen bond occurs. There is a great practical interest in a better understanding of the dissociation mechanism on a molecular basis, since dissociative nitrogen adsorption has been identified as the rate-determining step in the ammonia synthesis reaction on iron. 3 The kinetics of dissociative nitrogen adsorption on Fe(l 11) have been studied in detail previously 4 ; it was concluded that this process proceeds through a molecularly adsorbed species with a rather low activation energy. Later it was found 5 that this species (a state) is preceded at low temperatures by a more weakly held, physisorbed y state, which desorbs at about 85 K. In the present work, the a state has been identified as a rr -bonded molecule with both N atoms interacting with the surface and thus forming the immediate precursor to dissociation. This evidence is based on experimental x-ray photoemission spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (not discussed here) supplemented by calculations for the N 1 s core-hole spectra and high-resolution electron-energy-loss spectroscopy (HREELS) data. N 15 core-level photoelectron experiments using Al Ka radiation were performed in a Leybold Heraeus photoelectron spectometer equipped with a rotable cold finger. 6 The vibrational data were obtained in a different vacuum system with a high-resolution electronenergy-loss spectrometer. The Fe(lll) crystals were cleaned and annealed following previously described procedures. 7 In Fig. 1, a sequence of N 15 spectra for the three different nitrogen bonding states on Fe(lll), denoted by 7, a (molecular), and ft (atomic) is displayed. The y state spectrum (curve a) was recorded at 85 K in an ambient of P N2 = 5xlO~7 mbar and includes emission from some a-N 2 , 5 giving rise to the shoulder at ~ 399-eV binding energy (E B ). The intensities of the two peaks at E B = 405.9 eV and E B -401 eV depend on the ambient N 2 pressure at T^ 85 K, since y-N 2 is only weakly adsorbed (£^^24 kJ/mol). 5 A similar N ls-doublet structure has been observed by Kishi and Roberts 8 for N 2 on polycrystalline iron but was interpreted incorrectly as arising from two different adsorption states of N 2 . After heating the predosed surface in vacuum to 97 K, ...

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