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Abstract:
Angle resolved photoelectron spectra excited by synchrontron radiation from the BESSY storage ring have been obtained for adsorbed N2 on Fe(111) as a function of temperature. The two previously identified phases of molecular N2, i.e. the γ-and α-phase, can be clearly differentiated by their photoemission characteristics. From the angular profiles the γ-phase can be shown to consist of N2 molecules oriented perpendicular to the surface plane. The α-phase, on the other hand, shows spectra that are only consistent with N2 whose axis is strongly inclined from the surface normal. These findings corroborate the conclusions drawn from HREELS and XPS that the N2 in the α-phase is “π-bonded” to the surface. We also present the results of ab initio generalized valence bond calculations for the A3Σu+ excited, state of the N2 molecule and suggest that this state of N2 may be stabilized by the Fe(111) surface resulting in the species responsible for the α-phase. A microscopic model for the “π-bonding” state is proposed.
