Nitrogen valence electronic structure in the strong chemisorption limit: Molecular adsorption on Cr(110) and O/Cr(110).

Abstract

Nitrogen adsorption and dissociation on clean and oxygen-dosed Cr(110) surfaces have been studied with angle-integrated synchrotron-radiation ultraviolet photoelectron spectroscopy (UPS) and work-function measurements. At 90 K, nitrogen adsorbs molecularly via a mobile precursor state and exhibits UPS valence-orbital peaks at binding energies of 12.7 (weak), 8.4, and 7.1 eV with respect to the Fermi level; these positions and separations suggest an unusual ${\mathrm{N}}_{2}$(ads) valence electronic structure similar to that of ``\ensuremath{\pi}-bonded'' ${\mathrm{N}}_{2}$ on Fe(111). Thermal dissociation of ${\mathrm{N}}_{2}$(ads) commences at 100 K but remains incomplete until \ensuremath{\sim}265 K. Chemisorbed ${\mathrm{N}}_{2}$ increases the work function \ensuremath{\Phi} by 0.1\ifmmode\pm\else\textpm\fi{}0.1 eV, whereas annealing the adlayer to 300 K subsequently reduces \ensuremath{\Phi} by 0.7 eV due to dissociation. At 300 K, only dissociative chemisorption is observed but with a reduced sticking probability and a lower saturation coverage compared with thermally dissociating a molecular adlayer. A surface-sensitive Cr valence-band feature is identified which (i) shifts to increasing binding energy with ${\mathrm{N}}_{2}$ adsorption, but (ii) exhibits different coverage-dependent behavior for isoelectronic ${\mathrm{N}}_{2}$(ads) and CO(ads). Low coverages of chemisorbed oxygen do not inhibit ${\mathrm{N}}_{2}$ adsorption but co-adsorption effects are observed when a ${\mathrm{N}}_{2}$(ads) saturated surface is exposed to large oxygen doses. Consideration of these results in comparison with other nitrogen chemisorption studies shows that the bonding of nitrogen to Cr(110) is not well described by the usual \ensuremath{\sigma}-donation and \ensuremath{\pi}-backbonding concepts. A bonding geometry with both nitrogen atoms coordinated to chromium atoms is proposed in spite of the absence of surface hollow sites usually invoked to account for such a species.