Vibrational Anisotropy and Anharmonicity of N Atoms Bonded to Ni(100)

Angle- and temperature-dependent surface extended x-ray-absorption fine-structure measurements for the $\frac{p4g(2\mathrm{}\times 2)\mathrm{N}}{\mathrm{Ni}\mathrm{}\left(100\right)\mathrm{}}$ structure reveal a strong local vibrational anisotropy of the fivefold-bonded N atoms. At room temperature we find clear evidence for anharmonic vibrations of N relative to its four in-plane Ni neighbors while the vibrations along the fifth bond remain harmonic. This is directly linked to the strong surface reconstruction induced by the chemisorption process. The out-of-plane vibration of the N atom is discussed in terms of an Einstein model.