Photoemission line shapes of adsorbates: Conclusions from the analysis of vibration-resolvedN2spectra

Physisorbed ${\mathrm{N}}_2$ on Ni(111) has been studied by uv photoemission using rare-gas spacer layers. For 1${\mathrm{\pi }}_{\mathit{u}}$ ionization, vibrational fine structure has been resolved experimentally and analyzed in detail. The line shape of the electronic levels cannot be described by simple broadening of the corresponding gas-phase spectrum. Nearly no Lorentzian lifetime broadening could be observed; rather the vibrational peaks have a Gaussian line shape (narrowest FWHM, 0.21 eV). The observed peculiarities can be understood within a hopping model taking into account the deexcitation of the ionized molecule by hole transfer to neighbors.