A correlation between anomalous electronic and vibrational properties of Chemisorbed molecules

• Published in: Surface science 1984-04, Vol.139 (2), p.558-568
• Main Authors: Heskett, D., Plummer, E.W., Messmer, R.P.
• Format: Article
• Language: English
• Subjects: Applied sciences; Chemistry; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; General and physical chemistry; Materials science; Metals, semimetals and alloys; Metals. Metallurgy; Physics; Solid-gas interface; Specific materials; Surface physical chemistry
• ISSN: 0039-6028; 1879-2758
• DOI: 10.1016/0039-6028(84)90070-0

The photoelectron spectra of weakly chemisorbed CO and N 2 molecules exhibit a multitude of satellite lines due to multi-electron excitations, i.e., the simple single particle picture of photoionization does not apply to these systems. In all chemisorption systems for which data are presently available, the observation of multi-electron excitations in the valence spectrum is always accompanied by an anomalous behavior for the energy of the molecular vibration as a function of coverage. The energy of such a vibrational mode decreases or remains constant with increasing coverage in these weak chemisorption systems, in contrast to what is seen in “ordinary adsorbates” or predicted by theory for adsorbate dipole-dipole coupling. The correlation between the vibrational spectral data and photoemission data is so consistent that there would appear to be a common explanation for these electronic and vibrational properties. We offer a possible explanation based on: (a) recent ab initio theoretical calculations, which incorporate important electronic correlation effects, and (b) a model previously proposed by Pritchard for the infrared data. The proposed model consists of the following main features. In weakly chemisorbed diatomic systems the 2π orbital (unoccupied in the free molecule) is not involved in the bonding to the substrate in the neutral system. The screening of the hole produced by photoionization is then a non-adiabatic process in which the unoccupied 2π-like orbital (slightly above the Fermi energy) of the neutral ground state falls below the Fermi level and becomes occupied in the ion state. This leads to strong satellite lines in the valence spectra. If the 2π orbital in adsorbed CO or N 2 is not occupied in the ground state, then the 5σ orbital is primarily responsible for the bond to the substrate. Since this orbital is slightly antibonding with respect to the two atoms of the diatomic, the molecular stretching frequency can decrease as the adsorbate coverage increases and the adsorbate-substrate bond weakens (Pritchard's model).