Surface infrared spectroscopy

Alkali-metal-affected adsorption of molecules on metal surfaces

K shell excitation spectra of the aromatic molecules benzene and pyridine in the gas phase are compared to those for the solids (ices) and for monolayers chemisorbed on Pt(111). The gas phase and solid spectra are essentially identical and even the spectra for the chemisorbed molecules exhibit the same resonances. Because of the orientation of the molecules upon chemisorption the latter spectra show a strong polarization dependence as a function of x‐ray incidence. This polarization dependence in conjunction with a multiple scattering Xα calculation for the benzene molecule allows us to assign the origin of all K shell resonances. The resonances are found to arise from transitions to π* antibonding orbitals and to σ* shape resonances in the continuum. The shape resonances are characterized by potential barriers in high (l=5 and 6) angular momentum states of the excited photoelectron. The polarization dependence and energy position of the resonances allow the molecular orientation on the surface to be determined and show that the change in the carbon–carbon bond length is less than 0.02 A.

Resonances in the K shell excitation spectra of benzene and pyridine: Gas phase, solid, and chemisorbed states

Structural studies of alkali metal adsorption and coadsorption on metal surfaces

A systematic analysis of K shell excitation spectra of gas phase molecules containing B, C, N, O, and F reveals a correlation between the position of a characteristic K shell excitation feature, the σ shape resonance, and the intramolecular bond length. When referenced to the 1s ionization threshold the position of the σ shape resonance is found to vary linearly with the internuclear distance between the pair of atoms which gives rise to the scattering resonance. This linear relationship holds remarkably well within different classes of molecules characterized by the total number of electrons (sum of atomic numbers) for a given pair of atoms. The empirically derived rules are compared with the predictions of a full multiple scattering theory treatment. Comparison of empirical experimental and first‐principle theoretical results allows the prediction of the average intramolecular muffin tin potential in the presence of the core hole.

Determination of intramolecular bond lengths in gas phase molecules from K shell shape resonances

Intramolecular scattering resonances in near-edge x-ray-absorption fine-structure spectra of chemisorbed molecules are used to determine the molecular orientation, bonding, and bond length. In particular, we demonstrate that the position of the $\ensuremath{\sigma}$ shape resonance relative to the Fermi level is directly correlated with the intramolecular bond length of the chemisorbed molecule. By comparison with the known molecular distances in the gas phase, changes induced by the chemisorption bond are revealed.

Bonding and bond lengths of chemisorbed molecules from near-edge X-ray absorption fine-structure studies

The near edge x‐ray absorption fine structure (NEXAFS) spectra of ethylene on Pt(111) at 90 and 300 K have been measured and compared to the spectra calculated using the multiple scattering Xα method for cluster models of adsorbed ethylene and ethylidyne. The results indicate that ethylene is adsorbed in the di‐σ configuration (sp3 hybridization) at 90 K, and that the π* orbital retains its identity in this configuration, with no component along the C–C axis. The spectrum obtained at 300 K is attributed to ethylidyne (CCH3) and the calculated spectrum for a cluster consisting of this species bonded symmetrically to three Pt atoms is in good agreement with experiment. The calculations show that the multiple scattering molecular cluster approach is a useful technique for interpreting the NEXAFS spectra of adsorbed molecules.

Structure and bonding of chemisorbed ethylene and ethylidyne on Pt(111) from near edge x-ray absorption fine structure spectroscopy and multiple scattering calculations

R. J. Koestner

Papers

Bonding and bond lengths of chemisorbed molecules from near-edge X-ray absorption fine-structure studies

Structure and bonding of chemisorbed ethylene and ethylidyne on Pt(111) from near edge x-ray absorption fine structure spectroscopy and multiple scattering calculations