$\sigma$-bonding contribution of a strong $\pi$-acceptor molecule: Surface chemical bond of ${\text{SO}}_2$ on Ni(100)

We study the electronic structure and chemical bonding of the ${\text{SO}}_2$ molecule adsorbed on Ni(100) surface using x-ray emission spectroscopy at the $\text{O}1s$ absorption edge. Elemental and symmetry specificities of the method enable us to directly probe surface chemical bonding formed by strong hybridization of $\text{O}2p$ with $\text{Ni}3d$ orbital. Contrary to the expected $\pi$ bonding due to strong $\pi$ acceptor character of the ${\text{SO}}_2$ molecule, we find electronic structures with both $\sigma$ and $\pi$ characters just below the Ni Fermi level. The details of this complicated surface chemical bonding are discussed using a calculation based on density-functional theory.

Figure 1

(Color online) (a) $\text{O}2p\to 1s$ XES spectrum of multilayer ${\text{SO}}_2$ measured at an energy above the $\text{O}1s$ ionization threshold (540 eV). The occupied MOs corresponding to each peak and also the lowest unoccupied MO $3{\text{b}}_1$ $\left({\pi }^{\ast }\right)$ are depicted. (b) Emission angle dependence of XES spectra of saturated monolayer (0.4 ML) ${\text{SO}}_2$ on Ni(100). (c) Symmetry resolved XES spectra obtained from the spectra in (b). The elastic peak is subtracted using the spectrum obtained at excitation energy below the absorption edge. The inset shows the polarization dependence of $\text{O}1s$ XAS spectra of saturated monolayer ${\text{SO}}_2/\text{Ni}\left(100\right)$.

Figure 2

(Color online) (a) Simulated XES spectra of monolayer ${\text{SO}}_2$ on Ni(100) obtained by DFT calculations together with symmetry resolved XES spectra. Simulated spectra are plotted using a convolution of the Gaussian function with the half-width at half-maximum of 0.3 eV. (b) Calculated MOs of ${\text{SO}}_2$ on Ni(100) labeled in terms of corresponding MOs of the isolated ${\text{SO}}_2$ molecule.

Figure 3

(Color online) Assignment of calculated XES spectral features between $0–4.5\text{eV}$ binding energy in terms of symmetry resolved $\pi$ and $\sigma$ states. MO pictures for typical peaks are also depicted.