An HREELS study of π* and σ* negative ion resonances of c(8×2) benzoate on Cu(110)

For the $\text{c(8×2)}$ benzoate on Cu(110) system, multiple unoccupied states have been measured through negative ion resonances by high-resolution electron-energy-loss spectroscopy (HREELS). Based on the azimuthal and polar angle dependences, we assign contributions from $B_2$ and $A_1$ states to a resonant feature in the 〈001〉 direction and contributions from $B_1$ and $A_1$ states in the 〈110〉 direction. The $B_2$ state is observed close to 4.7 eV and the $B_1$ feature is centered at 6.5 eV. The angular and azimuthal dependence of the C–H stretch intensity in HREELS at resonance is consistent with their assignment and the results of ab initio calculations. Overtone intensity shows an exponential decay as a function of overtone quantum number. From the overtone frequencies, an anharmonicity of 0.0272 is obtained which leads to a dissociation energy of the C–H bond of about 81 kcal/mol.