Vibrational infrared spectrum of NH3 adsorbed on MgO(100). II. Interatomic potential calculations Original Research Article

Interatomic potential calculations are used to determine the main characteristics of the infrared spectrum of ammonia adsorbed on MgO. The frequencies and intensities of the vibrational bands for the single admolecule and for the addimer (NH3)2 compare fairly well with the results issued from ab initio calculations of paper I. This indicates that the semiclassical approach is convenient for describing adsorption characteristics, at least within the physisorption scheme for which electrostatic and dispersion-repulsion potential interactions are dominant. The bar infrared spectrum is then drawn and compared to the experimental spectra recorded at 300 K and at low and medium coverages. The frequency regions associated with the bands around 1100 and 3300 cm−1 and assigned to the ν2 and ν1 symmetric modes are interpreted in terms of vibration-libration Q peaks. The bands at about 1600 and 3400 cm−1, characteristics of the initially twofold degenerate ν4 and ν3 vibrations, exhibit rotational structures due to the quasi-free spinning motion of the protons in the adsorbed NH3 molecule. From this quantitative analysis, other adsorption sites which cannot be accounted for by a physisorption model, must be invoked to interpret the whole shape of the infrared profile.