The origin of inverse absorption bands observed in the far-infrared RAIRS spectra of SnCl₄ and SnBr₄ adsorbed on thin-film SnO₂ surfaces

The adsorption of SnCl₄ and SnBr₄ on polycrystalline SnO₂ has been studied using synchrotron radiation based far-infrared reflection absorption infrared spectroscopy FIR-RAIRS. In order to exploit the sensitivity advantages of the buried metal layer method, the SnO₂ is in the form of a thin film deposited on a tungsten foil substrate. Adsorption of SnCl₄ and SnBr₄ on an oxygen sputtered surface at 120 K results in spectra characteristic of condensed multilayers. In addition, both spectra exhibit an inverse absorption band centred at 355 cm^-1. Modified 4-layer, wavelength-dependent, Greenler calculations show that this inverse absorption band is induced by the presence of the adsorbate but is characteristic of the SnO₂ layer. The lack of any frequency shift upon changing the adsorbate from SnCl₄ to SnBr₄ rules out the possibility that the inverse absorption band is due to a dipole-forbidden parallel mode of the molecule excited via the interaction with free electron oscillations in the metal, resulting from the radiation induced oscillating electric field just below the surface.