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.