The torsional-rotational spectral structure of ethanol molecules adsorbed on polycrystalline silver substrate

Abstract

A laser photoacoustic spectrometer (LPAS) using a branch-tunable carbon dioxide laser source and a piezoelectric transducer as a detector was employed to study the adsorption of ethanol on a polycrystalline silver surface at 108 K. The strength of bonding between adsorbate and substrate was found to lie between physisorption and chemisorption according to the values of heat of adsorption (33.2 kJ mol-1, 38.8 kJ mol-1, 39.5 W mol-1 and 44.2 kJ mol-1 obtained by thermal-desorption spectroscopy). A fine spectral structure of seven peaks was obtained in the region of 1045-1054 cm-1, which bears striking resemblance to that obtained by Fourier-transform infrared spectroscopy for gaseous ethanol. Taking into consideration the results of the thermal-desorption study, which suggests weak bonding between ethanol and the silver surface, this fine spectral structure was attributed to the internal torsional rotation of the ethanol adsorbates. To our knowledge, this work is the first report on a torsional-rotational fine spectral structure of ethanol adsorbate on a silver surface. Thus our findings show that torsional-rotational structure exists not only in free molecules as in the gas phase, but also in the adsorbate phase. The set-up used in this experiment was capable of providing 1 cm-1 fine spectral resolution at 1 L exposure sensitivity. The LPAS technique developed here has potential for studying other systems as well.