THE TORSION ROTATIONAL SPECTRAL STRUCTURE OF ETHANOL MOLECULES ADSORBED ON POLYCRYSTALLINE SILVER SUBSTRATE

A laser photoacoustic spectrometer (LPAS) using a branch-tunable carbo n dioxide laser source and a piezoelectric transducer as a detector wa s employed to study the adsorption of ethanol on a polycrystalline sil ver surface at 108 K. The strength of bonding between adsorbate and su bstrate was found to lie between physisorption and chemisorption accor ding to the values of heat of adsorption (33.2 kJ mol-1, 38.8 kJ mol-1 , 39.5 kJ mol-1 and 44.2 kJ mol-1, obtained by thermal-desorption spec troscopy). A fine spectral structure of seven peaks was obtained in th e region of 1045-1054 cm-1, which bears striking resemblance to that o btained by Fourier-transform infrared spectroscopy for gaseous ethanol . Taking into consideration the results of the thermal-desorption stud y, 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 th e first report on a torsional-rotational fine spectral structure of et hanol adsorbate on a silver surface. Thus our findings show that torsi onal-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 ex periment was capable of providing 1 cm-1 fine spectral resolution at 1 L exposure sensitivity. The LPAS technique developed here has potenti al for studying other systems as well.