PHOTOCHEMICAL-REACTIONS OF WATER ADSORBED ON PT(111)

The photochemical reaction of water adsorbed on Pt(111) at 85 a has be en investigated by high resolution electron energy loss spectroscopy ( HREELS) and thermal desorption spectroscopy (TDS). Under the action of ultraviolet (UV) light, molecular desorption as well as dissociation of adsorbed water into OH and H has been observed. In addition to the known vibrational losses of adsorbed OH at 430 cm(-1) (Pt-O stretching ) and 1020 cm(-1) (OH bending), two new losses are observed at 340 and 820 cm(-1), which are assigned to a hindered translation and a second OH bending vibration, respectively. Using central force field calcula tions, a tilt angle of 15 degrees of the OH bond towards the Pt surfac e could be derived from the difference of about 200 cm(-1) between the two OH bending vibrations. From energetic considerations it is assume d that the photodissociation (threshold: 5.2 eV) occurs via a substrat e excitation. Thermal desorption spectra taken after UV irradiation of adsorbed water exhibit two peaks, one at 175 K and one at 180 K, indi cating both molecular desorption of water and recombination of OHad an d H-ad, respectively.