Photochemical reactions of water adsorbed on Pt(111)

The photochemical reaction of water adsorbed on Pt(111) at 85 a has been 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 calculations, a tilt angle of 15 degrees of the OH bond towards the Pt surface could be derived from the difference of about 200 cm(-1) between the two OH bending vibrations. From energetic considerations it is assumed that the photodissociation (threshold: 5.2 eV) occurs via a substrate excitation. Thermal desorption spectra taken after UV irradiation of adsorbed water exhibit two peaks, one at 175 K and one at 180 K, indicating both molecular desorption of water and recombination of OHad and H-ad, respectively.