PHOTOCHEMICAL PATHWAYS OF WATER ON PD(111) AT 6.4 EV

The photochemistry of water adsorbed on Pd(111) was studied by high-resolution electron energy loss spectroscopy (HREELS), temperature-programmed desorption (TPD), and time-of-flight (TOF) mass spectroscopy. UV laser irradiation (6.4 eV, 193 nm) of the water bilayer adsorbed on Pd(111) results in sequential dissociation to form surface hydroxyl groups first and later atomic oxygen. The first step is dominating for irradiation with up to 2 x 10(18) photons/cm2. In this case the dissociating hydrogen atom is retained on the surface. For prolonged irradiation the second step is observed accompanied by recombination of a dissociating hydrogen atom with a surface hydroxyl group leading to the desorption of molecular water. Beside these processes also direct desorption of molecular water is observed dominating the desorption yield during the initial stage of irradiation. In both cases water desorption is nonthermal, with a mean translational energy, <E(trans)>/2k, of (600 +/- 60) K. On c(4 x 2)CO and p(2 x 2)O covered Pd(111), only direct photodesorption is observed, with cross sections of (2.4 +/- 0.1) x 10(-20) cm2 and (2.6 +/- 0.3) x 10(-20) cm2, respectively. The photochemistry is attributed to transient attachment of photoexcited substrate electrons.