Time-of-flight spectroscopy (TOF) and REMPI-TOF (resonance enhanced multi-photon ionization-TOF) were applied to measure the angular and translational energy distribution, as well as the internal state resolved energy distribution of desorption and reaction products on some model systems. Desorption of hydrogen and deuterium from clean and modified Pd(111) surfaces was studied, where the palladium sample was part of a permeation source. Water formation by reaction of oxygen with hydrogen on palladium was investigated by using different types of hydrogen supply: molecular H-2 exposure and atomic H exposure from the gas phase, as well as H exposure by permeating hydrogen. Vanadium oxide nanostructures on Pd(111) were prepared and the influence on D-2 desorption and D2O production was investigated with the permeation technique. Additionally, deuterium desorption from sulfur and oxygen covered V(111) and V(100) surfaces was studied by TOF and REMPI-TOF spectroscopy. From the TOF spectra information concerning the reaction and desorption dynamics (activation barriers) could be gained.

The internal state distribution (rotation, vibration) and the translational energy distribution of state selected molecules can be measured with the help of a REMPI (resonance enhanced multi-photon ionisation)-TOF (time-of-flight) spectrometer. We have performed measurements of this type on hydrogen and deuterium molecules desorbing from an oxygen covered V(100) surface. The rotational state population of the hydrogen and deuterium molecules in the vibrational ground state exhibits an overpopulation for J<3 and an underpopulation for 39 for desorbing deuterium. The calculation of the mean rotational energy Erot leads in all cases to values significantly below the respective thermal equilibrium values (rotational cooling). For the vibrational state distribution we observe in all cases an overpopulation for molecules in the first excited vibrational state (vibrational heating). The translational energy distribution of desorbing deuterium strongly depends on the surface quality. On the flat, well annealed oxygen covered V(100) surface the mean translational energy of desorbing deuterium is hyperthermal, whereas the translational energy of molecules desorbing from a rough surface is closer to thermal.

The rotational, vibrational and translational energy distribution for hydrogen (deuterium) molecules desorbing from a vanadium(1 0 0) + O surface have been determined using resonance enhanced multi-photon ionization time-of-flight spectroscopy. The translational energy has been found to be hyper-thermal for all selected internal states. However, the translational energy Ekin for molecules in v ¼ 1 is smaller than for those in the vibrational state v ¼ 0. The influence of the rotational states J on the translational energy is more complex: for low J -states Ekin increases with J , whereas for high J -states Ekin decreases with increasing J . We have also measured the population of the individual rotational and vibrational states of the desorbing H2 (D2) molecules: In both cases the first excited vibrational state is overpopulated. The population of the J -states is hyper-thermal for low and high J but sub-thermal for medium J . 2003 Elsevier Science B.V. All rights reserved.

The rotational, vibrational and translational energy distribution for hydrogen (deuterium) molecules desorbing from a vanadium(100)+O surface have been determined using resonance enhanced multi-photon ionization time-of-flight spectroscopy. The translational energy has been found to be hyper-thermal for all selected internal states. However, the translational energy Ekin for molecules in v=1 is smaller than for those in the vibrational state v=0. The influence of the rotational states J on the translational energy is more complex: for low J-states Ekin increases with J, whereas for high J-states Ekin decreases with increasing J. We have also measured the population of the individual rotational and vibrational states of the desorbing H2 (D2) molecules: In both cases the first excited vibrational state is overpopulated. The population of the J-states is hyper-thermal for low and high J but sub-thermal for medium J.

The kinetics of adsorption and absorption of atomic hydrogen on a sulfur-covered V(100) surface has been studied using thermal desorption spectroscopy. The sulfur overlayer exhibited an inverse c(root 2x3 root2)R45 degrees structure, as observed by low-energy electron diffraction, corresponding to a coverage of 0.66 monolayer, The initial sticking coefficient for atomic hydrogen on this surface at 90 K is 0.16; for molecular hydrogen it is smaller than 1 x 10(-4). Most of the adsorbed hydrogen diffuses into the bulk, even at liquid N-2 temperature. The surface hydrogen desorbs in the temperature range of 150-300 K, whereas the absorbed hydrogen desorbs above 800 K. The dynamics of molecular hydrogen desorption has been studied with resonance-enhanced multiphoton ionization time-of-flight spectroscopy. It turns out that the hydrogen molecules desorbing from the vanadium surface at 950 K are vibrationally hot (60 times overpopulation of the upsilon = 1 state), but rotationally cold (T-rot = 550 K).

Adsorption and desorption of water as well as the interaction of water with atomic deuterium on a clean and oxidized Al(111) surface has been studied using temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Quantitative determination of the sticking coefficient for water reveals a value of unity on the clean and the oxidized surface. In the desorption spectra three different peaks of water and hydrogen can be observed. The alpha-peak for water at 160 K can be attributed to evaporation of water multilayers. Already at this low temperature parallel dissociation of water leads to the formation of aluminum hydroxides. The simultaneous desorption of molecular hydrogen can be explained by a reaction assisted process, due to the formation of hot H-atoms during the dissociation process of water. The beta-peak at about 320 K and the gamma-peak at 650 K, both for water and hydrogen, are due to decomposition/recombination of different aluminum hydroxides. Dosing a thick ice layer with atomic deuterium reveals only a weak interaction. The reaction coefficient for dissociation of the water molecule or the replacement of an H-atom of the water by a D-atom is about 4%. (C) 1999 Elsevier Science B.V. All rights reserved.