The dissociation probability of N 2 on Ru(001) increases from 5 × 10 -7 at a kinetic energy of 0.15 eV to 10 -2 at 4.0 eV. Vibrational excitation of the impinging nitrogen molecules enhances the dissociation more than the equivalent energy in translation. Its relative importance increases as the incident kinetic energy grows. The dissociation was found to be surface temperature independent at all incident kinetic energies, in agreement with theoretical predictions based on quantum mechanical nonadiabatic calculations. These simulations reproduce accurately the kinetic energy dependence of S 0 over the entire energy range, suggesting that N 2 tunnels from the molecular to the adsorbed atomic state through an effective barrier of 2.2 eV.

The interaction of charged particles with condensed water films has been studied extensively in recent years due to its importance in biological systems, ecology as well as interstellar processes. We have studied low energy electrons (3-25 eV) and positive argon ions (55 eV) charging effects on amorphous solid water (ASW) and ice films, 120-1080 ML thick, deposited on ruthenium single crystal under ultrahigh vacuum conditions. Charging the ASW films by both electrons and positive argon ions has been measured using a Kelvin probe for contact potential difference (CPD) detection and found to obey plate capacitor physics. The incoming electrons kinetic energy has defined the maximum measurable CPD values by retarding further impinging electrons. L-defects (shallow traps) are suggested to be populated by the penetrating electrons and stabilize them. Low energy electron transmission measurements (currents of 0.4-1.5 μA) have shown that the maximal and stable CPD values were obtained only after a relatively slow change has been completed within the ASW structure. Once the film has been stabilized, the spontaneous discharge was measured over a period of several hours at 103 ± 2 K. Finally, UV laser photo-emission study of the charged films has suggested that the negative charges tend to reside primarily at the ASW-vacuum interface, in good agreement with the known behavior of charged water clusters.

Water molecules adsorbed on SiO2/Si(100) at 140 K to form amorphous solid water (ASW) layers were utilized as a buffer for assisting the growth of gold nanoclusters. It was shown that the average height and diameter of the clusters deposited on the silicon oxide substrate following the buffer annealing/desorption increase as the buffer layer becomes thicker and as more gold is deposited. The clusters' height and diameter were determined by tapping mode AFM and high-resolution SEM imaging, respectively. Typical heights were between 0.5 and 4.5 nm, and the diameters were in the range of 3-9 nm for ASW layer thickness of 7-100 ML and gold deposition in the range of 0.2-1.2 A. The density of the clusters decreased from 65 x 10(10) to 8 x 10(10) cm (-2) in the same buffer layer thickness range. Significantly different morphology of the clusters is obtained when compared to those formed by direct deposition of gold on the silicon oxide surface and to those grown on top of Xe as buffer material.

The chemistry of methyl bromide on Ru(001) has been studied utilizing work function change (Δφ) measurements and temperature-programmed desorption (TPD) in the crystal temperature range of 82−1350 K. Employing a Δφ-TPD mode, chemical changes in the adsorbed state could be detected at temperatures below the onset for desorption. A decrease in work function of 2.15 ± 0.02 V has been measured at the completion of a monolayer coverage, which has been determined to consist of (3.6 ± 0.3) × 1014 molecules/cm2, equivalent to CH3Br/Ru = 0.22 ± 0.02. The onset for C−Br bond cleavage near 125 K was observed. 50% of the initial 1 monolayer methyl bromide molecules decompose to adsorbed methyl and bromine. A low-temperature increase in work function was found to precede dissociation or desorption as coverage increases. This change in work function is discussed in terms of several possible mechanisms, including multiple sites population, molecular rearrangement, and tilt angle, that change with coverage and surface temperature. A thermally activated flipping mechanism in which a fraction of the adsorbate molecules rearrange to adsorb with the methyl group facing the surface is found to be most consistent with the observed results. Sequential dehydrogenation of the methyl fragments, competing with minor methane production at higher coverages, was directly observed by employing the differential work function measurements. The corresponding surface temperature window for each of these decomposition steps has been determined, and the detailed reaction mechanism is discussed. Bromine atoms on Ru(001) were found to decrease the work function by 320 mV at a coverage Br/Ru = 0.3, indicating a complex charge redistribution upon adsorption. Deuterium preadsorption, which significantly passivates the surface, has been employed to better understand the various reactivity steps of the hydrocarbon fragments. Finally, work function measurements indicate the presence of strong interactions of the methyl bromide molecules with the metal surface up to the third layer. Alternating contributions to the work function of the first three layers are observed. This is understood in terms of an opposite adsorption geometry in which bromine faces the surface in the first layer, methyl in the second, and bromine again in the third. Upon heating, the third and fourth layers rearrange in a bilayer-like structure before the completion of the fourth layer, leading to higher stability of the combined two layers compared with the third alone. This structure is rather similar to that of methyl bromide in its molecular crystal.

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