Novel method for absolute quantification of the flux and angular distribution of a radical source for atomic hydrogen J.An intense atomic hydrogen source with a movable nozzle output Rev.An atomic hydrogen doser of the Bertel type was characterized in terms of the degree of dissociation and angular distribution of the effusing particles. In this doser hydrogen is dissociated in a tungsten tube which is heated by electron bombardment. Various experimental techniques were used to determine the degree of dissociation as function of temperature and gas flux. It is shown that simple equilibrium considerations cannot be applied to obtain the degree of dissociation accurately. Nevertheless, for sufficiently small gas flux and temperatures above 1850 K, the degree of dissociation approaches 100%. The angular distribution was determined by a gold foil on a goniometer as detector, which is sensitive to atomic hydrogen only. The experimental results were compared with Monte Carlo simulations. A strongly forward focused distribution is observed which allows efficient atomic hydrogen dosing. This doser was used to measure absolute initial sticking coefficients for atomic hydrogen on various single crystal metal surfaces: S 0 ͑H͒ϭ1.0 on Ni͑111͒, 0.9 on Ni͑110͒, 0.7 on Al͑111͒, 0.6 on Al͑100͒ and 0.5 on a polycrystalline gold foil, respectively.
We present a combined experimental and theoretical study of spin-orbit-induced spin splittings in the unoccupied surface electronic structure of the prototypical Rashba system Au(111). Spin-and angle-resolved inversephotoemission measurements reveal a Rashba-type spin splitting in the unoccupied part of the L-gap surface state. With increasing momentum parallel to the surface, the spectral intensity is lowered and the spin splitting vanishes as the surface state approaches the band-gap boundary. Furthermore, we observe significantly spin-dependent peak positions and intensities for transitions between unoccupied sp-like bulk bands. Possible reasons for this behavior are considered: initial and final-state effects as well as the transition itself, which is controlled by selection rules depending on the symmetry of the involved states. Based on model calculations, we identify the initial states as origin of the observed Rashba-type spin effects in bulk transitions.
The energy and angular distribution of deuterium molecules desorbing from a vanadium (111) surface modified either by oxygen or by sulfur has been studied, using time-of-flight spectroscopy. It has been shown that the desorption flux contains two contributions, a thermal and a hyperthermal contribution. The mean translational energy of the hyperthermal part can be described by 〈E〉=8.3⋅kTs and 5.8⋅kTs for the sulfur and oxygen covered V(111) surface, respectively. Interestingly, the mean translational energy of the hyperthermal contribution is independent of the desorption angle. The angular distribution of the hyperthermal desorption flux is forward focused and can be described by cos3.3 θ and cos4.3 θ functions for the sulfur and oxygen modified surface, respectively. From the angular flux distribution and the angle independent mean translational energy of the hyperthermal contribution one can conclude that normal energy scaling does not exist for this adsorption/desorption channel. This is mainly due to the strong geometric corrugation of the modified V(111) surfaces.
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