Thermal breakup of NH 3 adsorbed on W(211)

Abstract

Interaction of ammonia with a (211) tungsten surface is studied by flash desorption mass spectrometry, low energy electron diffraction, and measurements of work function change. The ammonia is adsorbed at room temperature. Upon heating, one-third of the hydrogen is evolved at 500° leaving a residue having stoichiometry NH 2. At about 800°K NH 2 groups are ordered without further decomposition into a centered rectangular array resting upon the substrate. The overlayer and substrate together form a C(4 × 2) structure. Onset of partial evaporation of the NH 2 layer begins at about 900°K with the rectangular unit mesh of the NH 2 array being continuously stretched in the direction parallel to the troughs of the substrate. The maximum stretching amounts to about 12%, with stoichiometry apparently remaining NH 2. At 1050°K, the rectangular array becomes unstable and there is a drastic surface rearrangement after which LEED patterns are similar to patterns produced by pure nitrogen, though stoichiometry seems to be still NH 2. Finally, heating to even higher temperatures causes destruction of this “pseudo-nitrogen” NH 2 structure. Nitrogen and hydrogen evaporate together in a pressure burst near 1200°K and the surface is completely clean at about 1300°K. NH 3 can be adsorbed as a second weakly held layer on top of an NH 2 primary layer. Kelvin method measurements have shown that the work function is hardly changed by a layer of NH 2, but NH 3 reduces the work function by about one volt after adsorption either on a clean surface or on a thermally prepared NH 2 layer.