Adsorption and Decomposition of Ammonia on a Single‐Crystal Tungsten (100) Surface

The interaction of ammonia with a single‐crystal tungsten (100) surface has been investigated at pressures below 10−7 torr with a combination of techniques, including low‐energy electron diffraction (LEED), flash desorption, mass spectrometry, and work‐function measurements. The results indicate that adsorption of NH3 at room temperature is nondissociative, occurs with a sticking probability >0.45, and lowers the work function φ by ∼1 eV. Raising the temperature to ∼800°K after saturation results in dissociation of NH3 and desorption of H2, leaving behind half a monolayer of NH2 in a C(2 × 2) structure with a Δφ of ∼−0.4 eV. This structure can be destroyed rapidly at ∼1375°K, with desorption of H2 and N2. The NH2 density can be increased to ∼1 monolayer by exposure to ammonia at ∼800°K. During this process Δφ returns to a value of ∼0, the structure changes from a C(2 × 2) to a(1 × 1), and there is evidence that the rate of hydrogen desorption is controlled by the breaking of a N–H bond. Adsorption and des...