THE STICKING AND DISSOCIATION OF NH3 ON W(110) - A 3-STATE MODEL

The kinetics of the adsorption of NH3 on W(110) and its subsequent dis sociation have been investigated using molecular beam techniques and t emperature programmed desorption (TPD) for surface temperatures rangin g from 140 to 700 K. NH3 shows a wide desorption peak around 270 K and a smaller peak at 170 K while H-2 and N-2, produced by dissociation, desorbed at 550 and 1350 K, respectively, with kinetic parameters simi lar to those reported for H and N generated by adsorption of H-2 and N -2. At normal incidence and for a surface temperature of 140 K, the NH 3 sticking coefficient was found to decrease from unity at a beam ener gy of 0.8 kcal/mol to 0.5 for a beam energy of 5.4 kcal/mol. The stick ing coefficient generally decreases with surface temperature to a valu e of 0.05 at 700 K, but, for a 5.4 kcal/mol beam, it exhibits a relati ve minimum near 300 K. The reflection coefficient of NH3, for an angle of incidence of 49 degrees, increases with temperature and incident b eam energy in agreement with the sticking measurements. The TPD peak p ositions, sticking and reflection data are all well reproduced by a th ree-state model based on simple kinetics. The model assumes that NH3 i nitially traps in a molecular state and that dissociation occurs by th ermal activation into an intermediate state. At no temperature is the sticking probability enhanced by increasing the kinetic energy of the incident molecules and there is no evidence for a direct dissociation channel which has a translational energy barrier less than 5.4 kcal/mo l.