Orientation dependence of the sticking probability of NO at Ni(100)

In a molecular beam experiment we have measured the sticking probability of NO at Ni( lO0) as a function of coverage and molecular orientation. The NO molecules in the 2 1 1, H1/212, 1⁄2> ground state are state selected by use of an electrostatic hexapole. The orientation was produced by a homogeneous electric field. The initial sticking probability is found to be higher for N-end collisions. The asymmetry of the initial sticking probability was found to depend linearly on the degree of molecular orientation. This effect is of the order of 70 % with respect to the degree of orientation. The orientation dependence of the sticking probability as a function of coverage shows that the asymmetry is nearly constant up to saturation coverage. These results indicate that a simple picture of the adsorption process including only trapping into a "precursor" state, and neglecting direct chemisorption, cannot explain the adsorption kinetics of NO at Ni( lO0).