Interaction of gas-phase oriented N2O with lithium metal:: evidence for an Eley-Rideal mechanism

The interaction of state-selected and oriented N2O molecules with one monolayer of Li on Rh(100) was studied by means of exoelectron emission analysis, time-of-flight mass spectroscopy and sticking measurements in a pulsed molecular beam experiment. State-selected and oriented N2O molecules are prepared using electrostatic hexapole and dipole fields applying the linear Stark effect. The sticking probability of N2O on Li/Rh(100) is found to be independent of the molecular state and orientation. The exoemission response on the pulsed beam reveals two different reaction channels: A direct channel which coincides with the arrival of the gas pulse and an indirect channel after the arrival of the gas pulses. The exoemission probability in the direct channel is strongly enhanced for vibrationally excited N2O molecules and further increases when N2O hits the surface with the O-end first. Furthermore, it is associated with the backscattering of N-2 molecules which depends on the molecular orientation, as well. From these observations we identify the direct reaction channel with an Eley-Rideal mechanism, i.e. an abstraction reaction that proceeds during the impact of the N2O where 'harpooning' is the key step that triggers the reaction. The indirect reaction channel, on the other hand, follows the Langmuir-Hinshelwood scheme where the reacting molecules are adsorbed on the surface with complete energy accommodation and thus lose the information on their molecular state and orientation in the gas phase. (C) 1999 Elsevier Science B.V. All rights reserved.