Dependence of the sticking probability on initial molecular orientation: NO on Ni(100)

Abstract

The sticking probability of NO at Ni(100) was examined using a beam of orientated NO molecules. It is found to be higher for N-end collisions. The asymmetry of the sticking probability has been measured to be a linear function of the molecular degree of orientation.It was determined to be A = 0.7 f 0.1 and nearly independent of coverage when normalized to the degree of orientation. The orientational dependence of the sticking probability as a function of coverage shows that the adsorption of the molecules cannot be described by a precursor model that neglects direct chemisotption.NO molecules are bond to most surfaces perpendicularly with the nitrogen atom directed towards the surface. The dynamics of the adsorption process should therefore be orientation-dependent due to anisotropic molecule-surface interactions. This leads to the question whether the sticking probability of the molecules can be influenced by a specific orientation of the molecular axis prior to adsorption. In theoretical studies a higher sticking coefficient was calculated for molecules approaching the surface in the favourable bonding configuration for NO/Pt(lll) and NO/Ag(lll) [l]. Recent experiments were focused on the effects of orientation in scattering and trapping/ desorption. Steric effects were observed in scattering experiments of orientated NO from Ag(ll1) [2] and of oriented CH,F from graphite [3]. Recently, Kuipers et al. [4] have measured steric effects in scattering and trapping/desorption of NO at Pt(ll1) from which they determined a higher trapping probability for an initial orientation with the N-end towards the surface. complemental to these scattering experiments we have examined steric effects in the sticking probability, which probes directly the chemisorption potential. As reported briefly [S], in the case of NO/Ni(lOO) for molecules with one particular degree of orientation we have observed a higher initial sticking probability when the surface is approached with the N-end first. However, to assess the order of magnitude of the effect measured one has to take into account the actual degree of orientation achieved. Therefore, we present here measurements of the sticking probability of nitric oxide on Ni(lOO) at different orientation field strengths. In addition, to study the influence of the molecular orientation on the adsorption kinetics we have examined the sticking probability as a function of coverage.Ni(100) was selected to study asymmetries in the sticking process because it is known that the NO molecules are chemisorbed perpendicularly with the N-end to the Ni surface (61 and have an initial sticking coefficient markedly different from unity and from zero (7]. We work at low target temperatures where the desorption probability from the chemisorbed state has been found to be negligibly small.Using the electrostatic hexapole technique [ 81, NO molecules can be state-selected for subsequent orientation in homogeneous electric fields 193 due to their permanent dipole moment. In our experimental apparatus a molecula...

Orientation dependence of NO sticking and scattering at Pt (100)

“…In contrast to previous investigations employing a continuous beam source [ 1,3 1, the source was now replaced by a pulsed nozzle in order to decrease the rotational temperature of the NO molecules. Since the degree of orientation depends strongly on the rotational state distribution of the NO molecules [ 10 1, an increase of the previous degree of orientation of (cos a) = 12% [ 1 ] can be expected. For instance, theoretically, NO molecules in the 2111,2]J=& a=& M,=$) ground state can reach (cos@)=33.3% and the ]J=$ G?=j, M,= $ ) state can result in (cos 0) = 20% in the limit of high electric field strength.…”

Steric Effects in the Scattering of Oriented CH3Cl Molecular Beam from a Graphite Surface: Weak Interaction of Physisorption

“…In particular, the steric effect on the interaction of polyatomic molecules from surfaces is one of the central topics in the fields of organic chemistry, catalytic chemistry, biochemistry, and so on. Although there are a lot of studies on the steric effects of diatomic molecules on surfaces, [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][26][27][28][29][30][31][35][36][37]41 the works on polyatomic molecules are rare. 1,[17][18][19][20][21][22][23][24][25][31][32][33][38][39][40] The steric effects of a polyatomic system was first reported by Novakoshi and McClelland in the CF 3 H scattering from Ag(111) surface.…”

Observation of orientation-dependent electron transfer in molecule–surface collisions

“…Thus, we target measurements at high kinetic energy to have the best chance of avoiding the obscuring effects of dynamical steering. Moreover, the experiment is further simplified at high kinetic energy as these conditions suppress sticking (20), an elementary surface process where strong orientation effects have been previously reported (21)(22)(23)(24)(25)(26)(27).…”