Steric effect in the scattering of hexapole-oriented beams of symmetric-top molecules by graphite(0001)

Abstract

The orientational dependence of the scattering of hexapole-focused, oriented molecule beams of six symmetric-top molecules (CH3Cl, CH3F, CHCl3, CHF3, t-BuCl, CH3CN) and CH3OH from the (0001) surface of a graphite crystal is reported. Experimental angular distribution data are well represented by a two-component model, consisting of a ‘‘trapped/desorbed’’ and a ‘‘direct’’ scattered contribution. The steric effect, defined as the difference (‘‘heads’’ vs ‘‘tails’’) divided by the average of the scattered signals, has been measured as a function of the scattering angle and the degree of orientation of the molecules. There is considerable diversity among the different molecules with respect to the direction and magnitude of the steric effect of the scattering (and trapping). In all cases, however, the magnitude of the steric effect is essentially a linear function of the degree of orientation. Limited data on the incident energy dependence of the angular distributions and the steric effect are also presented. A model which deconvolutes the steric effect for the trapped/desorbed and directly scattered components is introduced. It also provides an independent estimate of the trapping probability of the incident molecules.

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

“…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. 1 They determined the molecular orientation of the outgoing molecule after collisions.…”

“…18,20 These improvements in the experimental conditions enable us to separate the scattered beam profile into two components explicitly (vide infra). In all possible scattering geometry in our apparatus, the scattered CH 3 Cl beam intensity in the CH 3 -end approach is found to be higher than that in the Cl-end approach.…”

Supersonic Molecular Beam Experiments on Surface Chemical Reactions

“…[86][87][88] They demonstrated steric effects in the weak interaction of polyatomic molecules with more degrees of freedom than NO.…”

“…[99] As we mentioned above, Bernstein et al also studied extensively the steric effects in the interaction between alkyl halide molecules and HOPG. [86][87][88] Below 100 K, physisorption occurred on HOPG and its growth kinetics were discussed in detail. [100] On the other hand, above 100 K, no stable adsorption layer was formed on HOPG.…”

“…In a series of works by Bernstein and co-workers, the trapping probability was always higher for the side of the highest charge density. [86][87][88] The interaction between a permanent dipole and its image dipole may be described with a simple point-charge model. In the CF3H scattering from the HOPG surface, such a simple approximation succeeded in explaining the observed steric effects.…”