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

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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.