Surface temperature dependence of the steric effect in the scattering of oriented tert-butyl chloride and fluoroform molecules by graphite(0001)

The scattering of focused beams of hexapole-oriented {ital t}-BuCl and CHF{sub 3} molecules from a graphite (0001) surface has been studied over the surface temperature range 165 {lt} {ital T}{sub {ital s}}{lt}730 K. The detected number densities of the surface-scattered beams are compared for parallel vs antiparallel incident orientation of the molecular dipole with respect to the surface normal. From the fractional difference in scattered signals, i.e., the so-called steric effect, and from the scattering angular distributions of the unoriented molecules, the relative difference in scattering probabilities for opposite molecular orientations is obtained. For both molecules the magnitude of the relative difference in scattering (the steric effect) increases as {ital T}{sub {ital s}} decreases. At the lowest surface temperature of 165 K the relative difference in scattering probability reaches +1.5 for {ital t}-BuCl (and {minus}0.8 for CHF{sub 3}), approaching the largest possible magnitude of 2.0, as compared to +0.3 (and {minus}0.4 for CHF{sub 3}) at the highest temperatures. At all temperatures the steric effect for directly (near-specularly) scattered molecules depends linearly on the degree of molecular orientation. From the angular distribution measurements, it is found that the inelastic translational energy loss in the direct'' scattering process increases as {ital T}{sub {ital s}} decreases. Finally, the temperature dependence of the scattering probability of unoriented beams of {ital t}-BuCl and CHF{sub 3} has been obtained.