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

Abstract

The scattering of focused beams of hexapole-oriented t-BuCl and CHF3 molecules from a graphite (0001) surface has been studied over the surface temperature range 165 < Ts<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 Ts decreases. At the lowest surface temperature of 165 K the relative difference in scattering probability reaches +1.5 for t-BuCl (and −0.8 for CHF3), approaching the largest possible magnitude of 2.0, as compared to +0.3 (and −0.4 for CHF3) 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 Ts decreases. Finally, the temperature dependence of the scattering probability of unoriented beams of t-BuCl and CHF3 has been obtained.

Derivation of a temperature-dependent accommodation coefficient for use in modeling laser-induced incandescence of soot

“…At higher surface temperatures for which trapping probabilities are low, only the direct inelastic component is observed, resulting in a decrease in thermal accommodation with increasing surface temperature. This behavior has been observed for tert-butyl chloride [24], CHF 3 [24], Ar [16], and NO [14][15][16] scattered from graphite. The diffuse component decreases rapidly between 200 and 730 K for tert-butyl chloride [24] and CHF 3 [24] and disappears at surface temperatures above ∼400 K for Ar [16] and NO [14-16, 26, 27].…”

“…This behavior has been observed for tert-butyl chloride [24], CHF 3 [24], Ar [16], and NO [14][15][16] scattered from graphite. The diffuse component decreases rapidly between 200 and 730 K for tert-butyl chloride [24] and CHF 3 [24] and disappears at surface temperatures above ∼400 K for Ar [16] and NO [14-16, 26, 27]. For NO scattered from graphite, the sticking probability has been shown to decrease to zero at about the same surface temperature at which the diffuse component disappears [28].…”

“…Partitioning of scattered molecules into separate diffuse and quasi-specular populations has been observed for tertbutyl chloride [24], fluoroform (CHF 3 ) [24], N 2 [25], and NO [14][15][16]23] scattered from graphite. At higher surface temperatures for which trapping probabilities are low, only the direct inelastic component is observed, resulting in a decrease in thermal accommodation with increasing surface temperature.…”

“…8 demonstrate a decrease in α Trans with increasing surface temperature for both the points inferred from the data and the extrapolated values. Other data sets that provide information for t-butyl chloride [24], CHF 3 [24], and Xe [42] scattered from graphite as a func- Table 4, and the fits in (a) were used to derived the lines in (b) tion of surface temperature are not complete enough to derive translational accommodation coefficients.…”

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.…”