A schematic top-view of the UHV chamber that shows the essential components for dissociative sticking measurements.
Schematic diagram of the heated effusive molecular beam source.
Relative RGA signals from selected m/e ion currents during effusive molecular beam dosing of propane as a function of nozzle temperature while maintaining a total chamber pressure of 10−7 Torr. The dashed vertical line at a nozzle temperature of T n = 960 K marks the onset of propane pyrolysis.
Evolution of the carbon coverage buildup on Pt(111) at T s = 1014 K as a function of exposure to ethylene dosed as a background gas at T c = 300 K. The solid points are experimental data whereas the red solid line is a simulation based on globally fit functional form, S(θ c ) = S 0[1 − (θ C /θ C, max )], such that θ c (ε) = θ C, max [1 − Exp( − n S 0 ε/θ C, max )], where S 0 = 0.82, θ C, max = 2.57ML, and n = 2. The red dashed line gives the dependence for a constant dissociative sticking coefficient, S(θ c ) = S 0 = 0.82. The expanded scale inset shows that a constant S(θ c ) holds adequately well over the 0.1–0.4 ML range of θ c typically accumulated in alkane dissociative sticking coefficient measurements. The best linear fit passing through the origin for the experimental data of the inset is given by the black dotted line whose slope yields S = 0.80.
(a) Effusion from a trapped volume behind the calibrated leak for CH4, N2, Ar, and (CH3)3CH is monitored by the manifold Baratron pressure, p m . The inset shows that the fitted time constants vary as . (b) Pumping speed measurements for N2 over a range of chamber pressure shows a falling off of C c = C l p m /p at pressures belowp∼ 8 × 10−9 Torr. The upper axis provides an approximate p m for comparison to p based on a fixed average pumping speed for N2 of C c = 172 L/s.
Evolution of the carbon coverage buildup on Pt(111) at T s = 700 K when dosed by an effusive beam at T g = 700 K from a heated nozzle with a 0.0197 in. dia. orifice and 0.003 in. orifice wall thickness at a distance of 1 cm away from the surface. The exposure from background gas is εbkg = (1/R)εdir, and, under these conditions, the C coverage attributable to dissociative sticking from the background gas was ∼160 times less than that from the directed effusive beam. The line passing through the origin was fitted to the direct component of the total C coverage for data with θ c ⩾ 0.1 ML. The line's slope yields S dir.
Alkane dissociative sticking coefficients on Pt(111) for the impinging gas temperatures indicted (Ref. 5). The solid points (expts) and lines (ME-MURT simulations) are for normally directed effusive molecular beams. The open points (expts) and dashed lines (ME-MURT simulations) are for ambient gas impingement. (a) Methane/Pt(111); (b) Propane/Pt(111) for which the S n (T) points from direct effusive beam experiments are additionally labeled by encirclement.
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