Using angle and time resolved molecular beam techniques, an investigation of the low coverage adsorptiondesorption kinetics of NO on Pt(l!!) is made. The experiments are carried out over a crystal temperature range of 300 K < T, < 900 K. For T, > 500 K, the sticking probability s > 0.9. NO adsorbs molecularly with little dissociation « 5%). The desorption rate is found to be strongly dependent on the incident beam flux and trace amounts of chemisorbed oxygen « 1%) on the surface. These findings suggest that steps play the dominant role in low coverage kinetics for a nominally flat crystal. We present a model which incorporates the effect of steps, explains the nonlinearity of the desorption kinetics, and reconciles the disagreement between these results and previous molecular beam studies.
A kinetic model is presented which describes thermal desorption of molecules from surfaces and incorporates trapping at steps. The model is applied to desorption of NO from Pt(111). The results of three molecular beam experiments, which differ substantially from each other, are accounted for quantitatively by the model using one set of rate parameters. It is demonstrated that steps, even when present in small concentrations on accurately cut crystals, can play a dominant role in thermal desorption. The magnitude of the effect depends on the difference between the binding energies on the terrace and at steps, which for NO on Pt(111) is large (25±2 kcal/mol and 34±2 kcal/mol, respectively). Furthermore, it is shown that the rate parameters governing terrace desorption, escape from steps, and surface diffusion can all be extracted from modulated molecular beam studies carried out for a suitable range of conditions.
The classical hard ellipse model for atom–diatom rotationally inelastic (RI) collisions is generalized to include simultaneous vibrational excitation by assuming that this excitation is proportional to the square of the component of momentum transferred along the major axis of the ellipse. Calculations are presented which compare level-to-level RI angular distributions of vibrationally elastic (Δv = 0) and vibrationally inelastic (Δv = 1) scattering. These calculations reproduce the main features observed in recent measurements of level-to-level rovibrationally inelastic scattering for Na2 with Ar. With Δv = 0 and 1, the scattering distributions display rainbow structure whose angular position increases nearly linearly with Δj. In addition, the Δv = 1 scattering shows considerable suppression of small angle scattering (which also has small Δj). We emphasize that this suppression of forward scattering results from a kinematic exclusion of small angle scattering for impulsive inelastic collisions with a slowly rotating molecule irrespective of the origin of inelasticity.
We report measurements of differential cross sections of rotationally (Δj = 15 to 79)–vibrationally (Δv = 1) inelastic collisions in ground electronic state Na2 with Ar on a level to level basis. The experiment is performed using crossed supersonic molecular beams and two dye lasers, one for tagging the initial level (v = 0, ji = 7), and the second for detection of the final level (v = 1, jf = 7+Δj) and scattering angle ϑ. A description of the experimental apparatus and technique is given. The differential cross sections all exhibit a pronounced rise at some minimum angle to a maximum followed by a gradual decline out to ϑ = π. The angular positions of the rise and peak increase nearly linearly with Δj. The Δv = 1, Δj integral cross sections are roughly constant in size even though the inelasticity (ΔE) varies over a large fraction of the available initial kinetic energy (∼2400 cm−1); by comparison, the Δv = 0, Δj integral cross sections, over a similar range in Δj, follow a power law dependence in ΔE. The qualitative features of both the differential and integral cross sections appear consistent within the framework of a classical, sudden collision on a sharply repulsive potential.
We discuss two methods, one of them new, for recovering level-specific differential cross sections in crossed molecular beams experiments from the Doppler profiles of line shapes observed by laser induced fluorescence. The angular resolutions of the two methods are compared and shown to be complementary. An experiment using both methods can have moderately good angular resolution at all scattering angles. In the first method, which has previously been demonstrated experimentally, the Dopper profile is taken with the laser beam parallel to the relative velocity of the collision system. Good angular resolution is obtained between π/4 and 3π/4. In the second method, which is proposed here, the Doppler profile is taken with the laser beam perpendicular to this relative velocity, and the best angular resolution is obtained in the regions 0 to π/4 and 3π/4 to π. This method requires an integral transform to recover the cross section from the Doppler profile. A practical implementation of this transform is presented along with a numerical example showing its relative insensitivity to noise in the profile.
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