Molecular-beam scattering second

Very accurate potentials for rare gases, based on a large amount of experimental data in gas phase (second virial coefficient, molecular beam scattering cross sections, spectroscopic data) and in solid phase, have been proposed by Maitland and Smith [80] with the functional form proposed by Barker and Pompe [81]. This function, with its larger number of parameters, is much more flexible than the LJ or Buckingham potential and the attractive part is correctly described as in equation (36) while the repulsive one also includes an exponential function, multiplied by a fifth order polynomial. 

A very recent second example of a sudden change in A0f was found for the system N2-Ru(0 0 0 1) by Papageorgopoulos et al. [69]. N2 can dissociate on this surface, which is relevant for ammonia synthesis, see, e.g. [70,71]. The width A0f is plotted in Fig. 7. A trend very similar to that found for 02-Ag(l 1 1) is observed. Note the difference with Ar scattering in Figs 5 and 7. Clearly, the fast N2 sees a change of potential. In this case, this change in potential has been predicted and verified theoretically [70, 72]. The observation of this change for A0f and the connection to the shape of the interaction potential shows the power of the molecular beams method in the exploration of gas-surface dynamics. 

Utilization of both ion and neutral beams for such studies has been reported. Toennies [150] has performed measurements on the inelastic collision cross section for transitions between specified rotational states using a molecular beam apparatus. T1F molecules in the state (J, M) were separated out of a beam traversing an electrostatic four-pole field by virtue of the second-order Stark effect, and were directed into a noble-gas-filled scattering chamber. Molecules which were scattered by less than were then collected in a second four-pole field, and were analyzed for their final rotational state. The beam originated in an effusive oven source and was chopped to obtain a velocity resolution Avjv of about 7 %. The velocity change due to the inelastic encounters was about 0.3 %. Transition probabilities were calculated using time-dependent perturbation theory and the straight-line trajectory approximation. The interaction potential was taken to be purely attractive ... 

A computer-controlled modulated molecular beam source is used to investigate the kinedcs of the surface reactions which occur when bromine is reactively scattered by Pd(lll). The reaction products are atomic bromine and molecular bromine the latter species arises from an adatom recombination process and gives rise to a product vector modulated at twice the frequency of the incident beam (2u.) By making suitable measurements of the temperature dependence of the product vector phase shifts at w and 2u, the four kinetic paranwteis which characterise the first-order and second-order rate processes are obtained. These are A, =2.5X 10 s , = 177 kJ moP, = 3.6X 10 m s , 13 3 mol. The significance of these values is discussed... 

This means that the scattering is elastic of the kind often referred to as Rayleigh scattering. Second, each ray entering the system is scattered only once. This assumption is essential for obtaining the required relation between the intensity of the scattered beam and the pair distribution function. If multiple scattering occurs, then such a relationship involves higher-order molecular distribution functions. 

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