Scattering molecule

The site specificity of reaction can also be a state-dependent site specificity, that is, molecules incident in different quantum states react more readily at different sites. This has recently been demonstrated by Kroes and co-workers for the Fl2/Cu(100) system [66]. Additionally, we can find reactivity dominated by certain sites, while inelastic collisions leading to changes in the rotational or vibrational states of the scattering molecules occur primarily at other sites. This spatial separation of the active site according to the change of state occurring (dissociation, vibrational excitation etc) is a very surface specific phenomenon. 

In order to do this, we anticipate the form of the expression for Equation (10.31) will show that 1 /1q can be written as the product of two terms an optical-molecular factor we symbolize as and a geometrical factor 1 + cos where r is the distance from the scattering molecule and 0 is... 

We shall see in subsequent sections that measuring Rg as a function of 0 can be used to evaluate the radius of gyration of the scattering molecules, thereby providing more information about the polymer in addition to M and B. 

Observed angular distributions were quasi-specular and scattered rotational distributions were strongly dependent upon the incidence energy, both observations indicating the direct nature of the interaction. The most important observation of the work was the approximately Arrhenius surface temperature dependence of the vibrational excitation probability, exhibiting an effective activation energy close to the vibrational excitation energy of the scattered molecule (see Fig. 2). The authors also showed that the... 

Figure 12 An illustration of the cosine law distribution for frequency of (a) incident and (b) scattered molecules. The length of an arrow is proportional to the frequency of molecules incident or scattered at the angle indicated.

Thus, for Knudsen cosine scattering, / = 1, and for specular reflection, / = 0. Equation (59) may be solved for the drift velocity of the scattered molecule to give Uf = (1 - f)ut. The viscous force transmitted to the wall during gas collisions is the product of the number of collisions per second and the momentum change per collision,... 

The energy of an incident molecule will not normally be the same as that of the molecule when it is scattered from the surface, i.e., ZsP Ef There will be an accommodation to the surface and an exchange of energy with the surface. Complete accommodation or equilibration with the surface would imply that the scattered molecules have the same temperature as the surface. The energy accommodation coefficient, ac, is defined for each surface involved in the problem by the expression... 

While for a given surface the incident and scattered molecules have different energies (unless ac = 0), the molecule scattered from surface 1 is the molecule that is incident for surface 2. That is, since there are no gas-gas collisions when the molecule travels between the two surfaces, there is no mechanism to change the energy that it has when it leaves surface 1. Likewise, the energy of the molecules scattered from surface 2 is the same as the energy of those molecules incident upon surface 1. Thus, the temperatures are related by... 

Assuming in the first instance that the polymer molecules are quite independent, then the situation is analogous to that in the preceding section, except that the scattering molecules (polymer) are now surrounded by solvent molecules of refractive index n0 instead of by free space of refractive index 1.0. The analogue of Eq. (15) becomes... 

State-resolved inelastic scattering for a wide range of incident conditions ( ), d,) are measured for this system by combining molecular beam techniques with (2 + 1) ion TOF REMPI detection of the scattered molecules [58]. Energy transfer parallel to the surface is measured from the Doppler broadening of the REMPI spectra. Trapping... 

The Rayleigh theory does not apply when the scattering molecules are absorbing or when the atmosphere contains dust particles, water drops, or other particles with dimensions that are larger than ordinary gas molecules. 

One of the main goals of the crossed-beam experiment is to measure the internal energy AEvlh rol transferred to the molecule. In principle, this is possible in either of two ways. First, the scattered molecules could be detected and their product-state population analyzed. Infrared emission or absorption techniques may be considered, similar to those used in cell experiments.13 21 Although such studies would lead to the most detailed results (at least for polar molecules), under crossed-beam conditions they are impossible for intensity reasons, even if the possibility of measuring differential cross sections is renounced and the molecules in the scattering volume itself are detected. Detection via electronic molecular transitions may be invisaged. Unfortunately, the availability of tunable lasers limits this possibility to some exotic molecules such as alkali dimers. The future development of UV lasers could improve the situation. Hyper-Raman... 

In order to quantify the energy transfer to the e-h pairs, the energy distribution for directly scattered molecules was determined (Fig. 14b). Less than 10% of the incident kinetic energy is transfered to... 

The site dependence of the PES produces diffraction of scattered molecules, which we shall discuss in Section 5. The corrugation can also be probed by examining the incidence angle dependence of the dissociation. This is commonly discussed in terms of the scaling of the dissociation, writing... 

Figure 15 A swarm of classical trajectories incident on a model PES [49]. The site labelled bridge is initially attractive, but ultimately there is a barrier to dissociation at this site. At die atop site dissociation is activationless (downhill), but molecules can fail to take this path because they are initially steered to die bridge site. Some molecules can trap because die momentum normal to die surface is converted into parallel motion and rotations. After making several bounces, the trapped molecules dissociate or return to die gas-phase. For H2/Pd(l 1 1) die trapping channel contributes a large fraction of the scattered molecules [50].

Higher specificity of experiments can be obtained by the detection of scattered molecules in specific quantum states, by preparation of the molecules in the molecular beams in specific quantum states, or both. The first category has been discussed above. The experiments in the latter cases are of enormous complexity and will be discussed last. State-preparation of the incident molecules can be done in various ways as recently reviewed by Sitz [32]. Therefore, I will be very brief in this review and only discus systems that have been already mentioned earlier in this review. There are several ways to prepare the initial state of the molecular beam ... 

Another method to prepare the initial molecular state is by using fields and the Stark effect. In this way oriented beams of notably NO and several spherical top molecules can be prepared. This has already been introduced in Section 2. In the first experiment of this kind the state selector was not used to prepare the beam, but to determine the orientation of the scattered molecules [139]. In later experiments, the... 

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