Collisions with a surface

Another promising method for producing not only alignment but also orientation in a molecular beam uses spatial effects in collisions with the surface of a crystalline solid (see [208, 247, 281, 299, 354, 355] and references therein). These and other spatial effects are being studied intensely in the laboratories of Auerbach, Zare, Stolte and others, beginning in the 1980s. The idea of the method may be visualized as the bank shot of a billiard ball. 

The problem of extracting information on the polarization of the beam molecules under these conditions has been discussed in [207, 354]. 

In this sense considerable value ought to be attributed to the observation [353,355] of the emergence of orientation of the angular momentum in the N2/Ag(lll) scattering, described by means of polarization moments of rank n = 1 and 3. The orientation was measured along the direc- 

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The deposition of mass and charge selected ions onto surfaces is underway but is in its infancy. How do the ions survive the collision with a surface This question has a myriad of answers depending on many variables and will have a future in investigative studies. A soft landing is now a possibility (280) and allows the potential spectroscopic investigation of trapped ions. So far no transition metal ions have been examined using this method but it is only a matter of time. Soft landings via inert gas matrices also have potential in the surface deposition of mass selected clusters. 

The short lifetime of an excited species near a metal surface is most clearly demonstrated by the data in Fig. 2 and Table 1 which shows that the probability of an ion being neutralized upon collision with a surface is greater than 0.99 for a number of ions over a range of kinetic energies . Neutralization happens with high probability even though the time that the ion spends near the surface is very short. The time that a 100 eV He ion, for example, spends within 5 A of the surface is 10" -10" seconds and yet the probability of neutralization is 0.9983. 

Probability that an ion will dissociate upon collision with a surface versus ion energy, probability of dissociation is equivalent to the probability that the Nj will react chemically consequence of its collision. When is reacts chemically, it disappears from the gas phase. 

Molecules are in continuous motion, and in the gas phase they are moving several hundred miles an hour. For example, water molecules at room temperature are traveling at about 1340 mph and ethanol molecules at about 840 mph. They make between 3 to 5 billion collisions with each other or against a surface each second. These collisions with a surface are what we recognize as pressure. The pressure exerted by our atmosphere pressing down against the earth s surface at sea level is called an atmosphere (at 0 C). Table 3-1 indicates several ways of expressing an atmosphere plus a few other common conversion factors. 

According to kinetic theory, the pressure of a gas, P, will be proportional to the frequency of molecular collisions with a surface and to the average force exerted by a molecule in collision. 

Since catalysis involves molecules adsorbed on a surface, it is also useful to consider the expression for the number of hard sphere collisions with a surface (Figure 4.7). Then only the velocity component of a particle into the direction of a surface is of interest, and only values of the angle 9 smaller than nil contribute to a collision and the relative velocity for surface collisions becomes... 

LEIS is attractive as a surface-specific technique. Spectra are usually obtained using noble gas ion beams from 0.5 to 3 keV. Due to strong electron affinity of inert-gas ions, the probability of electron transfer is very high, even in the initial collision with a surface atom. After two or more collisions, most ions are neutralized so that a detector set to analyze only ions of the same type as those in the incident beam detects ions that have almost entirely had only one collision with a target atom. Projectiles entering the solid are discarded because they require several scattering events to return to the surface and exit. 

Surface-induced dissociation (SID) ° is an important experimental tool for determining structural properties of ions and energetic and mechanistic information concerning their dissociation pathways. In SID the ion is energized by collision with a surface. If electronic excitation is unimportant, the collision translational energy E, is partitioned between the final translational energy Ef, and transfer to the internal vibrational/rotational modes of the ion AEint and the vibrations of the surface... 

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