Collider experiments

In the following section, dealing with colloidal collider experiments, we present material indicating that the stability is as a result of a nonhomogeneous steric barrier with a thickness varying from 7 to 40 nm. 

ALICE Collaboration, Technical Proposal for a Laige Ion Collider Experiment at the CERN LHC, CERN-LHCC-95-71 (1995)... 

Drops coalesce because of coUisions and drainage of Hquid trapped between colliding drops. Therefore, coalescence frequency can be defined as the product of coUision frequency and efficiency per coUision. The coUision frequency depends on number of drops and flow parameters such as shear rate and fluid forces. The coUision efficiency is a function of Hquid drainage rate, surface forces, and attractive forces such as van der Waal s. Because dispersed phase drop size depends on physical properties which are sometimes difficult to measure, it becomes necessary to carry out laboratory experiments to define the process mixing requirements. A suitable mixing system can then be designed based on satisfying these requirements. 

Let us review this experiment. First, an amount of work was performed, W with the cylindrical stick. The amount of kinetic energy received by the white ball, (KE), was exactly fixed by W,. After the white ball collided with the red ball, the sum of the energies of the two balls, (KE)2 + (KEh, is exactly equal to (KE),. We can write... 

This discussion of threshold energy causes us to wonder what energies are possessed by molecules at a given temperature. We have already compared the molecules of a gas with billiard balls rebounding on a billiard table. When billiard balls bounce around, colliding with each other, some of them move rapidly and some slowly. Do molecules behave this way Experiment provides the answer. 

The experiment conducted by Rutherford and his co-workers involved bombarding gold foil with alpha particles, which are doubly charged helium atoms. The apparatus used in their experiment is shown in Figure 14-9. The alpha particles are produced by the radioactive decay of radium, and a narrow beam of these particles emerges from a deep hole in a block of lead. The beam of particles is directed at a thin metal foil, approximately 10,000 atoms thick. The alpha particles are delected by the light they produce when they collide with scintilltaion screens, which are zinc sulfide-covered plates much like the front of the picture tube in a television set. The screen... 

Such simple considerations led Scholten and Konvalinka to confirm the form of the dependence of the reaction velocity on the pressure, as had been observed in their experiments. Taking into account a more realistic situation, on the polycrystalline hydride surface with which a hydrogen molecule is dealing when colliding and subsequently being dissociatively adsorbed, we should assume rather a different probability of an encounter with a hydride center of a /3-phase lattice, an empty octahedral hole, or a free palladium atom—for every kind of crystallite orientation on the surface, even when it is represented, for the sake of simplicity, by only the three low index planes. 

Now that we have a model, we must check its consistency with various experiments. Sometimes such inconsistencies result in the complete rejection of a model. More often, they indicate that we need to refine the model. In the present case, the results of careful experiments show that the collision model of reactions is not complete, because the experimental rate constant is normally smaller than predicted by collision theory. We can improve the model by realizing that the relative direction in which the molecules are moving when they collide also might matter. That is, they need to be oriented a certain way relative to each other. For example, the results of experiments of the kind described in Box 13.2 have shown that, in the gas-phase reaction of chlorine atoms with HI molecules, HI + Cl — HC1 I, the Cl atom reacts with the HI molecule only if it approaches from a favorable direction (Fig. 13.28). A dependence on direction is called the steric requirement of the reaction. It is normally taken into account by introducing an empirical factor, P, called the steric factor, and changing Eq. 17 to... 

Gas density has a significant effect on the interactions among molecules of a gas. As molecules move about, they collide regularly with one another and with the walls of their container. Figure 5-13 shows that the frequency of collisions depends on the density of the gas. At low density, a molecule may move all the way across a container before it encounters another molecule. At high density, a molecule travels only a short distance before it collides with another molecule. As our Tools for Discovery Box describes, many scientific experiments require gas densities low enough to provide collision-free environments. 

In general one requires that gas and surface be equilibrated, such that they are at the same temperature. This may be a problem at loiv pressures, ivhere the gas molecules collide more often ivith the walls of the vacuum vessel than with the surface under study. Reducing the volume and increasing the pressure to the millibar regime by adding an inert gas helps to establish a region around the crystal where the gas is in thermal equilibrium with the surface. Such measurements are commonly referred to as bulb experiments. 

When a probe is inserted into a plasma, it will experience electrons and ions colliding with its tip. Due to the high mean speed of electrons, the flow of electrons is higher than the flow of ions. Consequently, the tip will charge up negatively until the electrons are repelled, and the net current then is zero. The probe potential then is the floating potential, Vfl. The electron current density Je then balances the ion current density 7,. At potentials lower than Vfl the ion current cannot increase further—in fact, only ions are collected from the plasma—and the ion saturation current /,s is measured. The plasma potential Vpi is defined as the potential at which all electrons arriving near the probe are collected and the probe current equals the electron current. Note that the plasma assumes the plasma potential in the absence of a probe hence probe perturbation at Vpi is... 

Experiments designed to probe these ideas have been carried out for NO colliding in high vibrational states on Au(lll). Before considering the results of these experiment, let us first look at how vibration might lead to unusual interactions with metal electrons. Molecules in vibrational states as high as NO(r = 15) undergo nuclear excursions that influence their... 

Figure 1 shows three configurations for colliding-beam experiments that have been used for studies of dissociative recombination. These are the inclined-beam apparatus at the University of Newcastle-upon-Tyne,30 the merged-electron—ion-beam experiment (MEIBE) at the University of Western Ontario,31 and an electron-cooler apparatus at the Manne Siegbahn laboratory in Stockholm.32 In the inclined-beam method (Figure la), the ion beam is accelerated to an energy of 30... 

Figure 3 shows another example where colliding-beam experiments have produced results in excellent agreement with each other. This particular reaction is the recombination of ions with electrons producing ion-pair products, i.e. ... 

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