Physical kinetics collision cross-section

Figure 5 (Kurachi and Nakamura, 1990) presents a survey of electron collision cross sections of CF4. In addition to the momentum-transfer cross section q , it shows the vibrational-excitation cross sections q T, and q (for two different vibrational modes), the (total) electronic-excitation cross section q, the dissociation cross section q j , the electron-attachment cross section qg, and the (total) ionization cross section 9,. Each of the cross sections is a function of the electron kinetic energy and reflects the physics of the collision process, which is being clarified by theory. The cross sections designated as total can be discussed in greater detail in terms of different contributions, which are designated as partial cross sections.

Chemical thermodynamic data and thermophysical properties of fluids are routinely evaluated in this manner. Computer programs have been developed that permit these checks to be carried out on large data sets and which select the recommended values through a least-squares or similar fitting procedure. Other fields amenable to this approach are atomic and molecular spectroscopy (see Spectroscopic Databases), nuclear physics, and crystallography (see Cambridge Structural Database). In still other cases, such as chemical kinetics and various collision cross-sections, theory can be used to place limits on data values. 

This expression has a particular physical meaning. If we set equal to zero and replace nd with the hard-sphere collision cross section of the kinetic theory of gases then we obtain the collision frequency Z (Hirschfelder et ai., 1959). 

For a molecule in the excited state, the effective cross-section for coil os can be much greater than those for kinetic collision. The optical collisions nay be defined as the minimum distance of approach over which the excited mole-cule can interact with another molecule to bring about a physical or chemical change. 

Different layers of differential information are available in ionizing collisions. The simplest quantity is the single differential cross section, dpositrons scattered, or the electrons ejected, into a particular solid angle are measured, irrespective of their kinetic energy and the fate of the undetected particle. McDaniel (1989, section 6.9A) stated that no measurements of this cross section have been made for electron impact since it has no physical content, owing to the presence of two electrons in the final state. However, this quantity is of interest in positron collisions. 

In this chapter we consider the physics of the positronium atom and what is known, both theoretically and experimentally, of its interactions with other atomic and molecular species. The basic properties of positronium have been briefly mentioned in subsection 1.2.2 and will not be repeated here. Similarly, positronium production in the collisions of positrons with gases, and within and at the surface of solids, has been reviewed in section 1.5 and in Chapter 4. Some of the experimental methods, e.g. lifetime spectroscopy and angular correlation studies of the annihilation radiation, which are used to derive information on positronium interactions, have also been described previously. These will be of most relevance to the discussion in sections 7.3-7.5 on annihilation, slowing down and bound states. Techniques for the production of beams of positronium atoms were introduced in section 1.5. We describe here in more detail the method which has allowed measurements of positronium scattering cross sections to be made over a range of kinetic energies, typically from a few eV up to 100-200 eV, and the first such studies are summarized in section 7.6. 

The main observables that relate directly to small scale physical processes are the spectral properties of the meteor emissions and the physical properties of ionization trails. Both luminosity and ionization are derived directly from hypervelocity collisions between vaporized meteoroid species and atmospheric contituents. From Table 1, it is seen that, for example, a Mg atom evaporated from a Draconid meteoroid has a kinetic energy of 50 eV, while the same atom evaporated from a Leonid meteoroid has a kinetic energy of 630 eV when it collides with an atmospheric molecule. In both cases, the translational energy available to reactions is sufficient for inelastic processes such as electronic excitation and collisional ionization. As will be pointed out in Sec. 3, the respective cross sections increase dramatically with collision energy. Both the visual magnitude and the radar echo signatures of equally sized Draconid and Leonid meteoroids will thus differ substantially. 

Modern gas-diffusion medium in low-temperature fuel cells is typically a highly porous carbon paper with porosity in the range of sgdl = 0.6-0.8 and with the mean pore radius in the order of 10 pm (10 cm). By the order of magnitude, the mean free path of molecules in atmospheric pressure air is = l/(A LO-fci ), where Nl = 2.686 10 cm is the Loschmidt number (number of molecules in a cubic centimetre of atmospheric pressure gas at standard temperature) and akin — 10 cm is the molecular cross-section for kinetic collisions. With this data we get 3 10 cm, or 3 10 pm. Obviously, mean pore radius in the GDL is nearly 3 orders of magnitude greater than I f and the physical mechanism of molecule transport is binary molecular diffusion. 

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