Hydrogen Atom Collision Processes

The traditional kinetic scheme of a CR model for atomic hydrogen (see, e.g., [12]) includes only the electron impact and radiative processes of H(n), n 1, 

In divertor regions with temperatures below a few eV, II ions may be formed by radiative electron attachment 

In the context of hydrogen neutral beam attenuation kinetics in fusion plasmas, and charge exchange core plasma diagnostics, the important hydrogen atom processes are those with highly stripped impurity ions, Aqz (where Z is the atomic number of impurity, and q is its charge) 

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As mentioned earlier, practically all reactions are initiated by bimolecular collisions however, certain bimolecular reactions exhibit first-order kinetics. Whether a reaction is first- or second-order is particularly important in combustion because of the presence of large radicals that decompose into a stable species and a smaller radical (primarily the hydrogen atom). A prominent combustion example is the decay of a paraffinic radical to an olefin and an H atom. The order of such reactions, and hence the appropriate rate constant expression, can change with the pressure. Thus, the rate expression developed from one pressure and temperature range may not be applicable to another range. This question of order was first addressed by Lindemann [4], who proposed that first-order processes occur as a result of a two-step reaction sequence in which the reacting molecule is activated by collisional processes, after which the activated species decomposes to products. Similarly, the activated molecule could be deactivated by another collision before it decomposes. If A is considered the reactant molecule and M its nonreacting collision partner, the Lindemann scheme can be represented as follows ... 

As indicated in Table 1, free radicals can be formed in the gas phase by the collision of energetic free electrons with monomer molecules. And in a related process, hydrogen atoms are produced together with a hydrogen depleted monomer molecule. The ease with which this process occurs appears to increase with increasing saturation of the monomer . Once formed, the hydrogen atoms can produce further free radicals by either hydrogen abstraction or addition to an olefin. 

Shimokawa, S., Namiki, A., Garno, M. N. and Ando, T. Temperature dependence of atomic hydrogen-induced surface processes on Ge(100) Thermal desorption, abstraction, and collision-induced desorption. Journal of Chemical Physics 113, 6916-25 (2000). 

Once reliable data for electron capture by positrons became available it was natural to compare the behaviour of the cross sections for this process with those for the analogous capture process in heavy positive particle impact, in particular, the cross section for the formation of atomic hydrogen in collisions of protons with various atoms and molecules, reaction (4.3). It is also pertinent to note that comparisons between the behaviour of protons and positrons are usually made at equal projectile speeds v rather than at equal energies. 

In addition to precision frequency metrology, 2S-nX spectra can provide a wealth of information about scattering lengths for excited atoms. Metatable collision processes, and photoassociation processes, should also be observable. In summary, there are lots of scientific opportunities for trapped ultracold hydrogen. 

Besides volume processes wall collisions of hydrogen particles can contribute to the vibrational population. A direct process is the interaction of already vi-brationally excited molecules with the surface (v) +wall —> ff2(w) mostly depopulating the vibrational levels. Further fundamental mechanisms are the Langmuir-Hinshelwood and the Eley-Rideal mechanism. They are based on recombining hydrogen atoms or ions Hads/gas + Hads —> H2(v). In the first case an adsorbed particle at the surface recombines with another adsorbed particle (Langmuir-Hinshelwood mechanism). In the second case one particle from the gas phase recombines with an adsorbed particle (Eley-Rideal mechanism). For these processes the data base is scarce and often not determined from plasma material interaction experiments. A dependence on particle densities, surface material and surface treatment as well as surface temperature can be expected. 

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