Helium atom recombination

A simple and well studied process which illustrates the interaction of two atoms 

The first of these has to go over a hump of more than 0.2 eV, while the second has no hump. 

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Chlorine atoms were produced by flowing a mixture of 5 % Cl2 in helium through a quartz tube, coated with a thin film of baked phosphoric acid to inhibit Cl atom recombination, and enclosed in a 2.45 GHz microwave cavity operating at 35 W. The purity of reactants was 99.5 to 97 %, and they were frequently subjected to several freeze-pump-thaw cycles. The reactants were flowed inside the reactor neat or diluted in helium (3% mixtures). 

The role of electron-electron interaction is one of the main topics of atomic, molecular physics and quantum chemistry. The normal helium atom is then naturally one of the most fundamental systems. Doubly excited states are as almost bound states of special interest since the role of the electron-electron interaction is important in describing energies and also autoionization rates. Dielectronic recombination processes where one of the two excited electrons falls down to a lower level while the other is ejected appears to be a fundamental process where electron-electron interaction plays a dominant role[6]. The recently built electron-cooler storage rings [7] have made it possible to study dielectronic recombination and thereby doubly excited states with high experimental accuracy. 

The second step in the reaction, dissociation of the Hej Rydberg molecule, is similar to dissociative recombination of He with a free electron. For this reason, Bates73 called this recombination mechanism Rydberg dissociative recombination. It enhances the overall loss rate of free electrons because the stabilization of He2 prevents the return of weakly bound electrons to the population of free electrons. The reaction plays the same role as the reaction of H with H2 that we discussed in Section IV.C. As has been discussed by Bates, the mechanism also provides an explanation for spectroscopic observations of atomic and molecular emissions in helium afterglows. There is direct evidence for the existence of a substantial population of weakly bound electrons in helium afterglows.74 Most likely, the weakly bound electrons are Rydberg electrons in He2 molecules. 

A wide variety of plasma diagnostic applications is available from the measurement of the relatively simple X-ray spectra of He-like ions [1] and references therein. The n = 2 and n = 3 X-ray spectra from many mid- and high-Z He-like ions have been studied in tokamak plasmas [2-4] and in solar flares [5,6]. The high n Rydberg series of medium Z helium-like ions have been observed from Z-pinches [7,8], laser-produced plasmas [9], exploding wires [8], the solar corona [10], tokamaks [11-13] and ion traps [14]. Always associated with X-ray emission from these two electron systems are satellite lines from lithium-like ions. Comparison of observed X-ray spectra with calculated transitions can provide tests of atomic kinetics models and structure calculations for helium- and lithium-like ions. From wavelength measurements, a systematic study of the n and Z dependence of atomic potentials may be undertaken. From the satellite line intensities, the dynamics of level population by dielectronic recombination and inner-shell excitation may be addressed. 

The question of the mechanism of recombination of ground-state halogen atoms, including iodine atoms, has again attracted some interest.188 In the presence of NO and helium, Troe et al. established the mechanism as (70) and... 

By the term dense, it is implied that there are no intentional interconnected pores in the membranes other than atomic interstices, atomic vacancies and dislocations. Such void spaces are too small to accommodate even molecular hydrogen, and dense membranes, of the type reviewed, transport hydrogen only in a dissociated form. Dense membranes block transport even of helium, and the absence of larger pores gives dense membranes hydrogen selectivity approaching 100%. Transport of hydrogen in a dissociated form implies that dense membranes must possess adequate catalytic activity for the adsorption and dissociation of H2 on the feed-side surface (retentate) as well as for the subsequent recombination and desorption from the permeate-side surface. 

The atomic and molecular emission bands in the negative glow in helium discharges were observed with comparable intensities at high pressure. The molecular emission was shown to be due to three-body recombination. Other studies of ions in plasmas from radiation emission may be of interest. ... 

Abstract Rate constants for charge transfer processes in the interstellar medium are calculated using ab-initio molecular calculations. Two important reactions are presented the recombination of Si + and Si + ions with atomic hydrogen and helium which is critical in determining the fractional abundances of silicon ions, and the C+ + S -> C + S+ reaction, fundamental in both carbon and sulphur chemistry. 

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