Electron capture to the continuum

Figs. 10 and 11 for helium. This feature is closely analogous to the one observed for ve = vp for electron capture to the continuum mechanism or to electron loss to the continuum which is often referred to as the cusp. The electrons with v, vr 0 in the low-lying continuum states of the target ion also form a sharp peak known as the target cusp. ... 

In Figs. 24 and 25 we show the measured double differential cross sections for electron emission at zero degrees in collisions of 100-keV protons with He and H2 [39] compared to CDW-EIS predictions [39]. Uncertainties associated with the experimental results vary from 1% near the electron capture to the continuum peak to about 15% near the extreme wings of the distribution. These results have been scaled to provide a best fit with CDW-EIS calculations. In both cases there is satisfactory agreement between the CDW-EIS calculations and experiment, particularly with excellent agreement for electrons with velocities greater than v, where v is the velocity of the projectile. For lower-energy electrons the eikonal description of the initial state may have its limitations, especially for lower-impact parameters. 

The first reported study of the behaviour of double differential cross sections for positron impact ionization was that of Moxom et al. (1992) these workers conducted a search for electron capture to the continuum (ECC) in positron-argon collisions. In this experiment electrons ejected over a restricted angular range around 0° were energy-analysed to search for evidence of a cusp similar to that found in heavy-particle collisions (e.g. Rodbro and Andersen, 1979 Briggs, 1989, and references therein), which would be the signature of the ECC process. 

Schultz, D.R. and Reinhold, C.O. (1990). Electron capture to the continuum and binary ridge structures in positron-hydrogen collisions. J. Phys. B At. Mol. Opt. Phys. 23 L9-L14. 

Fig. 6. Probability (per unit time) of Auger capture to the L shell of a N ion embedded in a FEG 7 us a function of the number of L-shell electrons bound to the ion before the capture. Fl is in atomic units. Two different values of the electronic density = 1.5 and 2 are shown. For ml > 5 the 2p KS orbital merges into the continuum and is no longer bound.

If two electrons are transferred to bound states on the projectile, as opposed to the continuum, once again a clear signature of a multifermion process is present. However for charge transfer there is no simple identification of the strength of the external probe to see if EIMP or IIMP is dominant. Thus one must rely on theory to see whether correlation plays a role, or not, in such collisions. And everyone must agree on what correlation is. See for example, Stohlterfoht et al. [7.1], for a discussion of electron correlation in double capture. 

Multiple hole creation may result from direct excitation or from Auger and Coster-Kronig processes (see [2]). Under ion bombardment direct ionization to the target continuum of states can compete with electron capture by the projectile. It is known, that X-ray spectra excited by heavy ions may show intense multiple emissions whose energies correspond to electron- or photon-excited satellites. A great number of experiments and theoretical calculations then refer to one-electron transition in multi-ionized atoms [4]. 

As the energy of the incident positron is increased beyond several keV it becomes increasingly valid to consider the mechanism of electron capture into either bound or continuum states of positronium in terms of... 

To capture the essence of the Feshbach resonance phenomenon, we will need to understand what happens to the ground vibrational state 4>o(R) of the ground electronic state, also depicted in Figure 1.13, because of the interaction with the continuum of states excited electronic state. The physical process described above can be formulated as a two coupled channels problem where the solution irg(R) in the closed channel (the ground state) depends on the solution ire(R) in the open channel (the excited state) and vice-versa. The coupled Schrodinger equations read... 

Another class of systems for which the use of the continuum dielectric theory would be unable to capture an essential solvation mechanism are supercritical fluids. In these systems, an essential component of solvation is the local density enhancement [26,33,72], A change in the solute dipole on electronic excitation triggers a change in the extent of solvent clustering around the solute. The dynamics of the resulting density fluctuations is unlikely to be adequately modeled by using the dielectric permittivity as input in the case of dipolar supercritical fluids. 

Capture may take place to bound states of the negative ion that undergo radiationless transitions to repulsive states of the negative ion resulting in dissociative electron capture. These radiationless intramolecular transitions (Auger transitions) are a result of overlap of the discrete states with a continuum of states of AB-. These types of processes are illustrated for diatomic molecules in Figure 2.2b. Electrons are first captured into the bound state represented by curve 1, and before autodetachment can take place an intramolecular radiationless transition occurs to curve 2 resulting in dissociation into A and B-. 

In the laboratory capture process, any of the various electron shells contribute to the capture rate however the K-shell gives the dominant contribution. At temperatures inside the sun, e.g. 1, = 15, nuclei such as 7Be are largely ionized. The nuclei however are immersed in a sea of free electrons resulting from the ionization process and therefore electron capture from continuum states is possible (see e.g., [45], [46]). Since all factors in the capture of continuum electrons in the sun are approximately the same as those in the case of atomic electron capture, except for the respective electron densities, the 7Be lifetime in a star, ts is related to the terrestrial lifetime rt by ... 

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