ECC cusp analysis

Continuum-orientation phenomena in ionization by positron impact

“…The idea of a similar orientation effect occurring in ion-atom collisions seems to be at odds with the common knowledge that the first few terms of the expansion (4) could correctly describe the ECC asymmetry. However, some late experimental results by Pregliasco et al (1994aPregliasco et al ( , 1994b of ionization of He by the impact of 50, 100 and 200 keV H + and 100 keV amu −1 He 2+ seem to give support to the presence of a strong orientation of the electron-ion pair at the lower collision energy, similar to the one proposed here for positron impact. Actually, while at 200 keV the ECC cusp was well described by using only two ( = 0, 1) terms in the expansion (4), more than four terms were needed at 50 keV.…”

“…It was theoretical predicted by Salin (1969) and Macek (1970), and experimentally discovered by Crooks and Rudd (1970) four decades ago. Since then, extensive research in this area (Meckbach et al 1981, Shakeshaft and Spruch 1978, Garibotti and Miraglia 1980, Briggs and Macek 1991, Pregliasco et al 1994a, 1994b, Sarkadi et al 1998 has established this process as a postcollisional effect that reproduces the zero energy divergence of the Coulomb continuum electron-projectile final state, being its position in the electronic spectra solely fixed by energy and momentum conservation (Barrachina 1997. The same mechanism was shown to operate in positron impact ionization, even though the mere existence of such a cusp was initially doubted due to contradicting results emerging from several theoretical and experimental groups (Mandal et al 1986, Brauner and Briggs 1986, Brauner et al 1989, Schultz and Reinhold 1990, Bandyopadhyay et al 1994, Kövér et al 1994.…”

“…It is clear that the parametric expansion ( 4) is not well suited to represent such an extreme anisotropy. In figure 4 we show the different coefficients, as defined by Pregliasco et al (1994aPregliasco et al ( , 1994b,…”