ABSTRACT
In this chapter, we consider resonant collisions involving e.g. two hydrogenlike atomic systems
(ZP, e1)i1 + (ZT, e2)i2 −→ (ZP; e1, e2)∗∗f + ZT. (19.1) This is simultaneous transfer and excitation i.e. the TE process, where a doubly excited (auto-ionizing) state is produced in the projectile after capture of the target electron e2. Here, there are two mechanisms that interfere through the resonant and non-resonant modes i.e. the RTE and NTE modes, respectively. The RTE mode occurs via capture of the target electron and simultaneous excitation of the projectile electron by means of the interaction 1/r12 between the two electrons. The NTE mode appears when the target electron is transferred by its interaction with the projectile nucleus P of charge ZP, whereas the excitation of the helium-like projectile formed in the exit channel comes from the interaction of the projectile electron e1 with the target nucleus T of charge ZT [187]. Process (19.1) is the prototype of the simplest example of the TE collision. Of course, the TE process can also occur with more complicated colliding particles, involving e.g. a multi-electron target. An example is the first work on TE by Tanis et al. [469] who measured the total cross sections for the S13+ −Ar colliding system. The doubly excited state of the projectile (ZP; e1, e2)∗∗f in the exit channel
of process (19.1) relaxes either by radiative decay via X-ray emission (TEX) or through the Auger mechanism (TEA), thus providing two different and complementary experimental approaches for understanding the TE phenomenon [470, 471]. Hereafter, the resonant and non-resonant TEA are denoted by RTEA and NTEA, respectively. The first experimental evidence of the resonant TEX mode (RTEX) was reported by Tanis et al. [469]. A similar measurement on the TE process via the RTEX mode was subsequently made with the H2 target by Schulz et al. [472]. In the theoretical studies by Brandt [473] and Feagin et al. [474], the RTE and NTE modes were considered as independent. However, the basic features of the CDW-4B method could obviously provide a more adequate description of the TE by a natural introduction of the critical interference effects between the RTE and NTE modes [184, 185]. Furthermore, the CDW-4B method can be of help in interpreting the experiment of Justiniano et al. [471]. Here, a very asymmetric collisional system S15+−H2 was investigated, where the state
( S14+
)∗∗ formed in the exit channel decays via the radiative emission lines Kα −Kα and Kα −Kβ that are
ION-ATOM
dominated by the RTEX mode. The CDW-4B method has been found [184] to be in good agreement with the experimental data of Justiniano et al. [471], as well as with the results of Brandt [473]. Furthermore, it has been reported in Ref. [184] that the interference between the RTE and NTE modes can be important if the TE process occurs in a nearly symmetrical collisional system such as the He+−H or He+−He encounters. These two collisions involving the TE process were studied experimentally [470] and theoretically [185, 188]. The latter two theoretical studies used the CDW-4B method. Here, the TE process is observed experimentally through the TEA mode. Agreement between the experimental data obtained using the 0◦ electron spectroscopy technique and the theoretical cross sections computed by the CDW-4B method for the TEA mode is not satisfactory. For certain auto-ionizing states, the total cross sections from the CDW-4B method underestimate the corresponding experimental data of Itoh et al. [470]. This could be due to competition between the direct and indirect transfer excitation (ITE). The direct TE is the customary TE, which we have already defined. In the ITE, forward emitted electrons are generated through two intermediate channels (a) target ionization and (b) simultaneous capture of the target electron and projectile electron loss. The direct and indirect TE cannot be distinguished if the electron ejected from the target is not detected by a coincident measurement. Ourdane et al. [188] revisited this problem using the CDW-4B method, but this time with a more consistent description of the final auto-ionizing state, by including the adjacent continuum components in addition to the discrete ones. Specifically, the final state used in Ref. [188] is described within the atomic resonant structures from the formalism of Fano [475] as a a linear superposition of bound and continuum orbitals.
