Yv. Popov et C. Dalcappello, THEORETICAL DEVELOPMENTS IN (E,2E) STUDIES OF EXCITED-STATES AND IN (E,3E) SPECTROSCOPY, Canadian journal of physics, 74(11-12), 1996, pp. 843-849
The theory of single and double ionization of atoms deals with one of
the most difficult problems in quantum mechanics: the scattering of a
few charged particles. A large number of different (e,2e) experiments
and theoretical calculations have helped us to understand the main phy
sical mechanisms and their effect on the shape of triple differential
cross section (TDCS). Recently the first deeply asymmetric (e,2e) expe
riments, leaving the residual ion in an excited state (which we indica
te in this paper by (e,2e)), and (e,3e) experiments have been perform
ed. These offer new challenges to the theory. A very preliminary surve
y of main theoretical methods currently used to explain the experiment
al measurements is presented here. It will be shown that small differe
nces in the choice of initial and final state models employed by diffe
rent authors lead to large effects in both the shape and absolute size
of the TDCS in the case of excitation ionization, even if these model
s give almost identical results for the (e,2e) case. A few physical me
chanisms contributing to the (e,2e) process are discussed in this pap
er. Special attention is given to the multichannel close-coupling meth
od. (e,3e) experiments allow us to study the final state wave function
with two continuum electrons. We obtain two unexpected results. First
, we found that the two-step mechanism contribution is comparable and
even bigger than that of shake-off. Second, the algorithms exploiting
the angular decompositions of many-body continuum wave functions do no
t work in the case of long-range potentials; this is a result of the f
ailure of the widely used diagonalization approximations in this case.
The physical considerations that support these and other results are
presented in this paper.