When a slow multiply charged projectile ion moving at a velocity below that of typical outer-shell atomic electrons collides with a many-electron neutral target, a large number of electrons can become active during the collision. Such collisions are dominated by the transfer of a number of target electrons to the projectile, resulting in the formation of projectile multiply excited states. Substantial progress has been made toward understanding one- and two-electron processes in slow ion-atom collisions during the past three decades ~see, e.g., Ref. @1#, and references therein!. Although slow collisions involving more than two active electrons have also been investigated for over two decades as well, our understanding of multielectron processes is by no means comparable to that of one- and two-electron processes. The vast majority of experimental studies of multielectron processes involved measurements of cross sections for projectile charge-change and recoil-ion production, both in a singles’ and in coincidence modes, and a limited number involved energy gain and visible photon spectroscopy ~see, e.g., @2‐5#, and references therein!. Moreover, since autoionization is a main decay mode of multiply excited states, Augerelectron spectroscopy has been employed in a singles’ mode @6,7# to study such collisions. While such measurements have played a significant role in understanding two-electron processes ~see, e.g., Ref. @8#, and references therein!, the situation is drastically different when many electrons are involved. For example, Benoit-Cattin et al. @6# obtained a singles’ electron spectrum for the 70-keV N 71 1Ar collision system. The analysis of the spectrum was rather difficult since it contained contributions from doubly, triply, quadruply, and quintuply excited states that rendered the interpretation nontrivial. During the last six years, however, Morgenstern and co-workers have made significant contributions @9‐12# toward understanding Auger-electron spectra obtained in multiple-electron capture processes by means of the coincident detection of Auger electrons and target ions. They obtained partial Auger spectra corresponding to the different target ion charge states that are much more informative than singles’ spectra. These spectra can provide further information if the final projectile charge state is also determined. From a theoretical point of view, understanding multielectron processes is a twofold problem. First, the different mechanisms involved in the collision process that lead to the production of the multiply excited states must be recognized and described. Second, the radiative and nonradiative properties of the resulting multiply excited states must be known. Concerning the first problem, quantum-mechanical or semiclassical treatment of collisions involving more than two electrons is prohibitively difficult due to the large number of channels involved. Therefore, extended classical overbarrier ~ECB! models have been developed @13,14# to account for multiple-electron capture processes. These models are limited to giving the capture state distribution on the projectile and possible simultaneous target excitation. There has been, until recently @15‐17#, a severe lack of theoretical work on the radiative and nonradiative properties of multiply excited states, due in part to the extremely large number of states that need to be taken into account, and in part to the lack of experimental data to which the calculations can be directly compared. Therefore, relaxation schemes @6,12# based on simple arguments, such as autoionization to the nearest continuum limits and minimum electron rearrangement ~twoelectron transitions!, have been invoked. This Rapid Communication reports triple-coincidence measurements of Auger electrons, scattered projectile, and target recoil ions in slow multiply charged ion-atom collisions. The measurements provide insights into the relaxation pathways of multiply excited states populated in such collisions.
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Nov 21, 2019
Rosa B Hughes-Currie + 4
Open Access
