PHYSICAL REVIEW A 82, 012704 (2010) Absolute cross sections for electron loss, electron capture, and multiple ionization in collisions of C 3+ with noble gases A. C. F. Santos, 1 G. M. Sigaud, 2,* W. S. Melo, 3 M. M. Sant’Anna, 1 and E. C. Montenegro 1 1 Instituto de F´ısica, Universidade Federal do Rio de Janeiro, Caixa Postal 68528, Rio de Janeiro, RJ 21945-970, Brazil 2 Departamento de F´ısica, Pontif´ıcia Universidade Cat ´ olica do Rio de Janeiro, Caixa Postal 38071, Rio de Janeiro, RJ 22452-970, Brazil 3 Departamento de F´ısica, Universidade Federal de Juiz de Fora, Juiz de Fora, MG 36036-330, Brazil (Received 28 April 2010; published 14 July 2010) Absolute charge-state-correlated cross sections for projectile electron loss, electron capture, and target multiple ionization in collisions between C 3+ ions and noble gases have been measured for energies between 1.3 and 3.5 MeV. The data have been compared with other similar absolute cross sections existent in the literature for several projectiles. Calculations for the single-loss−multiple-ionization channel have been performed for the screening mode, using both an extended version of the classical-impulse free-collision model and the plane-wave Born approximation (PWBA), and for the antiscreening mode within the PWBA. The energy dependence of the average number of target active electrons which contribute to the antiscreening has been described by means of a simple function, which is “universal” for noble gases but, in principle, projectile dependent. A method has been developed to obtain the number of active target electrons for each subshell in the high-velocity regime, which presented physically reasonable results. Analyses of the dependences of the single-capture and transfer-ionization (SC and TI, respectively) processes on the projectile charge states showed that, for He, equally charged bare and dressed projectiles have very similar cross sections; the latter thus acting as structureless point charges. A behavior similar to that in the SC has been observed for the pure single ionization of He by projectiles with different charge states and of the other noble gases by singly charged projectiles. It has been shown that the q 2 dependence of the pure-single and total-ionization cross sections, predicted by first-order models, is only valid for high-collision velocities. For slower collisions, the electron capture process becomes more relevant and competes with the ionization channel, a feature which grows in importance as the projectile charge state increases. DOI: 10.1103/PhysRevA.82.012704 PACS number(s): 34.50.Fa, 52.20.Hv I. INTRODUCTION One of the most important processes in atomic collisions in the low- to intermediate-velocity regime is the multiple ionization of atoms by dressed ions due to the wide range of applications in many fields, such as plasma physics [1], accel- erator technology [2], planetary atmospheres [3], radiotherapy and radiation dosimetry [4], heavy-ion-beam materials mod- ification and analysis [5], and nuclear fusion [6,7]. From the point of view of fundamental physics, this process has intrinsic scientific interest because it involves a variety of complex competing processes which may occur simultaneously with the target ionization—such as electron loss and capture by the projectile—besides the direct-ionization channel, presenting theoretical and experimental challenges, many of them not yet overcome. In this velocity region, the time-dependent dynamics of many-electron systems can seldom be described by perturbative models, especially in the case when the projectile ion also carries electrons [8–10]. Multiple ionization is, in itself, a complex process to describe, due to the fact that there are several possible pathways through which it may occur. The simplest case, namely, that of double ionization by swift ions, for instance, can be quite well understood in terms of four mechanisms [11,12]: (i) a shake-off process, where one electron is ionized directly by the projectile, with a second electron being ejected by a rearrangement in the final state; (ii) the ionization of one electron which, on its way out of the target, knocks out the * gms@vdg.fis.puc-rio.br second electron; (iii) the direct ionization of an inner-shell electron by the projectile, followed by a postcollisional Auger- like ionization; and (iv) the ionization of two electrons by the direct interaction with the projectile in the same collision. The cross sections for the first three processes are essentially proportional to the single-ionization cross section, while those for the latter—which is the dominant process in the low- to intermediate-velocity region [13]—depend on the projectile velocity and charge state quite differently than the single- ionization ones. However, even in this case, the dependences of the cross sections on the projectile charge states and/or the influence of competitive collision channels can be quite strong, as has been shown for the ratio between double and single ionization of the He target with and without the simultaneous electron loss or capture by dressed C and/or O projectile ions, in the low- to intermediate-velocity regime [14–18]. When more than two target electrons are removed, the problem becomes increasingly more complex, due not only to the nonlinear increase in the possible ways to reach a given target charge state but also to the crescent role that other competitive collision channels may play in these processes. From the theoretical point of view, probably the most widely used methods to describe the multiple ionization process lie within the framework of the independent-particle model (IPM), where it is assumed that the target electrons are ionized independently of each other and the different ionization probabilities are combined to provide the total cross sections by means of the binomial and multinomial distributions [10,19–24]. This approach has been also used, for instance, to take into account postcollisional time-delayed ionization of the target with quite successful results [25–30]. 1050-2947/2010/82(1)/012704(18) 012704-1 ©2010 The American Physical Society