Dominant screening process in the projectile electron loss for F- + Ar collisions

Brazilian Journal of Physics, vol. 36, no. 2B, June, 2006 518 Dominant Screening Process in the Projectile Electron Loss for F− + Ar Collisions M. M. Sant’Anna Instituto de Fı́sica, Universidade Federal do Rio de Janeiro, Cx. Postal 68528, Rio de Janeiro 21941-972, Brazil Received on 29 July, 2005 A comparison between projectile electron loss cross sections for negative, F− , and positive, He+ , projectiles is presented for collisions with Ar target. The behavior of the two collision systems is similar for the projectile electron loss with target ionization. For projectile electron loss without target ionization (the so-called screening electron-loss process), quite different situations are presented for the studied positive and negative projectiles. For He+ + Ar, the loss without target ionization collision channel is negligible for intermediate-to-low energies. On the other hand, for F− + Ar, this collision channel is the dominant one in the total projectile electron loss at intermediate-to-low velocities. The roles played by coupling with the electron capture by the projectile collision channel and by the very different binding energies for negative and positive projectiles are discussed. Keywords: Anion; Negative ions; Collision; Projectile electron loss I. INTRODUCTION Collisions between many-electron ionic projectiles and many-electron targets often occur in nature. Cross sections for these collisions are important parameters in the modeling of technological applications. However, a rigorous theoretical description of the multiple-ionization collision channels is a difficult task [1]. The experimental cross section data available are still scarce especially for anionic projectiles [2, 3]. Regarding projectile electron-loss the identification of two dynamically different collision processes, often called screening and antiscreening processes [4, 5], sheds light on the problem of the physical description of the collisions. The beam-attenuation experimental technique allows the determination of the total projectile destruction cross section [6–8]. This cross section corresponds to the sum of single and all multiple projectile-electron-loss collision channels, regardless of the target final charge state. The collision channel for which the projectile loses one or more electrons and the target remains in the ground state is therefore included in those measurements. This latter collision channel is often called screening projectile electron loss (also called projectile elastic loss), since the field of the target nucleus, screened by their electrons, ionizes the projectile with no target excitation or ionization [4, 5]. Projectile electron loss also takes place with markedly different dynamics, in the so-called antiscreening process (sometimes named two-center electron-electron correlation process). Here the projectile-electron-target-electron interaction is responsible for the projectile electron loss. The target electron is the ionizing agent of the projectile and, due to the energy and momentum transfer to the projectile, has a high probability of being ionized simultaneously with the projectile electron. Thus, coincidence measurements for projectile and target final charge states can at least partially separate experimentally the screening and antiscreening processes specifying the cross sections σ p,q for the projectile (p) and target (q), final charge states [9, 10]. For positive ions there are at least two factors that complicate this experimental approach to the problem. (i) The screening projectile electron-loss accompanied by the symmetrical process in the projectile frame of reference (namely the target direct ionization) produces the same final charge states as the antiscreening does. Experimental techniques like COLTRIMS can separate these collision channels (e.g. [11, 12]) but they will be undistinguishable in integrated cross sections obtained only by final charge-state coincidence measurements. The He+ + He and C3+ + Ne are examples of collision systems for which the simultaneous screening ionization of both target and projectile masks the antiscreening contribution to projectile electron loss. For He+ + He this is an important effect in the intermediate-to-low velocity range [9, 10]. For C3+ + Ne the effect is even stronger and the antiscreening contribution becomes negligible for low velocities [1]. (ii) The antiscreening process has an energy threshold similar to the one found in electron impact ionization [4, 5, 13]. The screening process is therefore, in principle, prominent below the antiscreening threshold. However, for low collision velocities electron capture by the projectile is very probable for positive ions and there is a strong coupling between the collision channels [1, 14, 15]. Anion projectiles offer a vast field to study the different dynamics of screening and antiscreening processes. This paper analyzes the F− + Ar collision system in the intermediate velocity range, from 0.3 to 1.5 atomic units. For this collision system the screening contribution dominates the total projectile electron loss cross sections for intermediate-to-low velocities, in opposition to the case of positive projectiles illustrated by the He+ + Ar collision system. Possible effects of the absence of electron capture collision channel and of high asymmetry in projectile and target binding energies are discussed. Details on the experimental determination of F− + Ar cross sections plus a comparison between F− and other anionic projectiles, regarding projectile electron loss, will be presented in future work [16]. Brazilian Journal of Physics, vol. 36, no. 2B, June, 2006 II. 519 ANTISCREENING: NEGATIVE VERSUS POSITIVE PROJECTILES Figure 1 compares the projectile electron loss with target ionization for F− + Ar [16] and He+ + Ar [17, 18] collision systems. Cross sections are shown as a function of the projectile velocity divided by (1) SCREENING: NEGATIVE VERSUS POSITIVE PROJECTILES 1000 σ (10 Figure 2 shows projectile charge-changing cross sections for He+ + Ar collisions. The projectile electron loss without target ionization (circles), which contains the screening contribution, is small at low velocities. Actually, DuBois [14] estimated this contribution to be zero within the experimental errors of his measurements. An estimate for the upper bound of these uncertainties is represented by the dashed line in Fig. 2. DuBois made his estimate by subtracting from the total electron loss cross sections the partial cross sections for channels with charged final states. Thus corresponding uncertainty was obtained combining in quadrature estimated experimental errors in total electron loss cross sections and in antiscreening electron-loss cross sections.The screening contribution increases with velocity and is of the same order of magnitude of the antiscreening (squares) for the higher velocities represented in Fig. 2. Electron capture by the projectile (tri (...truncated)