Saturation effect in the excitation of heliumlike Si projectiles in the intermediate velocity range

Abstract

The saturation effect in the excitation cross sections has been investigated for Si 12ϩ ions colliding with gas target atoms ͑atomic number Z t between 2 and 54͒ in the ͑8-12͒-a.u. velocity range, both experimentally and theoretically. The experimental excitation cross sections have been deduced from the measured Ly x rays following the decay of the excited atoms. Theoretical calculations have been performed using the first Born approximation, the symmetric eikonal continuum distorted wave ͑SECDW͒ approximation, and the Schwinger variational principle ͑SVP͒. Similar to the earlier observations on the heavier ion species, the saturation has been found in both the experimental and the SVP cross sections in the entire velocity range. The SECDW approximation, however, has been found to underestimate the cross sections at large values of Z t in the presently investigated systems. In the light of these findings, the role of higher-order effects within different theoretical models has been reviewed and the possible interaction mechanisms have been discussed.

Effect of Collective Response on Electron Capture and Excitation in Collisions of Highly Charged Ions with Fullerenes

“…This observation is found to be independent of the x-ray emission angles and is in agreement with the results at very high energy collisions (v p 36 a:u:) with thin solid foils [19]. In the case of S 13 ions, the K x-rays predominantly arise due to K-shell excitation to higher shells (see [30] for other second order processes) and in such cases the Ly ratios are shown in Fig. 3(b).…”

Saturation effect in projectile- and target- x-ray production in collisions between 2.5 MeV/u highly charged Cu ions and gaseous targets

“…The first-Born approximation (FBA) predicted Z 2 t dependence and failed to explain the observed saturation effect in the excitation cross section. The Schwinger variational principle gave an impressive agreement with the data at high-as well as low-energy experiments [4][5][6][7]. The symmetric eikonal continuum distorted wave (SECDW) calculation [8] is only high-energy approximation and breaks down at low velocity [6,7].…”

“…The Schwinger variational principle gave an impressive agreement with the data at high-as well as low-energy experiments [4][5][6][7]. The symmetric eikonal continuum distorted wave (SECDW) calculation [8] is only high-energy approximation and breaks down at low velocity [6,7]. Sigaud et al [9] have observed the saturation in the projectile electron loss cross sections for He + ions as a function of target atomic number.…”

“…The multiple processes, which also contribute to the excitation cross section, have also been separately identified. Tiwari et al [6,7] studied the projectile energy dependence for the K-shell excitation cross section of 45-110 MeV Si 12+ and 120 MeV S 14+ ions with velocities between 8 and 12.5 au in collisions with He, N 2 , Ne, Ar and Xe targets. A striking feature of all these measurements is the observed saturation in the excitation cross section as target atomic number Z t increases.…”

Experimental study of K-shell ionization of low-Zsolids in collisions with intermediate-velocity carbon ions and the local plasma approximation

“…For instance, LPA calculations for K-shell ionization of Al and Cu by proton impact reproduce the experimental data even for energies around 100 keV [34]. This relation between symmetry and impact velocities can be expressed in terms of the generalized perturbation strength S p = Z p /(v p Z t ) that was proposed by Tiwari et al [42]. To compare the deviations of the LPA predictions (σ LPA KI ) from the experimental data (σ EXP KI ), we plot the ratio, σ LPA KI /σ EXP KI , as a function of S p at 4 MeV u −1 (see figure 5).…”