Mixed-State Ionic Beams: An Effective Tool for Collision Dynamics Investigations

Abstract

The use of mixed-state ionic beams in collision dynamics investigations is examined. Using high resolution Auger projectile spectroscopy involving He-like ($1s^2\thinspace^1\!S, 1s2s\thinspace^{3,1}\!S$) mixed-state beams, the spectrum contributions of the $1s2s\thinspace^3\!S$ metastable beam component is effectively separated and clearly identified. This is performed with a technique that exploits two independent spectrum measurements under the same collision conditions, but with ions having quite different metastable fractions, judiciously selected by varying the ion beam charge-stripping conditions. Details of the technique are presented together with characteristic examples. In collisions of 4 MeV B$^{3+}$ with H$_2$ targets, the Auger electron spectrum of the separated $1s2s\thinspace^{3}S$ boron beam component allows for a detailed analysis of the formation of the $1s2s(^3\!S)nl \thinspace^2\!L$ states by direct $nl$ transfer. In addition, the production of hollow $2s2p\thinspace^{1,3}\!P$ doubly- and $2s2p^2\thinspace^2\!D$ triply-excited states, by direct excitation and transfer-excitation processes, respectively, can also be independently studied. In similar mixed-state beam collisions of 15 MeV C$^{4+}$ with H$_2$, He, Ne and Ar targets, the contributions of the $1s^2$, $1s2s\thinspace^{3,1}S$ beam components to the formation of the $2s2p\thinspace^{3,1}\!P$ states by double-excitation, $1s\rightarrow2p$ excitation and transfer-loss processes can be clearly identified, facilitating comparisons with theoretical calculations.

Pauli Shielding and Breakdown of Spin Statistics in Multielectron Multi-Open-Shell Dynamical Atomic Systems

“…which can be readily investigated experimentally since the initial, 1s2s 3 S, ionic core is metastable and therefore naturally found mixed-in with the ground state ions as provided by accelerators [13][14][15]. Then, the ratio R of the 1s2s2p 4 P/ 2 P ± SEC cross sections…”

“…Beam intensities on target ranged from 0.1 − 20 nA depending on incident energy and stripping conditions, while target gas pressures ranging from 5 − 40 mTorr were chosen to ensure single collision conditions. Using different electron stripper combinations [15,28], beams of C 4+ (1s 2 , 1s2s 3 S) mixed-state ions were prepared with different amounts of metastable 1s2s component so that the ratio R could be accurately determined by applying our two-spectra technique [25]. .…”

Investigation of electron capture in swift C4+ (1s2s 3S) collisions with gas targets using a zero-degree Auger projectile spectroscopy apparatus built with the L45 beam line at the ‘Demokritos’ 5.5MV tandem accelerator

“…This pre-excited beam component can be an invaluable asset, particularly for Auger projectile spectroscopy, since it can lead to the formation of doubly-excited projectile states through direct, single step collision processes such as SEC. Such additional pre-excited, metastable beam components allow for the investigation of ion-atom collision processes where projectile ions now have an initial K-shell vacancy [58]. Benis et al [59,60] were the first to observe that gas (rather than foil) stripping in the tandem accelerator terminal led to a much lower 1s2s 3 S content beam, thus providing with the capability to vary this metastable content.…”

“…for better control of the 1s2s 3 S content mostly when sufficient currents can be achieved with the already existing Foil Terminal Stripper (FTS) [58]. In cases when the required charged state ion beam delivered from the terminal is too weak (usually at the lowest collision energies required), an additional stripping point after the analyzing magnet is used known as the post-stripper.…”

“…The metastable components are expected to be produced statistically, in a ratio of 1:3. This means that 25% of the stripped 1s2s ions will result in 1s2s 1 S, whereas 75% will result in 1s2s 3 S. In the case of carbon, lifetimes for these states are τ 1s2s 3 S = 2.1 × 10 −2 s and τ 1s2s 1 S = 3.0 × 10 −6 s [58]. As stated earlier, there are two stripping locations for the ion beam, the terminal (≈26.35±0.5 m from target), and the post-stripper (≈12.1±0.5 m from target).…”