Explanation of graphs and tables

All measured cross sections are presented in graphs as a function of collision energy of E_cm in the center-of-mass system so as to compare results of different isotope species. Their numerical data are listed in tables as functions of three energies, E_lab/q, E_cm=E_lab×m₂/(m₁+m₂) and E_amu=E_lab/m₁, where E_lab, q, m₁ and m₂ are energy in the laboratory system, charge state of the projectile ion and masses of the projectile ion and the target, respectively.

Statistical uncertainties of presented cross sections were less than ±10% and systematic errors for each parameter measurements were commonly estimated to be around ±5% for the collision length, ±5% for target pressure and less than ±5% for signal counting, respectively.

0 0.5 1

∆V=V_i-V_c (volt)

-2 0 2 4 6 8 0.5·q eV

Beam intensity FWHM Ar⁸⁺ ion beam

Fig.3. Intensity curve of Ar⁸⁺

beam extracted from the mini-EBIS as a function of the difference (∆V=V_i-V_c) between potentials at the ion source (V_i) and the collision cell (Vc).

10^-2 10^-1 10⁰ 10¹ 10² 10³ 10⁴

Thus, the gross uncertainties of measured cross sections can be commonly estimated to be ±25%

at the most.

3.1. Part A: Ion-molecule reactions in hydrogen systems

Ion-molecule reactions in hydrogen systems were investigated by the experimental setup equipped with a Nier type ion source. Complicated ion-molecule reactions in hydrogen systems are discriminated by using isotope species and their cross sections are shown in the part A of the session 4 together with attenuation cross sections of σ(att) and contributions of δ from secondary processes. These cross sections are unpublished data measured during 1985-1986. In the cross section measurements for hydrogen systems, attenuation cross sections are much larger than sum of cross sections of ion-molecule reactions. This means that a lot of very slow ions, which are produced in ion-molecule reactions, cause secondary processes and most of them are confined in the OPIG for long times. Also, it should be noted that target excitation processes followed with

Fig.5. Resolved cross sections related to the following processes for the A2+/B2 system.

Fig.4. Resolved cross sections related following processes for the A⁺/B₂ system.

A⁺+ B2 → AB + B⁺+ 0 eV

10^-2 10^-1 10⁰ 10¹ 10² 10³ 10⁴

large energy loss which increases the attenuation cross sections.

By comparison of presented data, following cross sections and related processes are resolved for each hydrogen systems of A⁺/B₂, A₂⁺/B₂ and A₃⁺/B₂ types as shown in Fig.4-6, where A and B are element of H or D. The σΣ indicates the difference between σ(att) and sum of measured reaction cross sections.

3.2. Part B: Charge transfer of low-charged rare gas ions produced by a Nier type ion source

In the part B, single-, double- and triple-electron capture cross sections of low-charged rare-gas ions in collisions with rare gas atoms and atmosphere molecules, studied using a Nier type ion source again, are presented in graphs and tables as a function of collision energy of E_cm in the center-of-mass system.

Studies for symmetric collision systems of Ar²⁺/Ar, Ar³⁺/Ar, Kr²⁺/Kr and Kr³⁺/Kr were reported in detail [1] [2]. At low energies where the collision is dominated by an attractive potential of an induced dipole, symmetric resonant charge transfer process of A^q+ + A→A + A^q+

becomes to be most dominant even for q=2 and 3 and their cross sections well agree with a half of the Langevin cross section, σ^L=2πq(α/2Ecm)^1/2, where α is polarizability of the target atom [5].

Generally, a conventional electron impact type ion source provides doubly charged rare gas ions including low-lying metastable ions in states of ¹D2 and ¹S0 added to the ground state of

3P1/2,3/2. Practically, σ²¹ and σ²⁰ measured in collision systems of Ar²⁺/He and Kr²⁺/He except for Ne²⁺/He have strongly depended on the electron impact energy. Accordingly, the electron impact energy E_e used for production of the projectile ions is shown in each graph. From the pressure dependence of product ions, partial cross sections were determined of σ21(¹D₂) and σ21(³P) in Ar²⁺/He system and of σ21(¹S₀) and σ21(¹D₂) in Kr²⁺/He. It is worth noting that a very

Fig.6. Resolved cross sections related to the following processes for the A3+/B2 system.

steep threshold structure of σ²¹ in Ne²⁺/He near Ecm≈16 eV is quite different from others. On the other hand, σ²¹ in Ne²⁺/H2, Ne²⁺/ N2 increases with decreasing collision energy and goes up along the Langevin cross section at the low energy end.

3.3. Part C: Charge transfer of multiply charged ions produced by a small type electron beam ion source (Mini-EBIS).

In the part C, single- and double-charge transfer cross sections of multiply charged ions in collisions with helium atom and hydrogen molecule, which are measured with the Mini-EBIS, are presented in graphs and tables. An ion beam extracted from a plasma based ECR-type ion source very frequently accompanies ions in metastable states which cause significant and dramatic effects on the observed cross sections. However, the DC mode operation of the Mini-EBIS is very effective to suppress contamination of excited ions with a long life in multiply charge ion beams. The Mini-EBIS is operated at very low pressure lower than 10^-10 torr. At such low densities, there is little probability of metastable production via electron capture collisions and, even if excited ions with a long life are produced, they will be quenched during long ion-confinement times in the DC mode operation. Practically, no existence of long-living excited ions has been found in beams of multiply charged ions extracted from the Mini-EBIS except for C²⁺ and O²⁺ beams. Some of presented data were already discussed in [6] for He²⁺/He and He²⁺/H₂, in [4] for C⁴⁺, N⁴⁺ and O⁴⁺/He, in [9] for C⁴⁺/H₂, in [10] for Ar^q+/H₂ (q=6-9,11), in [11] for Ar^q+/He (q=6-9,11) and I^q+/He (q=24-26), and in [13] for C^q+/He and H₂ (q=2-5). In these measurements, it should be noted that the transfer ionization processes have been ignored since product ions are identified by m/q’ for the final charge states of q’.

Although both of He and H₂ targets have two electrons, measured cross sections for He target are quite different from those for the H₂ target. Especially, single- and double-charge transfer cross sections of C^q+, N^q+ and O^q+ (q=2~6) with He strongly depend on the collision energy and the charge states of q in contrast to those with H2.

In charge transfer collisions of multiply charged ions with many electron targets, contribution of multi-electron transfer processes becomes significant and their cross sections have a characteristic minimum structure in their energy dependence around few eV. Their cross sections are not presented here. They are reported in [7] for He²⁺/CO, N2 and O2, and Kr^q+/Ne, N₂, O₂ and CO (q=7-9), in [8] for C⁴⁺/Ar and C_nH_2(n+1) (n=1∼4), in [9] for C⁴⁺/H₂, N₂ and O₂, in [12] for Ar^q+/Ne,

At low collision energies, the collision dynamics changes drastically giving rise to various new effects on the charge transfer reactions. The Coulomb electric field created by the ionic charge polarizes neutral targets and an ion-induced dipole leads to a mutual attraction between the collision partners. The induced dipole interaction can capture the ion in a spiral trajectory close to the collision center at sufficiently small impact parameters and the trajectory takes a

circular orbit around the target at an appropriate impact parameter [5]. At sufficiently low collision energies where the collision is dominated by the ion-induced dipole, the orbiting cross section is enlarged in inverse proportion to the collision velocity. In various thermal reactions of singly charged ions, almost all reaction rates are constant. This signature is well known as the orbiting effects due to the ion-induced dipole interaction.

As the ionic charge-state becomes large, it is expected that the enhancement of cross section due to the orbiting effects can be observed at energies far much higher than thermal energy. This expectation was my initial motivation to start cross section measurements with multiply charged ions at low energies. It has been successfully confirmed that almost all measured charge transfer cross sections tend to increases at the lower end in the energy range studied [10] [11]. Only for Ar⁶⁺-H2 collisions, reported data using Penning trap at near thermal energy are available [14]. The thermal energy data places just on an extrapolation line of presented single charge transfer cross sections converging to the Langevin cross section σ^L in the lower energy side [5]. This agreement newly gives rise to a fundamental interest how far the cross section goes up with decreasing collision energy. Furthermore, quantum mechanical calculation has predicted that the orbiting resonance leads to drastically increase the interaction time and it makes a sharp peak in the electron capture cross section [15] [16]. The experimental verification of the orbiting resonance is still one of future interests with highly charged ions at low energies.

In slow collisions of He²⁺and Kr⁸⁺ highly charged ions with H2, N2, O2 and CO, charge transfer processes followed by fragmentation have been perfectly resolved by a new triple coincidence technique for three particle detection using twin OPIG systems [17]. Some new phenomena relevant to collision dynamics and oriented fragmentation have been found at low energies. The OPIG technique is very useful to trap and transport slow charge particles.

Recently, Groningen’s group also has applied the OPIG in state-selective electron capture experiments with slow highly charged ions [18] [19].