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RIKEN Accel. Próg. Rep. 24 (1990)

IM-2-19. Multiply Charged lons and Cluster lons Produced from Frozen Targets under Energetic Heavy lon Impact

H. Tawara, T. Tonuma, H. Kumagai, T. Matsuo, and H. Shibata

In the present work we have measured positive as well as negative ions produced under the impact of a few picoampere 60 MeV Ar^ę+ (q = 4, 12, 13) ions provided from RIKEN linear acceler-ator on frozen gas targets. The target (molecu-lar) gases are frozen on thin foils cooled at the liquid nitrogen temperaturę. The gas pressure measured at a vacuum chamber wali is about (5-7) X10^-7 Torr, with a background pressure of 1X1Q-⁷ Torr.

Fig. 1. A typical positive ion spectrum obtained from frozen C0₂ target under 60 MeV Ar⁴⁺ ion impact.

A typical example of positive ion spectra from a frozen C0₂ target is shown in Fig. 1. In addition to singly as well as multiply charged atomie ions and molecular ions, a variety of cluster ions are observed at fairly high intensities. These cluster ions are clearly formed from different core ions produced in preceding collisions with energetic heavy ions. In the present case, the following singly charged cluster ions are dominant: [C0₂ (C0₂)*]⁺, [C0(C0₂)_n]+, [0₂(C0₂)*]⁺, [C (C0₂)*]⁺, and [0(C0₂) _n]⁺. Surprisingly the most intense peak is found to be due to 0₂⁺ ions, which have never been observed in gaseous targets, followed by the cluster ions with 0₂⁺ ion core. In the frozen C0₂ targets, the 0₂⁺ ion is formed probably through recombination of 0⁺ ion with another O atom, both being the dissociation Products in the frozen target, during migration to the surface. It is also noted that the intensities of these cluster ions decrease smoothly with a little fluctuation as the number of molecules clustered around the core, n, inereases, but the “magie number” indicating cluster ions with special sta-bilities could be hardly observed. This is in con-trast to cluster ion produced in expanding gas targets.⁰

A similar sharp difference is also observed in the production of multiply charged ions: relative-ly intense peaks corresponding to 0^I+ (z = 1-4) ions and even a tracę of 0⁵⁺ ions are seen in a spectrum, meanwhile only very weak C²⁺ and practically no C³⁺ ions are observed. On the other hand, in a previous gaseous target spectrum,²⁾ multiply charged carbon ions are morę intense by an order of magnitude than corresponding multiply charged oxygen ions.

A morę dramatic difference is seen in negative-ly charged cluster ion spectra. In the present frozen target experiment, most intense peaks correspond to O" and [0(C0₂)„]^_ cluster ions, whereas only a series of [C0₂(C0₂) cluster ions are observed in expanding gas targets.³⁾

Similar measurements are under way for other frozen gas targets. Presently, the detailed production mechanisms and stabilities of ions involving cluster ions from frozen targets are not well understood, though the stabilities of the ions seem to be strongly influenced by the density of targets depending on gas or frozen phases.

It is concluded that the frozen targets can provide a variety of cluster ions as well as atomie and molecular ions, sharply different from those produced in gaseous targets, some of which have been observed only in space but not in laboratories.

References

1)    O. Echt, K. Sattler, and E. Recknagel: Phys. Lełł., 90A, 185 (1982).

2)    T. Matsuo, T. Tonuma, M. Kasę, T. Kambara, H. Kumagai, and H. Tawara: Chem. Phys., 121, 93 (1988).

3)    M.L. Alexander, M.A. Johnson, N.E. Levinger, and W.C. Lineberger: Phys. Rev. Lett., 57, 976 (1986).