No abstract

Experimental results are presented for ionization and fragmentation of C60 bombarded by various projectile ions of H+, Li1,2+, B1,2+, O2+, Si1,2+, Ag2+ and Au3+ with velocities 0.9v0-3v0 (v0: Bohr velocity). Mass distributions of light fragment ions Cn+ (n = 1-12) are found to change drastically depending on the projectile species or atomic number Zp. The result indicates that multifragmentation of C60 takes place at small impact parameters where the projectile nuclear charge is less screened. It is found that for heavy projectiles multiple ionization is apt to occur and lighter fragment ions are produced predominantly. Assuming the multifragmentation caused by penetration of the projectile through C60, energy deposition to C60 is calculated, and internal energy and ionization degree of C60 after collisions are estimated. It is concluded that the final fragment pattern is governed by the internal energy.

Absolute cross sections have been measured for the first time for H + -impact ionization and fragmentation of C 60 at proton energies ranging from 0.2 to 2.0 MeV. The cross sections were in fairly good agreement with other electron-impact data in the high-velocity region, indicating that the charge transfer is negligible in the present energy range. Theoretical cross sections for plasmon excitations were also calculated and compared with the experimental ionization cross sections. Agreement between them was within a factor of two, indicating that the plasmon excitation contributes to the total ionization by about 50%. A careful analysis of the energy dependence of the cross sections for C q+ 60 and C (q−1)+ 58,56 ions indicates that these ions are produced from the same initial excited states C * 60 created in collisions with H + ions.

By means of a time-of-flight technique, we have measured cross sections for production of C 1-3+ 60 and C 2+ 58,56 ions from a gas-phase C 60 target bombarded by 0.4-5.0 keV electrons. The results were in fairly good agreement with other data at overlapping energies below 1 keV. Semiclassical calculations of the single-ionization cross sections were also performed up to 10 keV using the Deutsch-Märk formula proposed for the C 60 molecule. A fairly good agreement between the experimental and theoretical cross sections was obtained both in magnitude and in energy dependence. It was found that the double and triple ionization cross sections both decrease monotonically and exhibit no hump structure, indicating that the inner-shell ionization does not play an important role in electron-impact multiple ionization of C 60 .

We have measured positive fragment ions produced in collisions of 2 MeV Si qϩ (qϭ0, 1, 2, 4͒ projectiles with a C 60 molecular target. The measurement was performed with a time-of-flight coincidence method between fragment ions and charge-selected outgoing projectiles. For all the charge-changing collisions investigated here, the mass distribution of small fragment ions C n ϩ (nϭ1-12) can be approximated fairly well by a power-law form of n Ϫ as a function of the cluster size n. The power derived from each mass distribution is found to change strongly according to different charge-changing collisions. As a remarkable experimental finding, the values of (loss) in electron loss collisions are almost the same for the same final charge states k irrespective of the initial charge q, exhibiting a nearly perfect linear relationship with k. We also performed calculations of the projectile ionization on the basis of the semiclassical approximation and obtained inelastic energy deposition for individual collision processes. The estimated energy deposition is found to have a simple correlation with the experimentally determined values of (loss).