New highly charged fullerene ions: Production and fragmentation by slow ion impact

Unraveling the Metastability of C n2+ ( n = 2-4) Clusters

Pure carbon clusters have received considerable attention for a long time. However, fundamental questions, such as what the smallest stable carbon cluster dication is, remain unclear. We investigated the stability and fragmentation behavior of C _n²⁺ ( n = 2-4) dications using state-of-the-art atom probe tomography. These small doubly charged carbon cluster ions were produced by laser-pulsed field evaporation from a tungsten carbide field emitter. Correlation analysis of the fragments detected in coincidence reveals that they only decay to C _n-1⁺ + C⁺. During C₂²⁺ → C⁺ + C⁺, significant kinetic energy release (∼5.75-7.8 eV) is evidenced. Through advanced experimental data processing combined with ab initio calculations and simulations, we show that the field-evaporated diatomic ¹²C₂²⁺ dications are either in weakly bound ³Π_u and ³Σ_g^- states, quickly dissociating under the intense electric field, or in a deeply bound electronic ⁵Σ_u^- state with lifetimes >180 ps.

Theoretical study of the stability of multiply charged C70 fullerenes

We have calculated the electronic energies and optimum geometries of C(70) (q+) and C(68) (q+) fullerenes (q=0-14) by means of density functional theory. The ionization energies for C(70) and C(68) fullerenes increase more or less linearly as functions of charge, consistent with the previously reported behavior for C(60) and C(58) [S. Diaz-Tendero et al., J. Chem. Phys. 123, 184306 (2005)]. The dissociation energies corresponding to the C(70) (q+)-->C(68) (q+)+C(2), C(70) (q+)-->C(68) ((q-1)+)+C(2) (+), C(70) (q+)-->C(68) ((q-2)+)+C(+)+C(+), C(70) (q+)-->C(68) ((q-3)+)+C(2+)+C(+), and C(70) (q+)-->C(68) ((q-4)+)+C(2+)+C(2+) decay channels show that C(70) (q+) (like C(60) (q+)) is thermodynamically unstable for q>or=6. However, the slope of the dissociation energy as a function of charge for a given decay channel is different from that of C(60) (q+) fullerenes. On the basis of these results, we predict q=17 to be the highest charge state for which a fission barrier exists for C(70) (q+).

Structure and electronic properties of highly charged C60 and C58 fullerenes

We present a theoretical study of the structure and electronic properties of positively charged C60(q+) and C58(q+) fullerenes (q = 0-14). Electronic energies and optimum geometries have been obtained using density-functional theory with the B3LYP functional for exchange and correlation. We have found that closed- and semiclosed-shell C60(q+) ions (q = 0, 5, and 10) preserve the original icosahedral symmetry of neutral C60. For other charges, significant distortions have been obtained. The C58(q+) fullerenes are, in general, less symmetric, being C58(8+) the closest to the spherical shape. Most C60(q+) fullerenes follow Hund's rule for spin multiplicity, while most C58(q+) fullerenes are more stable with the lowest spin multiplicity. The calculated ionization potentials for both kinds of fullerenes increase almost linearly with charge, except in the vicinity of C60(10+) and C58(8+). We have also explored the region of the potential-energy surface of C60(q+) that leads to asymmetric fission. Minima and transition states corresponding to the last steps of the fission process have been obtained. This has led us to conclude that, for 3 < or = q < or = 8, C2(+) emission is the preferred fragmentation channel, whereas, for higher q values, emission of two charged atomic fragments is more favorable. The corresponding fission barrier vanishes for q > 14.

Event-by-event analysis of collision-induced cluster-ion fragmentation: sequential monomer evaporation versus fission reactions

The most abundant decay channels have been studied quantitatively for high-energy (60 keV/amu) cluster ions H (3) (+)(H (2))(m = 1-14) colliding with He atoms employing a recently developed multicoincidence technique for the simultaneous detection of the correlated fragments on an event-by-event basis. This allows us to identify decay reactions and their underlying decay mechanisms responsible for the occurrence of the U-shaped fragmentation pattern.