Transition from during- to post-collision multifragmentation in cluster surface impact

Abstract

Multifragmentation of C 60 − was studied by measuring the kinetic energies of outgoing C n − ͑n =2-12͒ fragments following C 60 − collisions with a gold surface at sub-keV impact energies. We observe the transition from a multifragmentation with common average energy for all fragments ͑"during-collision" event͒ to a one with a common average velocity for all fragments ͑"post-collision" event͒. The energy distributions for both multifragmentation modes were successfully modeled by treating the fragmenting cluster as a hot "gas" consisted of various C n fragments and moving with some center of mass velocity.Fragmentation modes of highly energized microscopic systems, ranging from large macromolecular ions to hot atomic nuclei, are of fundamental physical interest. Their importance is quite often related with the universal patterns revealed and shared by the measured distributions ͑mass, energy, etc.͒. 1 These patterns may be applicable for interpreting and predicting fragmentation processes of different objects over very large scales of size and energy. 2,3 Of special interest in this context are multiparticle breakup ͑multifragmen-tation͒ phenomena which were studied already for several molecular species but mainly in the complete impact disintegration limit ͑shattering͒, where the projectile is shattered into its smallest constituents. The so-called shattering transition 4-6 from the region of delayed evaporationlike emission of elementary subunits to complete disintegration was observed so far in weakly bound clusters such as ͑NH 3 ͒ n H + ͑n =4-40͒, 6 I 2 ͑CO 2 ͒ n − ͑n =1-50͒, 7 antimony clusters Sb n + ͑n =3-12͒, 8 and also covalent molecular ions such as Si͑Cd 3 + ͒, 9 peptide ions, 10 and fullerenes. 11-13 Fullerene molecules and C 60 specifically have recently turned into a model system for studying different types of fragmentation phenomena following extreme collisional and optical excitations. 14-16 Shattering-type fragmentation was reported by Beck et al. 11 for C 60 + ions impacting a graphite surface over the 200-1800 eV energy range based on the evolution of mass distributions alone. The common assumption in the literature is that surface-impact-induced multifragmentation event is completed when the fragments are still in close contact with the surface. This should result in uncorrelated velocities of the outgoing fragments. However, since the reported experimental characterization of these fragments is very limited, not much is known about the actual dynamics of the multifragmentation event. Recently we have shown that the sub-keV multifragmentation of C 60 − impacting a gold surface at near-grazing incidence is a fully correlated event, where all scattered C n − fragments have nearly the same velocity. 12,13 Here we show that by changing the scattering angle ⌿ by 90°, from near grazing ͑⌿ = 45°͒ to near normal ͑⌿ = 135°͒, we observe a dramatic transition from a common-velocity multifragmentation event to a commonenergy one ͑with corresponding kinetic energy losses of 60% and over 99%͒. Our measurem...

Kinetic energy losses of C60−2n+(n= 1--5) fragments in sub-keV impact of CFleischer1, Bekkerman2, Tsipinyuk3 et al. 2012Phys. Rev. BSelf Cite2000View full textAdd to dashboardCite

“…5,6 The multifragmentation/shattering processes leading to the formation of the small odd/even C ± m ions were also studied 5,12-17 along with detailed measurements and analysis of their KEDs. [13][14][15][16][17] It is important to emphasize that this paper is dealing only with the formation mechanisms of the larger, even-sized, C + 60−2n (n = 1-5) ions dominating the spectra below 300 eV impact energy. We are not concerned here with the more violent formation mechanism of the smaller C + m ions for which postcollision multifragmentation was reported for the 300-900 eV impact energy range.…”

“…We are not concerned here with the more violent formation mechanism of the smaller C + m ions for which postcollision multifragmentation was reported for the 300-900 eV impact energy range. 13,14 The mass-resolved KEDs of scattered C + 60 ions and fragment C + 60−2n (n = 1-5) ions following C − 60 impact on a gold target at primary energies E 0 = 200, 250, and 275 eV are presented in Fig. 2.…”

“…A more elaborate description can be found in earlier publications. 13,17 C − 60 anions were collided at kinetic energies of 200-275 eV with an atomically clean polycrystalline gold surface maintained under ultrahigh vacuum conditions. The C − 60 ions were generated using a two-stage all-ceramic differentially heated source, by passing fullerene vapors (generated in the first oven stage) through a high-temperature ceramic capillary nozzle combined with a soft (thermal) electron pickup.…”

“…Up to a certain impact energy and vibrational excitation (typically well below the cohesion energy) the energized molecule is assumed to recoil intact and, following fast intramolecular vibrational energy redistribution, undergo unimolecular (metastable) delayed emission of its subunits. Over the last 20 years, the C 60 molecule emerged as a leading model system for exploring general collisional fragmentation phenomena of a large complex species with many degrees of freedom 1 and specifically unimolecular decay 2-12 and multifragmentation 5,10,[12][13][14][15][16][17] following surface impact-induced deformation and vibrational excitation. This situation is mainly related with the high level of characterization of the molecule and the distinct massspectral fingerprint associated with the sequential emission of C 2 units (or single-step emissions of even-numbered C 2n fragments) resulting in large, even only, C 60−2n (n = 1, 2, 3,. .…”

Effect of target surface on the elastic properties of fast fullerenes

“…In the case of a stiff/soft molecule, a small/large amount of the energy is transferred to internal degrees of freedom and the outgoing kinetic energy is large/small. During the last two decades, a fair number of studies have been devoted to elastic properties, stability, fragmentation, and charge transfer during scattering of hyperthermal fullerenes from surfaces [15][16][17][18][19][20][21][22]. However, for angles of incidence Φ in ≥ 10…”