Direct Evidence for the Sequential Decay C60z+-->C58z+-->C56z+ -->

Ne2+[II (1/2)u]: radiative decay and electronic predissociation

Metastable fragmentation of neon dimer ions has been investigated by measuring and analyzing high resolution kinetic energy release distributions. Data were obtained by a modified MIKE (mass-analyzed ion kinetic energy) scan technique. Due to the high energy resolution it was possible to distinguish two different reaction mechanisms in the micros time regime which produce Ne+ ions with different kinetic energy distributions. Theoretical studies based on ab initio calculations of potential energy curves allowed the assignment of the reactions to specific electronic transitions in the excited Ne2+ ion. The unusual bimodal kinetic energy release distribution arises from competition between radiative and non radiative decay of the long-lived Ne2+ II(1/2)u state.

Sputtering of C60 Fullerenes Physisorbed on Ar, Xe, H2O, O2, and C8F18 Matrix Films Studied with Time-of-Flight Secondary Ion Mass Spectrometry

The ionization and fragmentation of C(60) fullerenes were investigated using matrix films covered with C(60) molecules and bombarded with 1.5-KeV He(+) ions. C(+), C(60)(+), and C(60)(++) ions were sputtered from the C(60) molecules that were physisorbed on Ar and Xe matrix films, whereas the sputtering of C(60) on the O(2) and C(8)F(18) matrix films induced an additional emission of ion adducts, such as (OC(60))(+) and (FC(60))(+), as well as the fragment ions, C(60-2n)(+) (n = 1-10). Very few ions were sputtered from the C(60) molecules that were adsorbed on the H(2)O matrix film and the Ni(111) substrate. The ions are thought to be created at the surface when C (C(60)) collides with the Ar, Xe, O, and F species via the electron-promotion mechanism, and the formation of quasi-molecules is manifested from the emission of the ion adducts. The fragmentation occurs during the interaction with the reactive species at the surface, and the delayed ionization/fragmentation of the internally excited C(60) molecules in the gas phase has negligible contribution in the present experiment. The matrix effect arises from the suppressed neutralization of the C(60)(+) ion because of the localization of a valence hole. The C(60)(+) ion undergoes neutralization on the H(2)O film because the hydrogen bond has some covalent character.

Modelling of C(2) addition route to the formation of C(60)

To understand the phenomenon of fullerene growth during its synthesis, an attempt is made to model a minimum energy growth route using a semi-empirical quantum mechanics code. C(2) addition leading to C(60) was modelled and three main routes, i.e. cyclic ring growth, pentagon and fullerene road, were studied. The growth starts with linear chains and, at n = 10, ring structures begins to dominate. The rings continue to grow and, at some point n>30, they transform into close-cage fullerenes and the growth is shown to progress by the fullerene road until C(60) is formed. The computer simulations predict a transition from a C(38) ring to fullerene. Other growth mechanisms could also occur in the energetic environment commonly encountered in fullerene synthesis, but our purpose was to identify a minimal energy route which is the most probable structure. Our results also indicate that, at n = 20, the corannulene structure is energetically more stable than the corresponding fullerene and graphene sheet, however a ring structure has lower energy among all the structures up to n≤40. Additionally, we have also proved that the fullerene road is energetically more favoured than the pentagon road. The overall growth leading to cage closure for n = 60 may not occur by a single route but by a combination of more than one route.

Quantifying electron transfer during hyperthermal scattering of C60+ from Au(111) and n-alkylthiol self-assembled monolayers

A tandem time-of-flight mass spectrometer with an intermediate surface was used to quantify electron transfer during glancing incidence scattering of hyperthermal C(60) (+) (E(coll)=250-500 eV, theta(in)=75 degrees ) from (i) self-assembled monolayers of n-alkylthiols on gold (of various chain lengths), (ii) partly fluorinated alkylthiols on gold, as well as (iii) clean gold surfaces. Self-assembled monolayers (SAMs) behave as insulating layers with their thicknesses determining the electron tunneling probability during collision. Correspondingly, a roughly exponential dependence of the neutralization probability on the chain length n was found. A pronounced dependence of the neutral yield on the primary beam kinetic energy indicates that dynamic SAM deformation and associated projectile penetration depth also play a role in determining electron transfer efficiency. Results are consistent with the molecular deformability of SAMs as determined with other experimental methods.

The anomalous shape of the cross section for the formation of SF3+ fragment ions produced by electron impact on SF6 revisited

The partial ionization cross section for the formation of SF(3) (+) fragment ions following electron impact on SF(6) is known to have a pronounced structure in the cross section curve slightly above 40 eV. We used the mass-analyzed ion kinetic energy (MIKE) scan technique to demonstrate the presence of a channel contributing to the SF(3) (+) partial ionization cross section that we attribute to the Coulomb explosion of doubly charged metastable SF(4) (2+) ions into two singly charged ions SF(3) (+) and F(+), with a threshold energy of about 45.5 eV. Thus the observed unusual shape of the SF(3) (+) partial ionization cross section is the result of two contributions, (i) the direct formation of SF(3) (+) fragment ions via dissociative ionization of SF(6) with a threshold energy of 22 eV and (ii) the Coulomb explosion of metastable SF(4) (2+) ions with a threshold energy of about 45.5 eV. A detailed analysis of the MIKE spectrum reveals an average kinetic energy release of about 5 eV in the Coulomb explosion of the SF(4) (2+) ions with evidence of a second channel corresponding to an average kinetic energy release of about 1.1 eV.