Fabian Seitz

Ions colliding with clusters of fullerenes - Decay pathways and covalent bond formations

We report experimental results for the ionization and fragmentation of weakly bound van der Waals clusters of n C60 molecules following collisions with Ar(2+), He(2+), and Xe(20+) at laboratory kinetic energies of 13 keV, 22.5 keV, and 300 keV, respectively. Intact singly charged C60 monomers are the dominant reaction products in all three cases and this is accounted for by means of Monte Carlo calculations of energy transfer processes and a simple Arrhenius-type [C60]n(+) → C60(+)+(n-1)C60 evaporation model. Excitation energies in the range of only ~0.7 eV per C60 molecule in a [C60]13(+) cluster are sufficient for complete evaporation and such low energies correspond to ion trajectories far outside the clusters. Still we observe singly and even doubly charged intact cluster ions which stem from even more distant collisions. For penetrating collisions the clusters become multiply charged and some of the individual molecules may be promptly fragmented in direct knock-out processes leading to efficient formations of new covalent systems. For Ar(2+) and He(2+) collisions, we observe very efficient C119(+) and C118(+) formation and molecular dynamics simulations suggest that they are covalent dumb-bell systems due to bonding between C59(+) or C58(+) and C60 during cluster fragmentation. In the Ar(2+) case, it is possible to form even smaller C120-2m(+) molecules (m = 2-7), while no molecular fusion reactions are observed for the present Xe(20+) collisions.

Polycyclic aromatic hydrocarbon-isomer fragmentation pathways: Case study for pyrene and fluoranthene molecules and clusters

We report on measurements of the ionization and fragmentation of polycyclic aromatic hydrocarbon (PAH) targets in Xe(20+) + C(16)H(10) and Xe(20+) + [C(16)H(10)](k) collisions and compare results for the two C(16)H(10) isomers: pyrene and fluoranthene. For both types of targets, i.e., for single PAH molecules isolated in vacuum or for isomerically pure clusters of one of the molecules, the resulting fragment spectra are surprisingly similar. However, we do observe weak but significant isomer effects. Although these are manifested in very different ways for the monomer and cluster targets, they both have at their roots small differences (<2.5 eV) between the total binding energies of neutral, and singly and multiply charged pyrene and fluoranthene monomers. The results will be discussed in view of the density functional theory calculations of ionization and dissociation energies for fluoranthene and pyrene. A simple classical over-the-barrier model is used to estimate cross sections for single- and multiple-electron transfer between PAHs and ions. Calculated single and multiple ionization energies, and the corresponding model PAH ionization cross sections, are given.

First results from the Double ElectroStatic Ion-Ring ExpEriment, DESIREE

We have stored the first beams in one of the rings of the double electrostatic ion-storage ring, DESIREE at cryogenic and at room temperature conditions. At cryogenic operations the following parameters are found. Temperature; T= 13K, pressure; p 10(7) and storage lifetime of a C-2(-) beam; tau = 450 S.

Bond formation in C+59 – C60 collisions

In this work, we show that keV-ions are able to remove single carbon atoms from individual fullerenes in clusters of C60 molecules. This very efficiently leads to the formation of exotic C+119 dumbbell molecules through secondary C+59 + C60 collisions within the fragmenting cluster. Such molecular fusion processes are inherently different from those induced by photons where only products with even numbers of carbon atoms are observed. Thus, ion collisions ignite unique and hitherto overlooked secondary reactions in small aggregates of matter. This relates to the question on how complex molecules may form in e.g. space.

The DESIREE project – a status report

We are in the process of constructing the double electrostatic storage ring DESIREE at Stockholm University and AlbaNova University Centre. DESIREE with its two 8.8 meters circumference ion storage rings with a common section for tuning the collision energy down to 10 meV will be operated at cryogenic temperatures. The two separate ion source platforms, with maximum floating potentials of 100 keV and 25 keV, respectively, will be equipped with different types of ion sources, e.g. electrospray sources, and equipment for cryogenic cooling of ions before injection in DESIREE. In this poster presentation we will give a status report of the project.

Interaction of multiply charged ions with isolated polycyclic aromatic hydrocarbon molecules

The interaction of multiply charged ions (He2+, O3+, and Xe20+, v ≤ 0.6 a.u.) with gas phase pericondensed polycyclic aromatic hydrocarbon (PAH) molecules (pyrene and coronene) was studied. Positively charged ionic products were analyzed with a linear time-of-flight mass spectrometer.

Molecular isomer effects in ionization and fragmentation of PAH monomers and clusters: pyrene and fluoranthene

We have investigated collisions between Xe20+ ions and the isolated polycyclic aromatic hydrocarbon (PAH) C16H10 isomers pyrene and fluoranthene or pure clusters of either pyrene or fluoranthene. We show fragment distributions and intensities for both cases and rationalize the results by means of a classical over-the-barrier (COB) model in conjunction with quantum chemistry calculations of vertical ionization energies of pyrene and fluoranthene monomers. We find that comparatively small differences in the first and second ionization energies for these isomers have significant effects on both the monomer and cluster fragment distributions.

The Double ElectroStatic Ion-Ring ExpEriment, DESIREE

We describe the status of the DESIREE facility, which is an electrostatic ion-storage device consisting of two storage rings with a common section, where ion beams of opposite charges stored in the two rings are merged. The electrostatic storage rings are mounted in a single common vacuum vessel, which is isolated inside an outer vacuum chamber and cooled by means of four cryogenerators. This unique system will allow for studies of interaction among ions of opposite charges at well controlled conditions both in terms of internal degrees of freedom and relative velocity. An advanced ion-beam production system for intense pulses of cold complex molecular ions is also presented.