E. Bodewits

Ion-polycyclic aromatic hydrocarbon collisions: kinetic energy releases for specific fragmentation channels

We report on 30 keV He2+ collisions with naphthalene (C10H8) molecules, which leads to very extensive fragmentation. To unravel such complex fragmentation patterns, we designed and constructed an experimental setup, which allows for the determination of the full momentum vector by measuring charged collision products in coincidence in a recoil ion momentum spectrometer type of detection scheme. The determination of fragment kinetic energies is found to be considerably more accurate than for the case of mere coincidence time-of-flight spectrometers. In fission reactions involving two cationic fragments, typically kinetic energy releases of 2-3 eV are observed. The results are interpreted by means of density functional theory calculations of the reverse barriers. It is concluded that naphthalene fragmentation by collisions with keV ions clearly is much more violent than the corresponding photofragmentation with energetic photons. The ion-induced naphthalene fragmentation provides a feedstock of various small hydrocarbonic species of different charge states and kinetic energy, which could influence several molecule formation processes in the cold interstellar medium and facilitates growth of small hydrocarbon species on pre-existing polycyclic aromatic hydrocarbons.

Multiple Ionization of Free Ubiquitin Molecular Ions in Extreme Ultraviolet Free-Electron Laser Pulses

The fragmentation of free tenfold protonated ubiquitin in intense 70 femtosecond pulses of 90 eV photons from the FLASH facility was investigated. Mass spectrometric investigation of the fragment cations produced after removal of many electrons revealed fragmentation predominantly into immonium ions and related ions, with yields increasing linearly with intensity. Ionization clearly triggers a localized molecular response that occurs before the excitation energy equilibrates. Consistent with this interpretation, the effect is almost unaffected by the charge state, as fragmentation of sixfold deprotonated ubiquitin leads to a very similar fragmentation pattern. Ubiquitin responds to EUV multiphoton ionization as an ensemble of small peptides.

Dynamics of ion guiding through nanocapillaries in insulating polymers

We review recent studies of dynamic properties concerning the ion guiding through nanocapillaries etched in polyethylene terephthalate (PET) and polycarbonate (PC). Typical lengths of the capillaries were 10 μm with diameters ranging from ~100 - 400 nm. The temporal evolution of the intensity and the angular distribution of the transmitted ions were studied by measuring transmission profiles as a function of the charge inserted into the capillaries. Tilt angles of the capillaries axis with respect to the incident beam direction were 3° and 5°. The mean emission angle of the transmission profiles exhibit pronounced oscillatory structures both for PET and PC. However, for PC nearly an order of magnitude more charge is required to induce the oscillations. In contrast to PET, with capillaries in PC we observed a strong decrease of the profile intensities with irradiation time. This observation provides evidence for blocking effects on the ion transmission. The experimental results are interpreted by simulations of the ion trajectories guided in 3 dimensions by the electrostatic field within the capillaries. This field was determined from the charge deposited at the walls of the capillaries taking into account the removal of the charges by means of a non-linear conductivity law.

Density effects on the blocking of ions guided through insulating PET capillaries

Guiding of 3-keV Ne7+ through untilted nanocapillaries in polyethylene terephthalate (PET) has been measured. The fraction of the transmitted ions is found to decrease with increasing charge insertion into the capillaries. This blocking effect is shown to be strongly dependent on the capillary density.

Role of charge patches in ion guiding through nanocapillaries in a PET polymer

We studied the dynamic properties of ion guiding through nanocapillaries in insulating polyethylene terephthalate (PET). The angular distribution of the transmitted ions was measured as a function of time. The temporal evolution of the angular transmission profiles was acquired for the capillary diameters of 200 and 400 nm. The tilt angle was varied from 0° to 6.5°. The transmission profiles exhibit significant changes in position as time varies. This observation is explained by the formation of temporary charge patches produced in the interior of the capillary besides the primary charge patch created in the entrance region.