G. Brinkmalm

Formation of fullerenes in MeV ion track plasmas

Abstract MeV primary 127 I 14+ ions from the Uppsala EN-tandem accelerator were used to bombard a polymer — poly (vinylidene difluoride). Positive ions of even numbered carbon clusters (C + n , n =40−120) are ejected as a result of the interaction of fast MeV ions with the organic solid. The distribution of cluster sizes suggests that stable, closed carbon-cage structures — fullerenes — are formed. Initial radial velocity distributions of the desorbed carbon species are consistent with preferential ejection backward into the direction of the incoming MeV ions. Correlation between the directions of incoming particles and ejected clusters suggests that the latter (including C + 50 and C + 60 ) originate in the infratrack plasma produced by the fast primary ions interaction with the sample. A main conclusion resulting from this study is that carbon cluster ions form as a result of a single primary ion impact and that they are ejected from an axially expanding infratrack plasma region.

HEAVY-ION-INDUCED SPUTTERING AND CRATERING OF BIOMOLECULAR SURFACES

We measured the electronic-stopping-power dependences of mega-electronvolt atomic-ion-induced sputtering and cratering of bio-organic targets. We employed a collector method to study relative total ...

Damaging of C60 films by MeV heavy ions

Abstract Thin films of C60 (99.99% purity) have been irradiated with 55 MeV 127I10+ ions in a fluence range from 2.5 × 109 to 3 × 1012 ions/cm2. Two methods have been employed to assess the modifications induced by the MeV ion irradiation: in situ plasma desorption mass spectrometry analysis and off-line micro-Raman spectroscopy. The yields of secondary low mass carbon cluster ions and intact C60 ions have been determined at different fluences. Damage cross-sections for the C60 molecules have been extracted from the ion yield curves as a function of MeV ion fluence. The secondary ion spectra of the C60 targets also show peaks corresponding to larger fullerenes with masses at least up to 2500 u. The evolution of the ion yields of these higher mass carbon clusters gives an evidence that they are not originally present in the target. They may be formed by coalescence reactions of C60 molecules as a result of an individual MeV ion impact.

MeV ion sputtering of polymers: correlation between secondary ion radial velocity distributions and heavy ion track structure

Abstract Systematic investigations of the initial radial velocity distributions of low mass positive and negative secondary ions, sputtered electronically from thin films of polyvinylidene fluoride and polystyrene, are reported. 72.3 MeV 127 I 13+ primary ions bombard the targets at 45° angle of incidence. Sputtered secondary ions in an individual MeV ion impact are analysed in a high resolution time-of-flight mass spectrometer. The accurate mass measurements of all ion peaks in the range from 1 to 100 m/z provide unequivocal determination of the chemical composition of these ions, forming homologous series, C n H ± m and C n H m F ± p . Plots of both the initial mean radial velocity (〈 v x 〉) and kinetic energy (α 〈 v 2 x 〉) as a function of the ion m / z results in a periodic pattern. Ions with lower hydrogen content exhibit wider velocity distributions (i.e. higher 〈 v 2 x 〉) and (〈 v x 〉) directed towards the primary ion trajectory. Ions with higher hydrogen content have lower mean kinetic energies and 〈 v x 〉 directed away from the incident ion trajectory. We argue that the 〈 v x 〉 and 〈 v 2 x 〉 periodic behaviour, connected to the chemical constitution of the ions, reflects the radial profile of the deposited energy density in the heavy ion track.

A plasma desorption time-of-flight mass spectrometer with a single-stage ion mirror: improved resolution and calibration procedure

Abstract A first order electrostatic ion mirror has been constructed and mounted on a time-of-flight spectrometer coupled to the Uppsala EN-tandem accelerator at The Svedberg Laboratory. In electronic sputtering of CsBr with MeV 127 I ions a highest mass resolution around 10500 at a full-width at half maximum for the cluster ions (CsBr) 2 Cs + has been obtained. The resolution for organic molecules is somewhat lower than this value but still significantly better than in a linear system. The mirror also provides great simplification of the calibration procedure and an improvement in mass accuracy compared with a linear spectrometer. For stable ions, and with the help of an einzel lens, the transmission approaches 100% in the mirror. The mirror is equipped with two stop detectors which make it possible to study the decay of metastable ions.

Damage cross sections and surface track dimensions of biomolecular surfaces bombarded by swift heavy ions

Damage cross sections and surface track dimensions of biomolecular surfaces bombarded by swift heavy ions

Quantitative determination of kinetic energy releases from metastable decompositions of sputtered organic ions using a time-of-flight mass spectrometer with a single-stage ion mirror

Abstract A method has been developed for determining the kinetic energy released when metastable organic ions, produced by particle-induced desorption—ionization, decompose in a time-of-flight mass spectrometer having a single-stage ion mirror. To the best of our knowledge, this is the first report of a fully developed, quantitative procedure for this particular combination of ionization method and mass analysis. In order to obtain the kinetic energy released in a specific metastable decay, the rate constant for the unimolecular reaction has to be estimated, and the widths of the precursor and charged fragment ion peaks have to be measured. The rate constant for a specific decomposition reaction is determined by deflecting all ions away from the optic axis at positions of increasing distance along the flight path through the first field-free region of the spectrometer and by counting the neutral fragments that reach the detector located at the back end of the mirror. The widths of the precursor ion peak and the charged fragment ion peak are measured respectively after both types of ion species have independently followed precisely the same flight path in space and time. With a single-stage mirror, the latter condition is met by reflecting the precursor and fragment ions with mirror potentials that are in proportion to the respective masses of the ions. Theoretical, experimental, and error analyses are described and illustrated with examples.

Ion desorption from organic and inorganic targets induced by 100 keV gold cluster ions

Abstract The emission of different secondary ions from inorganic (CsBr) and organic (103 u peptide — valinomycin) targets is monitored as a function of the number of atoms of impacting Au−n cluster ions (n = 1 to 5) at constant energy of 100 keV. A method for determination of the multiplicities (defined here as the number of ions of the same type emitted per impact event) is described. The multiplicity is a measure of the ion emission probability in a single impact event and thus may provide evidence for “cooperative” effects in polyatomic ion impact induced sputtering.

Plasma desorption mass spectrometry in studies of formation and sputtering of fullerenes by MeV atomic ions

Abstract An account is presented on plasma desorption mass spectrometry (PDMS) studies of different carbon-containing organic solids utilizing megaelectronvolt (MeV) atomic ions from the Uppsvala EN-tandem accelerator. Positive ions of even-numbered carbon clusters (C+n, n = 40 to > 200) are ejected as a result of the interaction of the fast MeV ions with the target. The distribution of cluster sizes suggests that stable, closed carbon-cage structures — fullerenes - are formed. Among the investigated materials that produce carbon clusters are poly(vinylidenefluoride) and fluorinated fullerenes — C60F2m. For comparison purposes data from C60 targets have been also collected and analyzed. PDMS has been used for the in situ assessment of the damaging of C60 films by MeV heavy ions. Results on delayed electron emission from C−60 sputtered by MeV ions from C60 fullerene targets are also presented. A model of fullerene formation as a result of MeV ion interactions with the organic solid, including the yield dependence on primary ion charge state, is summarized. Both the data and the model suggest that fullerenes are formed as a result of a single primary ion impact and that they are ejected from an axially expanding infratrack plasma region. Results on different types of coalescence reactions in synthetic C60 fullerene targets and in blends of pure (synthetic) C60 with polystyrene leading to ejection of higher mass positive fullerene ions (C+k, k from 60 to more than 200) are also reported. The coalescence reactions are induced by the interaction of a single MeV ion with the solid. We argue that our data contribute to elucidating some general patterns of the fullerene formation mechanism.

Electronic sputtering of carbon clusters from ion beam irradiated organic films

Abstract Positive ions of even numbered carbon clusters C+2n (n > 20) are ejected as a result of the interaction of fast MeV ions with poly(vinylidene fluoride) (PVDF). Experiments provide circumstantial evidence that these ejected carbon clusters have a three-dimentional structure, consisting of pentagons and hexagons, i.e. they are fullerenes. We have studied the effect of macroscopic carbonisation induced by ion bombardment on the ejection of carbon cluster ions (C+n, n = 1–33; C+2nn = 21–50) from PVDF. Damage cross-section values are extracted from the fluence dependence of the secondary ion yields. The yield of fullerene ions from PVDF decreases as a function of MeV ion fluence. Accordingly, macroscopic carbonisation of the polymer is detrimental and is not a prerequisite to fullerene emission, confirming earlier findings that these carbon clusters are formed and ejected in a single MeV ion impact basis. For comparison, the sputtering of carbon cluster ions originating from films of poly(tetrafluorethylene), poly(styrene), poly(ethylene terephthalate) and fluorinated fullerene have also been studied.