C. Mair

Surface-induced reactions of acetone cluster cations

The occurrence of two different chemical reactions initiated by the surface impact of acetone dimer, trimer, and tetramer cations (energy 20–70 eV) on a stainless-steel surface (covered with hydrocarbons) was observed. The reaction product is the protonated acetone ion, formed in (i) an intracluster ion–molecule reaction, and in (ii) a hydrogen pickup reaction of the cluster ion with the surface material. Only the monomer product ions (and small amounts of their dissociation products) could be observed; the spectra did not show any presence of clustered product ions. A simple model based on the Brauman double-well potential is suggested to explain the formation of the two product ions. In accordance with predictions from molecular dynamics simulations, this appears to be the first observation of competitive chemical reactions of a cluster ion driven by energy transfer in a surface collision.

Low-energy collisions of CF3+ ions with a hydrocarbon covered surface: scattering, chemical sputtering, and reactive collisions

Abstract The interaction of CF 3 + ions with a stainless steel surface covered with hydrocarbons has been studied with a recently constructed BESTOF tandem mass spectrometer as a function of the collision energy from about 0 up to 60 eV. Whereas the efficient reflection of projectile ions at low collision energy and the efficient ejection of chemically sputtered ions at higher collision energies found here are in semiquantitative agreement with a previous report, results obtained here also demonstrate that reactive ion surface collisions are an important feature of CF 3 + impact on the hydrocarbon-covered surface leading to the formation of various fluorohydrocarbon ions such as CH 2 F + , CH 2 CF + , and CHF 2 + .

Surface-induced dissociations and reactions of acetonitrile monomer, dimer and trimer ions

Dissociations and reactions induced by impact of acetonitrile monomer ions (CH3CN+, CD3CN+), dimer ions [(CH3CN)2+, (CD3CN)2+] and trimer ions [(CD3CN)3+] on a hydrocarbon-covered stainless-steel surface were investigated over the projectile energy range of 3–70 eV. Both simple dissociations of the projectile ion and chemical reactions of H-atom transfer from the surface material (followed by dissociations of the protonated projectile ion formed) were observed for the monomer ions. Results obtained for the dimer ions (CD3CN)2+ indicate the formation of the protonated acetonitrile ions via surface-induced reactions in two ways: (i) an intracluster ion–molecule reaction followed by dissociation to form CD3CND+, and (b) a hydrogen pick-up reaction from the surface material during the interaction of the dimer ion with the surface leading to CD3CNH+. A simple model based on the Brauman double-well potential—suggested earlier to explain the occurrence of analogous reactions in acetone cluster ion/surface intera...

High-resolution analysis of the kinetic energy distribution of fragment ions produced by dissociative ionization of propane

We report measurements of the kinetic energy distributions of fragment ions produced by electron impact dissociative ionization of propane using a two-sector-field mass spectrometer in conjunction with the retarding field method. This technique achieves a higher energy resolution, albeit at much reduced ion signals and at a significant loss of energetic fragment ions compared to the conventional ion deflection technique, which has been used extensively by other groups in the past. The higher energy resolution used in the present experiments results in a complete separation of the thermal/quasi-thermal fragment ions from the energetic fragment ions. This, in turn, reveals that the energetic fragment ions have a much narrower kinetic energy distribution than previously assumed. Because of the comparatively weak ion signals, further studies such as separate threshold cross-section measurements and appearance energy determinations for, respectively, the “slow” and “fast” fragment ions could only be carried out for the CH3+ and C2H4+ fragment ions.

Surface-induced reactions of Cn+, 50⩽n⩽60

Abstract Singly charged fullerene ions C n + , where 50⩽ n ⩽60, have been collided with a hydrocarbon-covered stainless steel surface. Fragmentation and pickup reactions are measured as a function of the collision energy. Compared to C 60 + , ions with n ⩽58 fragment much more strongly. This is attributed to the fact that the initial excitation energy in these projectiles, which are formed from C 60 by electron impact ionization, is higher. However, identical values of 6.8%±0.5% are derived for the conversion efficiency of translational into internal energy.

Edge plasma-relevant ion–surface collision processes

Abstract Graphite tiles or carbon-coated structures are largely used as wall or divertor materials in magnetically confined fusion devices. Laboratory work on plasma–wall interaction has so far mainly focused on obtaining physical and chemical sputtering yields for such materials. Much less is known on electron emission and molecular fragmentation and reactions during ion impact on graphite surfaces, despite the fact that both processes might play a decisive role in plasma–wall interaction. We therefore have measured such data for fusion-relevant ion species (e.g., Hn+, n=1–3 and Cz+, z=1,2,4,5) in the eV to keV impact energy region typical for fusion edge plasma conditions. As a target surface graphite tiles from the Tokamak experiment Tore Supra in CEA-Cadarache/France have been used. Implications of the experimental results for plasma edge modeling are also discussed.

The role of internal energy of polyatomic projectile ions in surface-induced dissociation

Abstract The effect of initial internal energy on the extent of surface-induced fragmentation was investigated in a tandem mass spectrometer for CH 3 + and CH 4 + ions prepared in different ion sources. The different initial internal energy content had a considerable effect on the extent of fragmentation of the surface-excited projectile ions: ions from a Nier source with a large amount of initial internal energy fragmented at much lower collision energies than internally relaxed projectile ions from a Colutron source. A quantitative estimation of this effect showed that the initial internal energy content of the projectile ions was entirely preserved in the projectile ion during the ion/surface collision.

Charge exchange and surface-induced dissociation of doubly charged molecular ions C6H52+, C6H62+ and C7H82+ upon impact on a stainless steel surface

Abstract Interactions of doubly charged molecular ions C6H52+, C6H62+ and C7H82+ with a hydrocarbon covered stainless steel surface have been investigated in the collision energy range of about 0–50 eV. The mass and energy selected beam of dications was focused on to the surface and reaction products were monitored with a TOF spectrometer. More extensive fragmentation, with a low threshold of dissociation, has been observed in the case of C6H62+ as compared to C6H6+. But fragmentation pattern and structure of fragments are quite similar in singly and doubly charged ions of C6H6. Intact singly charged parent ions, produced by partial charge exchange of C6H62+ with the surface, are detected only at low collision energies. Moreover, also in the case of doubly charged C6H52+ and C6H62+ ions singly charged protonated ions have been observed at low collision energies. All other product ions were singly charged fragment ions, except C7H62+ produced by surface impact of the parent ion C7H82+. The possible major reaction sequence responsible for the observed fragmentation pattern seems to be partial charge exchange followed by unimolecular fragmentation.

Surface induced dissociation of HD2+: a non-statistical behavior in the H+ and D+ fragmentation channels

Abstract Surface induced dissociation (SID) of HD 2 + upon impact on a Tore Supra tokamak graphite tile was studied in the collision energy range from 20 to 100 eV. SID of the projectile ion HD 2 + on this graphite surface yields exclusively atomic fragment ions H + and D + , whereas other possible fragment ions HD + and D 2 + were completely absent. In contrast to the statistical ratio of 2:1 of D:H in the HD 2 + ion, the relative abundances of the measured H + and D + fragment ions were about of equal magnitude over the whole collision energy range studied. This non-statistical decay can be accounted for by vibrational predissociation through a minimal centrifugal barrier at a total angular momentum of about J ∼27–30.

Spontaneous and induced dissociation of singly and multiply charged fullerene ions

Using newly developed experimental techniques we have studied spontaneous (metastable) dissociation, electron-induced dissociation, and surface-induced dissociation reactions of singly and multiply charged fullerene ions. In particular, we will discuss results concerning (1) competition in spontaneous decay reactions between evaporation of neutral and ionized fragments, thereby extending our own previous studies on the spontaneous decay of multiply charged fullerene ions, (2) electron induced dissociation for singly and doubly charged C60 ions with special emphasis on the time dependence of kinetic energy release distribution thereby extending the recent cross section measurements of Salzborn and co-workers [Phys. Rev. Lett. 71 (1993) 3439], and (3) surface induced dissociation of singly and doubly charged C581 ions thereby extending previous studies by Kappes and co-workers [J. Chem. Phys. 104 (1996) 3629, 3638] on singly charged C60 ions. (Int J Mass Spectrom 192 (1999) 267‐280)