INFLUENCE OF THE CHARGE STATE AND OF THE INTERNAL by g4509244

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									       INFLUENCE OF THE CHARGE STATE AND OF THE INTERNAL ENERGY OF THE
           PROJECTILE ION ON THE OUTCOME OF ION/SURFACE COLLISIONS


  A.Qayyum, T.Tepnual, L.Feketeova, W.Schustereder, G.Hasan, C.Mair, S.Matt, B.Concina, Z.Herman,
                                     P.Scheier and T.D.Märk


        Institut für Ionenphysik, Universität Innsbruck, Technikerstr.25, A-6020 Innsbruck, Austria



     We have conducted first detailed studies to        fragment ions observed for these doubly charged
investigate the influence of charge state and           projectile ions were singly charged, except C7H62+
internal energy of a molecular ion on its surface-      produced by surface impact of the parent ion
induced dissociation using our tandem mass              C7H82+.
spectrometer system BESTOF [1]. In short, a beam             Besides potential energy, we have also
of singly or doubly charged molecular ions is           addressed here the role of internal energy of the
generated using an electron impact ionization           projectile in the surface-collision process. For this
source or a gas discharge source, mass and energy       purpose ions such as CH3+, CH4+, CH5+, C2H4+ and
analyzed ions are then focused onto a surface and       C2H5+ were produced by a discharge ion source
secondary reaction products are monitored using a       (internally relaxed) and a Nier-type electron impact
secondary TOF spectrometer.                             ion source (internally excited) . From the onset and
     Reactive interactions of doubly charged            shape of measured ‘collision energy resolved mass
molecular ions CO22+, COS2+, CS22+, C4H32+,             spectrum’ curves and calculations of excitation
C6H62+, C6H52+, C7H82+, C7H72+ and C7H62+ with a        energy distributions in the projectile ions one could
stainless steel surface have been investigated in the   conclude that (i) ions from the Nier type source
collision energy range of about 0 to 50 eV. Distinct    had a higher internal energy than those ions
trends in the charge exchange and surface-induced       generated in the discharge source and (ii) that this
dissociation (SID) of these dications were found.       initial energy is fully used for subsequent projectile
Extensive fragmentation at already a rather low         dissociation in addition to projectile internal energy
collision energy threshold is observed in doubly        acquired in the surface collision.
charged CO2 and COS ions as compared to the                  In addition, SID of HD2+ on a graphite surface
respective singly charged ions. The product ion         has been shown to yield exclusively atomic
mass spectra of triatomic dications CO22+        and    fragment ions H+ and D+, whereas other possible
COS2+ are dominated (at least up to 7 eV collision      fragment ions, HD+ and D2+, were completely
energy in case of CO22+) by the charge separation       absent. In contrast to the statistical ratio of 2:1 of
reactions yielding the ion pairs O+ + CO+ and S+ +      D:H in the HD2+ ion, the relative abundances of the
CO+, respectively. In the case of CS22+ and C4H32+      measured H+ and D+ fragment ions were about of
no fragment ions can be detected, the major             equal magnitude over the whole collision energy
reaction being the double charge exchange with the      range studied. This non-statistical decay can be
surface. None of these dications showed partial         accounted for by vibrational predissociation
charge exchange with the surface to produce singly      through a minimal centrifugal barrier at a total
charged parent ions. In contrast all of the doubly      angular momentum of about J ~27-30.
charged hydrocarbon ions C6H62+, C6H52+, C7H82+,
C7H72+ and C7H62+ were observed to produce              References
corresponding singly charged parent and fragment        [1] C. Mair, T.Fiegele, F.Biasioli, R.Wörgötter,
ions by partial charge exchange with the surface        V.Grill, M.Lezius and T.D.Märk, Plasma Sources,
followed by unimolecular fragmentation. All the         Science and Technology, 8 (1999) 191

								
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