T. Ast

Collisions of polyatomic ions with surfaces

Abstract Surface-induced dissociation (SID) of polyatomic ions is reviewed. Instrumentation is described, including hybrid, tandem quadrupole and tandem time-of-flight instruments built especially to study polyatomic ion—surface collision phenomena. In addition, in-line devices that allow SID to be performed on conventional tandem mass spectrometers are noted, as are experiments which utilize ion cyclotron resonance instruments. The extent of energy deposition accompanying surface collisions is characterized and compared with data from gaseous collisions at non-zero scattering angles. The large internal energies available through SID are shown to facilitate structural characterization of molecular ions of polynuclear aromatic hydrocarbons and peptides. Applications to ion strucutre and chemistry are also illustrated. Emphasis is given to competing ion—surface collision phenomena, including charge-changing collisions and ion—surface reactive collisions. These latter processes are shown to provide information on adsorbates and to be thermochemically controlled. When certain projectile ions, especially small fluorocarbons, undergo charge exchange with surface adsorbates, they are released from the surface as ions. The influence of the nature of the surface on these processes and on SID is discussed. Current trends in research on ion—surface collisions are identified.

Chemical modification of fluorinated self-assembled monolayer surfaces by low energy reactive ion bombardment

Reactive collisions of low energy (<100-eV) mass-selected ions are used to chemically modify fluorinated self-assembled monolayer surfaces comprised of alkanethiolate chains CF3(CF2)11(CH2)2S— bound to Au. Typical experiments were done by using 1-nA/cm2 beams and submonolayer doses of reactant ions. Characterization of the modified surface was achieved by in situ chemical sputtering (60-eV Xe+·) and by independent high mass resolution time-of-flight-secondary ionization mass spectrometry (TOF-SIMS) (15–25-keV, Ga+) experiments. Treatment with Si35C14+· produced a surface from which Xe+ sputtering liberated CF235C1+ ions, which suggested Cl-for-F halogen exchange at the surface. Isotopic labeling studies that used Si35Cl237Cl2+·; and experiments with bromine-containing and iodine-containing projectiles, confirmed this reaction. High mass resolution TOF-SIMS spectra, as well as high spatial resolution images, provided further evidence as to the existence of halogen-exchanged species at the bombarded surface. Analogous Cl-for-F halogen substitution was observed in a model gas-phase reaction. The ion-surface reaction is suggested to proceed through an intermediate fluoronium ion in which the projectile is bonded to the target molecule. The most significant conclusion of the study is that selective chemical modification of monolayer surfaces can be achieved by using reactive ion beams, which lead to new covalent bonds at the surface and in the scattered ions.

Low-energy Collisions of Methane Ions at a Fluoroalkyl Monolayer Surface

Reactive scattering of low-energy (<100 eV) CH n + (n = 0-4) ions with a fluorinated self-assembled monolayer (F-SAM) surface produces fluorine-containing scattered ions, including CF + , C 2 HF 2 + and C 3 F 3 + . Evidence is provided that these ion-surface reactions occur by surface-induced dissociation (SID) prior to new bond formation. Chemical sputtering of the ion-beam modified surface is consistent with the modification of the fluorinated SAM surface by the methane derived ions occurring at the terminal CF 3 group. Studies with 13 C-labeled ions show the formation of 12 CF + and 13 CF + in nearly equal abundance. The participation of carbon atoms arising from both the surface and the projectile suggests the intermediacy of a symmetrical fluoronium collision complex, such as [F 13 CF 12 CF] + , involving the SID fragments of the projectile and the terminal CF 3 group of the surface. A 20% translational to vibrational (T → V) energy conversion is estimated from the inelastic collisions which lead to SID, a result which is consistent with previous studies using other ions on the same surface