V. Grill

Collisions of ions with surfaces at chemically relevant energies: Instrumentation and phenomena

An overview of gaseous ion/surface collisions is presented, with special emphasis on the behavior of polyatomic projectile ions at hyperthermal collision energies (1–100 eV) and the instrumentation needed for such studies. The inelastic and reactive processes occurring during ion/surface collisions are described in terms of several archetypes, viz., elastic and quasielastic scattering, chemical sputtering leading to release of surface material, inelastic scattering leading to surface-induced dissociation (SID) of the projectile, ion/surface reactions, and soft landing. Parameters that are important in ion/surface interactions are discussed, including the interaction time, the conversion of translational to internal energy, the translational energies of the scattered ions, the effects of scattering angle, and the influence of the nature of the surface. Different types of tandem mass spectrometers, built specifically to study ion/surface collision phenomena, are discussed and the advantages and disadvantages of the individual designs are compared. The role of SID as a technique in bioanalytical mass spectrometry is illustrated and this inelastic collision experiment is compared and contrasted with gas-phase collision-induced dissociation, the standard method of tandem mass spectrometry. Special emphasis is placed on reactive scattering including the use of ion/surface reactions for surface chemical analysis and for surface chemical modification.

High resolution dissociative electron attachment to gas phase adenine.

The dissociative electron attachment to the gas phase nucleobase adenine is studied using two different experiments. A double focusing sector field mass spectrometer is utilized for measurements requiring high mass resolution, high sensitivity, and relative ion yields for all the fragment anions and a hemispherical electron monochromator instrument for high electron energy resolution. The negative ion mass spectra are discussed at two different electron energies of 2 and 6 eV. In contrast to previous gas phase studies a number of new negative ions are discovered in the mass spectra. The ion efficiency curves for the negative ions of adenine are measured for the electron energy range from about 0 to 15 eV with an electron energy resolution of about 100 meV. The total anion yield derived via the summation of all measured fragment anions is compared with the total cross section for negative ion formation measured recently without mass spectrometry. For adenine the shape of the two cross section curves agrees well, taking into account the different electron energy resolutions; however, for thymine some peculiar differences are observed.

Bond selective dissociative electron attachment to thymine

Free-electron attachment to thymine and partially deuterated thymine, where D replaces H at all carbon atoms, is studied in the electron energy range from about 0 to 15 eV. The formation of fragment anions that are formed by the loss of one or two H (D) atoms is analyzed as a function of the incident electron energy using a crossed electron/neutral beam apparatus in combination with a quadrupole mass spectrometer. By using partially deuterated thymine and quantum-chemical calculation a bond selectivity for the loss of one and two hydrogen atoms is observed that is determined only by the kinetic energy of the incident electron.

Discrimination effects for ions with high initial kinetic energy in a Nier-type ion source and partial and total electron ionization cross-sections of CF4

Abstract Fragment ions produced by dissociative electron impact ionization of molecules often have large amounts of kinetic energy (up to several electronvolts). Presently performed computer simulations of the ion trajectories in a Nier-type ion source show strong discrimination effects in the extraction characteristics of such ions, leading to huge errors in measured fragmentation patterns and absolute partial ionization cross-section functions. Using the simulation results a correction function (extraction efficiency vs. fragment kinetic energy) is claculated here for a given potential distribution of a Nier-type ion source extraction system. This correction procedure is applied to previously measured partial ionization cross-sections of CF4. The corrected partial and total CF4 cross-sections are in excellent agreement with other recent measurements using advanced methods.

The working principle of the trochoidal electron monochromator revisited

Abstract On the basis of a number of different experiments employing various principles, we have demonstrated that the energy resolution of the trochoidal electron monochromator used in our laboratory the past 5 yr is not independent from the electron energy used, that is, the very high nominal-energy-resolution close-to-zero electron energy in the range of several meV deteriorates quickly with increasing electron energy reaching values of up to 100 meV at ∼1-eV electron energy. Carrying out extensive electron trajectory calculations with the Simion program, we were able to show that our variant of a trochoidal monochromator does not only operate on the trochoidal dispersion principle but also involves a retarding field component right after the dispersion region in achieving this high-energy-resolution close-to-zero energy. This retarding field is, however, weakened at higher electron energies (caused by the influence of the electron-acceleration field) leading to the decrease in energy resolution with increasing electron energy. On the basis of further simulations, we have designed and constructed a new monochromator avoiding this and other deficiencies. This new monochromator currently has an energy resolution of ∼45 meV independent of the electron energy. Further improvements are under consideration.

Low energy electron attachment to SF5CF3

Low energy electron attachment to the potent greenhouse gas SF5CF3 is studied at high energy resolution by means of mass spectrometric detection of the product anions. A large dissociative electron attachment (DA) cross-section forming SF5−+CF3 is observed within a very narrow energy range close to zero eV. In addition, comparatively weak resonances are observed near 1 eV yielding the fragment ions CF3− and F−. Some implications for the atmospheric lifetime of SF5CF3 and hence its global warming potential (GWP) are considered.

Energy partitioning in collisions of slow polyatomic ions with carbon surfaces

Abstract Energy transfer in collisions of slow polyatomic ions with carbon surfaces (Tore Supra carbon tile and highly oriented pyrolytic graphite) was investigated over the incident energy range 10–23 eV. Mass spectra and translational energy and angular distributions of product ions were used to determine the partitioning of the incident energy of the projectile ion into internal excitation of the projectile, product translational energy, and fraction absorbed by the surface. The ethanol molecular ion was used as a model polyatomic ion. For the incident angle of 60° (with respect to the surface normal) the peak values of the respective energy fractions were 6% for excitation of the projectile, 24%–28% for product ion translational energy, and 70%–66% absorbed by the surface. Similar values for energy transfer were found earlier for energy transfer at stainless steel surfaces covered by hydrocarbons and surfaces covered by a self-assembled monolayer (SAM) of C 12 alkane chains. The occurrence of chemical reaction products (protonated ethanol and its fragment ions) formed by H-atom transfer from the surface material for all the above mentioned surfaces (the carbon surfaces, stainless steel with a hydrocarbon layer, alkane SAM) indicated that the carbon surfaces were covered with a layer of hydrocarbons, too, and thus the present results provide information of interest for energy transfer on carbon tiles covered with hydrocarbon films used in fusion research.

Chemical modification of fluorinated self-assembled monolayer surfaces using low-energy ion beams for halogen and pseudohalogen transfer

Isocyanatosilane-derived cations, Si(NCO) n + , (n = 1-4) were mass-selected and reacted with fluorinated self-assembled monolayer (F-SAM) surfaces at low collision energy ( < 100 eV). Examination of the scattered products revealed a variety of reactions from simple fluorine atom abstraction (e.g. formation of SiF + ) to halogen-pseudohalogen exchange products (e.g. formation of SiF(NCO) 2 + and SiF 2 (NCO) + from Si(NCO) 3 + ). Bare Si + reacts predominantly by single F-atom abstraction, but small amounts of SiF 2 + and SiF 3 + were also observed. The correlation of reactivity with thermochemistry and the role of the electronic configuration of projectile ions in multiple F-atom abstraction processes are discussed. Chemical modification of the F-SAM surface itself was shown to be a consequence of the ion-surface collisions. For example, in situ Xe + chemical sputtering analysis of F-SAM surfaces exposed to Si(NCO) n + ion beam bombardment revealed the presence of CF 2 NCO + (m/z 92). This result suggests that NCO groups are incorporated into the fluorocarbon chains at the surface, probably via a pseudohalogen-by-halogen exchange mechanism. Surface modification was also achieved using the dicyanatochloro-methyl cation, C(CN) 2 Cl + . Prolonged reactive collisions of C(CN) 2 Cl + upon a F-SAM surface led to the incorporation of both chlorine atom and cyanato groups into the surface, and these were released as CF 2 CN + (m/z 76) and CF 2 Cl + (m/z 85) on Xe + sputtering analysis. Isotopic labeling experiments confirmed Cl substitution into the surface fluorocarbon chains.

SOFT LANDING OF IONS ONTO SELF-ASSEMBLED HYDROCARBON AND FLUOROCARBON MONOLAYER SURFACES

Abstract Low energy (≤ 10 eV) ion beams can be soft landed onto a C 12 -hydrocarbon self-assembled monolayer (H-SAM) surface. The H-SAM surface causes much less collateral fragmentation of the deposited ions under the same conditions than the corresponding fluorinated self-assembled monolayer (F-SAM) surface. The energy dependence for release of ions deposited onto an F-SAM monolayer surface shows that smaller ions are ejected more readily than sterically bulky species. The threshold energy for release of the soft-landed species by Xe +· coincides with that of the typical chemical sputtering product CF 3 + , implying that the deposited species are strongly held inside the SAM matrix and that C–C bond cleavage assists in efficient release of the trapped ions. In cases where fragment ions of the deposited projectile are released from the surface, it is shown, by varying the energy and nature of the releasing projectile ion, that dissociation primarily occurs upon impact of the soft-landed ion and not upon its release. Examination of various odd- and even-electron ions confirms the earlier conclusion that the former are efficiently neutralized and only the latter can be soft landed as ions onto F-SAM surfaces.

The observation of unusual resonance channels in the electron attachment to mixed argon-oxygen clusters

Abstract An additional electron attachment resonance peak, not observable in pure oxygen cluster anions, has been found at an energy of about 11.5 eV for both the stoichiometric Ar m (O 2 ) n − and the nonstoichiometric Ar m (O 2 ) n −1 O − cluster anions (produced by electron attachment to mixed argon-oxygen clusters). Two possible competing mechanisms are proposed to explain the appearance of these resonance peaks, i.e. the multiple-collision electron-scavenging mechanism and the core-excited resonance attachment mechanism. From the peak-width analysis it follows that in Ar m (O 2 ) 2 − the first mechanism is dominant, while in Ar m (O 2 ) n −1 O − the second mechanism seems to be the only one occurring.