J . Phys. Chem. 1994,98, 245-251
245
Energy Disposal and Target Effects in Hyperthermal Collisions of Ferrocene Molecular Ions at Surfaces S. A. Miller, D. E. Riederer, Jr., and R. G. Cooks' Department of Chemistry, Purdue University, West Lafayetre. Indiana 47907
W. R. Cho, H. W. Lee, and H. Kang' Department of Chemistry, Pohang Institute of Science and Technology, Pohang, Korea 790-600 Received: July 1 , 1993; In Final Form: October 6, 1993e
Ferrocene molecular ions undergo inelastic collisions at Si( 100) and alkanethiol self-assembled monolayer surfaces (SAMs). Dissociation of the activated projectile follows its recoil from the surface; viz., surfaceinduced dissociation (SID) occurs by a two-step mechanism, and the fragmentation pattern is independent of the nature of the surface. On the other hand, the SID efficiency for collisions at the S A M surfaces is much greater than that for the Si( 100) single-crystal surface as a result of a decrease in neutralization at the organic surfaces. The fragmentation pathways of a ferrocene molecular ion are elucidated as a function of collision energy and presented as energy-resolved mass spectra (ERMS) on the S A M andSi( 100) surfaces. Translational to vibrational energy partitioning is similar for Si(100) and a hydrocarbon surface (ca. 13%) but greater for a fluorocarbon surface (ca. 20%). The ferrocene molecular ion displays a high SID efficiency (total scattered ion yield) on the organic surfaces compared to other common projectile ions due to its low ionization energy. Because charge exchange between the projectile molecular ion and the surface is minimized, ferrocene does not undergo ion/surface reactions.
(Fe(CSH5)2*+) collides at surfacesas a function of collision energy Introduction and surface composition. In particular, a bare silicon metal is Collisions of polyatomic ions with surfaces at low energies compared to surfaces bearing organic groups. The dissociation (1-100 eV) have a variety of consequences which include (i) products of ferrocene are discussed in terms of the degree of elastic scattering, (ii) inelastic scattering followed by dissociation neutralization and conversion of translational to vibrational (Tof the projectile ions, a process known as surface-induced V) energy. dissociation (SID), (iii) charge exchange, including partial neutralization of doubly-charged ions, and (iv) chemical sputtering Experimental Section of adsorbates from the surface.' These processes have recently been investigated in several types of mass spectrometersincluding Instruments. Surface-induced dissociation of ferrocene was hybrids,= sect0rs,7~time-of-flight instruments,1s13 F T - I C R ' S , ~ ~ ~ ' ~ performed at Purdue using a hybrid BEEQ tandem mass andquadrupoleion traps.16 The subject of polyatomic ionlsurface spectrometer (B = magnetic sector, E = electric sector, Q = collisions has also been reviewed.17 Surface-induceddissociation quadrupole mass filter) which has been described previously6 has proven to be a useful alternative to the use of gas-phase (see Figure la). Gaseous ferrocene was introduced into the ion collisions to achieve collision-activated dissociation, and it has source from a cooled solids probe held at approximately 290 K, the particular advantage of depositing high internal energies.2J8 the samplewas ionized by 70-eV electron impact (unless otherwise Its potential value in chemical analysis has been demonstrated4J9 noted), and the resulting ions were accelerated to 2 keV. Next, although to date its most significant applications have been for the parent ion of interest, for example, the ferrocene molecular distinguishing isomeric ions.2G22 ion ( m / z 186), was mass and kinetic energy selected using the In another line of investigation,gaslsurface collisions at clean first two sectors of the instrument (the magnetic sector B and the metal surfaces have been studied extensively under ultrahighelectric sector E). The mass-selected ion beam was then vacuum (UHV) conditions, using molecular beams.23 However, decelerated by an eight-element lens system to a selected kinetic in most of these studies, collision energies were limited to a few energy in the range 1&120 eV and directed onto a target. The electron volts, and relatively few investigations2426have been target is located in thecenter of a custom-built,UHV-compatible, conducted in the energy region of 10-100 eV. While ion beams collision chamber which is held at a pressure of approximately allow ready access to this energy regime, scattering studies 4X Torr. The primary ion beam typically produced a 1.5employing reactive ions are relatively rare25,26due in part to nA ion current, measured prior to the collision chamber using a instrumental difficulties with low-energy ions and partly to the Faraday cup. The nominal ion current measured at the target perception that such slow-moving ions would be neutralized at surface ranged from 100 pA to 1.5 nA and increased with the the surface. However, recent s t ~ d i e s ~have ~ , ~demonstrated * the collision energy. The kinetic energy spread in the primary beam opposite behavior; i.e., scattered ion yields are higher for lowwas f 2 eV (fwhm). Secondary ions were selected for those of energy (
