Collisions of fluorocarbon ions at solid surfaces: Electronic excitation, surface‐induced dissociation and chemical sputtering

Abstract

Collisions of C,F:. at self-assembled hydrocarbon, deuterated hydrocarbon and fluorocarbon surfaces yield fragment ions which are characteristic of both electronic excitation and vibrational excitation. Direct electronic excitation is indicated by loss of F , which has been shown previously to be diagnostic of this type of excitation process. Electronic excitation is favored by low-energy collisions at the hydrocarbon surface. Even the change to the corresponding deuterated surface produces a large effect in favor of the normal vibrational excitation process. This change in mechanism with the nature of the target shows up as a dramatic isotope effect in the surface-induced dissociation (SID) mass spectra. The control over the excitation process exhibited by the effective mass of the target is probably exerted through its effect on the relative velocity of the collision partners. The fluorinated surface is more effective than the others in conversion of translational into internal energy and in minimizing ion loss through neutralization and other processes which compete with SID. The fluorinated surfaces yield spectra that are largely free from chemical sputtering, a process which occurs even at ultra-high vacuum for stainless-steel surfaces which are not rigorously cleaned. The internal energy deposition associated with chemical sputtering increases with increasing collision energy. Several of the fluorocarbon fragment ions generated from perfluoropropylene have also been examined at self-assembled monolayer surfaces and they are well behaved in their SID and chemical sputter-

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

“…The influence of the nature of the surface can be illustrated by an unusual case in which the excitation mechanism is affected by the nature of the surface in a way that controls the extent of electronic vs. vibrational excitation. 69 In one system, as subtle a change as using a self-assembled hydrocarbon adsorbate versus the corresponding deuterated surface is sufficient to cause a large effect in favor of normal vibrational excitation over electronic excitation of scattered fluorocarbon ions. 69 One of the most important intrinsic parameters involved in ion/surface collisions is the interaction time.…”

“…69 In one system, as subtle a change as using a self-assembled hydrocarbon adsorbate versus the corresponding deuterated surface is sufficient to cause a large effect in favor of normal vibrational excitation over electronic excitation of scattered fluorocarbon ions. 69 One of the most important intrinsic parameters involved in ion/surface collisions is the interaction time. The simplest way to estimate the interaction time is to employ a trajectory length of about 2 nm and a velocity of about 10 7 cm s Ϫ1 and then assume uniform motion of the projectile ion in the vicinity of the surface ͑neglecting deceleration and acceleration due to the repulsive interaction potential and ion trapping in the surface potential͒.…”

Effect of polyatomic ion structure on thin-film growth: Experiments and molecular dynamics simulations

“…Several studies have examined reactive and inelastic scattering of fluorocarbon ions off surfaces. [22][23][24] For example, CF x ϩ scattering off various surfaces found energy transfer to the surface to be very efficient and dependent upon ion and surface structure. 23 Energy transfer has also been studied for C 3 F 5 ϩ ions scattering off fluorocarbon ͑FC͒ surfaces and the kinetic to internal energy transfer was found to be ϳ20%.…”

Differentiation of isomeric oxygenated adsorbates using low-energy ion/surface collisions

“…The typical dissociation pathway for molecular ions formed from ethers or alcohols upon ionization by electron impact is also due to C-C cleavage of the alpha bond. Dissociation of gas-phase ions after electron impact ionization and that of surface molecules by chemical sputtering both occur as a result of energetic charge exchange [14], and therefore the same principle which governs the fragmentation of ions generated through electron impact is expected to hold true for the chemical sputtering of fragment ions from surfaces. The scattered ion mass spectrum resulting from 70 eV collisions of CF 3 + with the CD 3 O-SAM surface is shown in Fig.…”

“…The internal excitation achieved with fluorocarbon monolayer surfaces [12] is significantly greater than that for hydrocarbon-covered surfaces [13]. Reasons suggested for the effectiveness of fluorocarbon surfaces over the other surfaces include the chemical inertness of these molecules, the larger ionization energy which suppresses neutralization, and the greater effective mass which increases momentum transfer [14].…”