Thermal Desorption Induced by Kiloelectronvolt Ion Bombardment of Thiol-Bound Self-Assembled Monolayers on Gold

Abstract

Time distributions of neutral molecules desorbed from a chemisorbed self-assembled monolayer of phenylethanethiol on gold have been measured subsequent to 8 keV Ar + and H 2 + ion bombardment. These distributions show that, regardless of the projectile used, most of the ejected molecules leave the surface with thermal kinetic energies (∼0.03 eV). The shapes of the distributions have a strong surface temperature dependence over the range 240-300 K. This behavior is well described by a convolution of the Maxwell-Boltzmann distribution and the rate equation for first-order desorption. The results imply that kiloelectronvolt ion bombardment initiates a process which breaks the adsorbate-surface bond, leaving the resulting physisorbed molecules to evaporate after attaining thermal equilibrium with the substrate. A mechanism for this gentle cleavage of the adsorbate-substrate bond is proposed.

Adsorbate Geometry Distinction in Arenethiols by Ion/Surface Reactive Collisions

“…36,37 The discovery of ion/ surface reactions that are sensitive to closely related chemical species should make it possible to extend the scope of this methodology for adsorbate characterization. The fact that ion/ surface inelastic processes can provide data even for fractional monolayer coverages should increase the potential value of this approach.…”

Ion-induced erosion of organic self-assembled monolayers

“…Only a minor fraction of emitted molecules has higher kinetic energies. A careful analysis using dierent projectiles showed that these molecules are sputtered with kinetic energies of the order of 1 eV by a direct momentum transfer from the incident ion to the gold substrate and ®nally to the organic layer [7].…”

“…We have postulated that low energy emission of PEM fragments can be explained by chemical reactions stimulated by the primary ions [7]. Direct bond scission by projectile impact can be excluded since it would cause the molecules to eject with high kinetic energy.…”

“…Bond scission by chemical reactions is more gentle than direct bond scission by ion impact and is more likely to form products which may be trapped at the surface and ®nally evaporate with low kinetic energies. Direct scission of the sulfur±gold bond is endothermic by approximately 2±3 eV, while cleavage by reaction with a hydrogen radical is estimated to be exothermic by 1.7 eV [7]. If evaporation is involved, the process should be temperature dependent.…”

Coverage-Dependent Molecular Ejection from Ion-Bombarded C6H6/Ag{111}

“…The velocity distributions follow Maxwellian behavior with a temperature that closely track the substrate temperature. 47 For solid organic materials such as tryptophan though most molecules eject due to a ballistic process, emission of molecules for long times after ion impact is observed. 48 These data suggest that long-lived excited states may be involved in the desorption event.…”