The Mechanism of Hydrogen Formation Induced by Low-Energy Electron Irradiation of Hexadecanethiol Self-Assembled Monolayers

Abstract

The desorption of molecular hydrogen during low-energy electron irradiation of self-assembled monolayers containing n-alkanethiols has been previously reported, yet to date, there is no consensus as to the mechanism for the formation of this ubiquitous product. In this study, mixed monolayers containing known ratios of perhydrogenated and perdeuterated alkanethiols were chemisorbed to Au(111)/mica substrates and used as targets for low-energy electron irradiation; by measuring the electron-stimulated production of H(2), D(2), and HD as a function of the film composition, we unambiguously show that the desorbing molecular hydrogen is formed via a two-step bimolecular reaction process. The initial electron-molecule scattering event produces a reactive atomic fragment, which then abstracts a hydrogen atom from a nearby molecular site to produce the measured bimolecular yields; the contribution of one-step unimolecular dissociation channels to the overall molecular hydrogen yields is below the approximately 5% detection limit. The dependence of the electron-induced modifications to the film on the incident electron energy suggests that the primary event is dissociative electron attachment, and that the primary reactive fragment is most likely H(-). Quantitative analysis of the product yields shows that while approximately 80% of the molecular hydrogen is formed by this bimolecular mechanism within the film, the remaining 20% is formed from reactive atomic fragments that are ejected from the film and subsequently react with residual H(2)O adsorbed on the chamber walls.

Electronically induced modification of thin layers on surfaces

“…Nearly temperature independent radiation effects are those which do not involve transport of large fragments; abstraction of H atoms is the best example (see [10] for a description of the mechanism of electron stimulated H abstraction; although primary photons are applied in our study, secondary electrons with energies similar to those of [10] will stimulate the majority of abstraction processes even in our case, see below). Abstraction of H atoms leaves bond vacancies which recombine either by double bond formation (if at the same molecule) or by cross-linking (if at neighboring particles) [8].…”

“…Here we focus on the last point, using them for investigations of basic temperature effects on beam induced modifications. Many studies of irradiation effects in SAMs by electrons and to a lesser extent also by photons are reported in the literature, from which we cite three articles in which the reader can find most of the relevant references [8][9][10]. The majority of these previous reports are room temperature studies, thus neglecting any low-temperature effects.…”

The preparation of three new partially deuterated hexadecanethiols for applications in surface chemistry

“…Deuteration of an otherwise perhydrogenated molecule can be used to distinguish the vibrational properties and ordering characteristics at specific sites along the chain axis without significantly perturbing the van der Waals forces that provide the film's stability. Previous theoretical 2 and experimental work 3,4 has shown that isotopic segregation is not expected for these systems, and as such mixtures of hydrogenated and deuterated species can be considered as thermodynamically ideal solutions and adsorbed films. By diluting deuterated components within fully hydrogenated isotopomers it has been possible to identify and quantify the dynamic intermolecular and intramolecular coupling of the vibrational modes of two dimensional film, 4 and thus clarify the nature of the overall spectrum.…”

“…We have recently used perdeuterated alkanethiols to explore the mechanisms of molecular hydrogen formation when SAMs are exposed to lowenergy (0-15 eV) electron beams. 3 By measuring the relative yields of HD and D 2 formed as the mole fraction of perdeuterated thiolates was varied, we have shown that the reaction is bimolecular in nature, with a primary hydrogen (deuterium) fragment produced by electron impact, and the molecular hydrogen being produced by the reactive encounters of these fragments with adjacent molecules; by unambiguously showing that the unimolecular fragmentation channels 5 has negligible probabilities, the interpretation of electron-beam induced chemistry in lithographic processes becomes more reliable.…”

Herstellung mikroskaliger Proteinresistenzgradienten durch Elektronenstrahl‐Lithographie

“…Diese Methode beruht auf der strahlungsinduzierten Austauschreaktion (irradiation-promoted exchange reaction, IPER) an selbstorganisierenden Monoschichten. [17][18][19] Wie wir und andere zuvor demonstriert haben, werden Thiolat-basierte SAMs auf Gold durch Röntgenstrahlung, [20,21] UVLicht [22,23] oder Elektronenbeschuss [18,[24][25][26] …”