Controlling the outcome of reactions is a central issue of chemical research. Physical tools can achieve this if they are able to precisely dissociate specific bonds of a molecule. However, to control synthesis, such tools must induce the formation of new bonds between two reactants to yield a more complex product. In the ideal case of an atom efficient synthesis, this product would contain all or at least most of the initial material. An electron beam is a physical tool that is capable of preparing molecules in reactive states or, at low electron energies, of initiating highly selective bond dissociation. The resulting fragments in turn can react with other molecules to yield stable products. This tutorial review focuses in particular on such low-energy electron-initiated molecular syntheses and their applications in the modification of surfaces. It thus emphasizes strategies towards the controlled and predictable formation of more complex products from small reactants initiated by interaction with low-energy electrons either through selective bond dissociation or formation of specific reactive molecular species. However, selective bond dissociation is not always desirable. This is briefly illustrated by the case of electron beam induced deposition where additional strategies may be required to control product formation.

The smallest catalyst: A new strategy to control chemical synthesis by exposure to low-energy electrons relies on the electrostatic attraction caused by the soft ionization of one of the reaction partners. This approach was used to induce a reaction between C(2)H(4) and NH(3) yielding aminoethane. The reaction resembles a hydroamination except that the electron beam replaces the catalyst used in the organic synthesis.

Thermal desorption spectrometry (TDS) was applied to investigate reactions initiated by electron exposure of multilayer condensed films containing both an olefin and an amine or ammonia. In all cases, electron-induced hydroamination reactions leading to addition of ammonia or an amine to the CC double bond of the olefin were detected. The dependence of the hydroamination product yield on the electron energy was studied for mixtures of ethene with ammonia, ethylamine, and diethylamine and points, in all three cases, to an ionization-driven process. Concurrent reactions include the formation of larger hydrocarbons as well as fragmentation of the reactants and, in particular, of the amines so that higher substituted analogues are obtained even without the presence of an olefin. The mechanisms of these reactions are reviewed and an ionization-driven olefin dimerization is proposed that closely resembles the previously described hydroamination. The results underline the general concept of electron-induced synthesis driven by ionization but also demonstrate how the complexity of the reactions increases as the reactants become larger.

The formation of the prebiotically relevant molecule formamide under electron exposure of ammonia and carbon monoxide was studied at cryogenic temperatures of 30−35 K. Postirradiation thermal desorption spectroscopy was used to study the energy dependence of the reaction. A resonant process centered around ∼9 eV and a threshold type increase of the yield above ∼12 eV were observed. On the basis of the absence of particular side products such as urea and ethanediamide and supported by quantum chemical calculations, reaction mechanisms related to the two observed energy regimes of formamide production are proposed. Below the ionization threshold, electron attachment to ammonia and the subsequent dissociation of the radical anion trigger the reaction sequence. At higher energies, electron impact ionization and addition of the formed radical cation to a neutral molecule ultimately lead to the formation of formamide.

Self-assembled monolayers (SAMs) of 10-undecene-1-thiol on Au were functionalized with nitrogen-containing groups using an approach in which multilayer ammonia (NH(3)) films were deposited at low temperature onto the SAMs and subsequently exposed to 15 eV electrons. The result of this process was investigated after removal of the remaining NH(3) by annealing to room temperature using high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). HREELS shows that the CC double bonds disappear during electron exposure, while XPS gives evidence that about 25% of the terminal double bonds of the SAM were functionalized. Also, XPS shows that a sufficiently thick NH(3) layer protects the underlying SAM from electron-induced damage. The process suggested here thus represents a particularly gentle approach to the functionalization of ultrathin molecular layers. Thermal desorption spectrometry (TDS) and electron-stimulated desorption (ESD) experiments on condensed layers of NH(3) reveal production of N(2) but show that significant amounts of the initial NH(3) as well as N(2) produced during electron exposure desorb. Hydrogen released upon formation of N(2) is held responsible for the reduction of double bonds and protection of the SAMs from damage.