Richard Balog

Reactions in condensed formic acid (HCOOH) induced by low energy (<20 eV) electrons

The interaction of low energy (< 20 eV) electrons with a five monolayer (ML) film of formic acid (HCOOH) deposited on a cryogenically cooled monocrystalline Au substrate is studied by electron stimulated desorption (ESD) of negatively charged fragment ions. A comparison with results from gas phase experiments demonstrates the strong effect of the environment for negative ion formation via dissociative electron attachment (DEA). From condensed phase formic acid (FA) a strong H desorption signal from a resonant feature peaking at 9 eV is observed. In the gas phase, the dominant reaction is neutral hydrogen abstraction generating HCOO- within a low energy resonance, peaking at 1.25 eV. ESD studies on the isotopomers HCOOD and DCOOH indicate effective H/D exchange in the precursor ion at 9 eV prior to dissociation. The evolution of the desorption signals in the course of electron irradiation and the features in the thermal desorption spectra (TDS) of the electron irradiated film suggest the formation of CO2 at electron energies above 8 eV.

Low energy electron interaction with free and bound SF5CF3: Negative ion formation from single molecules, clusters and nanofilms

The interaction of free electrons with the potent greenhouse molecule SF5CF3 is studied under different degrees of aggregation: single molecules at collision free conditions, clusters within a supersonic molecular beam and condensed molecules. Electron collisions with single molecules are dominated by SF5− formation produced via dissociative electron attachment (DEA) within a resonance located below 2 eV. In clusters, undissociated parent anions SF5CF3− (and larger complexes containing undissociated anions) are observed in addition to the fragment ions. This indicates that (i) SF5CF3 possesses a positive adiabatic electron affinity and (b) low energy attachment is partly channeled into nondissociative processes when the molecule is coupled to an environment. Electron impact to condensed phase SF5CF3 exhibits a remarkably strong F− desorption signal appearing from a pronounced resonance located at 11 eV while in the gas phase at 11 eV only a weak DEA signal is observed. Electron induced desorption from sub...

Synthesis of Cl2 induced by low energy (0–18 eV) electron impact to condensed 1,2-C2F4Cl2 molecules

The interaction of low energy electrons (0–18 eV) with C2F4Cl2 molecules condensed in multilayer amounts on a cryogenically cooled Au substrate generates Cl2, as can be seen from the energy and temperature dependence of the Cl− desorption signal. The cross section for Cl2 formation exhibits two pronounced resonant features with maxima near 0 and 10 eV, dropping to essentially zero in the energy range between the resonances (near 3 eV). The energy dependence of the reaction cross section qualitatively follows that of Cl− desorption, which itself can be correlated to dissociative electron attachment (DA) processes in the gas phase. While at 10 eV excited negative ion resonances and electronically excited states of neutral C2F4Cl2 may be involved in the process, in the low energy region (<2 eV) the reaction can nonly be initiated by dissociative electron attachment. Possible reaction pathways are discussed.

Electron stimulated desorption of Cl− from adsorbed and condensed Cl2: Effects of environment and orientation

Electron stimulated desorption (ESD) of Cl(-) from condensed molecular chlorine in the energy range 0-15 eV is studied. Cl2 is deposited in either multilayer amounts directly on a cryogenically cooled gold crystal or in sub-monolayer quantities on rare gas films (Xe, Kr) or ammonia ice films. Cl(-) desorption from multilayer films shows an intense resonance peaking at 5.5 eV and a comparatively smaller feature at 3 eV in qualitative agreement with an earlier ESD experiment. The desorption signal is enhanced by about one order of magnitude when a 0.2 monolayer (ML) Cl2 is adsorbed on a multilayer rare gas film. In this case, the desorption signal shows two clearly separated resonances peaking at 2.5 and 5.5 eV closely resembling dissociative electron attachment (DEA) from gas phase Cl2. These resonances can be associated to the transitions Cl2(1Sigmag+) --> Cl2(-)(2Pig) and Cl2(1Sigmag+) --> Cl2(-)(2Piu), respectively, both final states representing core excited resonances. The shape of the resonance around 5.5 eV splits into different peaks when changing from grazing incidence of the electron beam to an impact angle of 45 degrees with respect to the surface normal. On the basis of the pronounced angular dependence of the Cl(-) intensity reported from gas phase DEA this observation is compatible with a situation in which the molecules are oriented along the surface normal. Compared to the noble gas films, ESD from sub-monolayer Cl2 on top of a multilayer NH3 film is suppressed while the overall shape of the yield function is approximately preserved. None of the present experiments show a Cl(-) desorption signal below 2 eV while the charging behaviour of the film indicates that electron attachment is still operative in this energy domain. This suggests that the transient anion in its electronic ground state (Cl2(-)#(2Sigmau+)) is still formed by low energy electron attachment but is subjected to effective energy dissipation creating either stabilized ions (Cl2(-)(2Sigmau+)) or fragment ions (Cl(-) (2P)) with insufficient kinetic energy to leave the surface.

Low Energy (< 3eV) Electrons as a Soft Tool for Surface Modification

One of the ultimate goals in chemistry has always been to prepare a molecular system in a particular way in order to induce a specific bond cleavage which is one of the necessary prerequisites to control a chemical reaction. We mention mode selective multi-photon excitation by IR lasers or state selective excitation in the UV region. The selectivity for bond breaking, however, is often deteriorated by effective energy redistribution within the molecule or, in the case of molecules at surfaces and in the condensed phase, by energy dissipation. A very timely subject of research along that direction is the control of a reaction by the application of ultra-short laser pulses. The idea here is to find the optimum pulse shape (and hence the optimum phase correlation between the frequencies) by means of evolutionary algorithms to push a reaction into the desired direction1,2. Here we demonstrate that the direction and extend of a condensed phase reaction can effectively be controlled by low energy electrons.