Electron scattering with ethane adsorbed on rare gas multilayers: Hole transfer, coulomb decay, and ion dissociation.

Positive ion desorption following electron impact dissociative ionization of ethane adsorbed on Ar, Kr, and Xe multilayers has been studied as a function of incident electron energy from threshold to 100 eV. Based on the dependence of ion yields on the identity of the rare gas, it is likely that the majority of ethane molecules undergo indirect ionization following hole transfer from the ionized underlying rare gas. This has also been corroborated by density of states calculations showing the energetic alignment of the outer valence states of ethane and the condensed rare gas ionization energies. Due to the near-resonant nature of charge transfer for single-hole states, the ethane molecular ion is excited to different final ionic states on different rare gases, which leads to differences in ion desorption yields and branching ratios. The quantitative yields increase with increasing ionization energy gap between the rare gas and ethane, in the order Ar > Kr > Xe. The large increase in yields from 25 eV onwards for all rare gases is likely due to the formation and decay of two-hole states on neighboring rare gas and ethane molecules due to interatomic and intermolecular Coulomb decay (ICD) and not electron transfer mediated decay (ETMD). The ICD and ETMD pathways become accessible when the incoming electron has sufficient energy to excite the inner valence ns level of the rare gas to a Rydberg state or ionize it. The experimental findings are supported by calculations of thresholds, density of states for the final configurations of these processes, and coupling strengths for hole transfer between ethane and rare gases. The fragment ion branching ratios vary with energy from threshold to about 35 eV, showing the fragmentation pattern changes with the mode of hole transfer and availability of excess energy. Sigma C-C bonds are more likely to break than C-H bonds in the mid-20 eV range, and this effect is most pronounced for Xe, followed by Kr, and then Ar.

Origin of Resonant Character in the Electron Impact Two-Body Neutral-Fragmentation of Methane.

The observation of peaks in the threshold region of two-body neutral fragmentation of methane molecule, i. e., CH4 →CH3 +H, by low energy electron (LEE) impact has been an enigma. The prevailing explanation that this resonant behavior is due to excitation energy transfer is unsatisfactory since this process is not expected to show peaks in the cross-sections unless there is the involvement of electron-molecule resonances. Our first-principles calculations now reveal that the observed peaks could be explained as due to the formation of negative ion resonances, which dominantly dissociate into two neutral fragments and a free-electron. This case of methane is a pointer to the possibility that such reactions contribute significantly to neutral radical production from molecules by LEE impact in comparison to dissociative electron attachment, and in general could play a significant role in electron-based chemical control.

Direct Damage of Deoxyadenosine Monophosphate by Low-Energy Electrons Probed by X-ray Photoelectron Spectroscopy.

Low-energy (3-25 eV) electron interactions with multilayers of 2'-deoxyadenosine 5'-monophosphate (dAMP) were probed using X-ray photoelectron spectroscopy (XPS). Understanding how electrons damage the nucleotide dAMP, which is a building block of DNA, can give insight into how the DNA undergoes radiation damage. Chemical modifications to the constituent units of the nucleotide were revealed in situ through monitoring of the O 1s, C 1s, and N 1s elemental transitions. It is shown that direct electron irradiation causes decomposition of both the base and sugar subunits, as well as cleavage of glycosidic and phosphoester bonds. Incident electrons undergo inelastic energy losses, including creation of core-excited resonances above 3-4 eV. In the condensed phase, these resonances decay via autoionization, producing electronically excited targets and <3 eV electrons. The excited states dissociate and the slow (<3 eV) electrons are captured by neighboring molecules, forming molecular shape resonances that can lead to bond rupture. Since the observed chemical changes were similar at all incident electron energies studied, they can be primarily attributed to formation and decay of transient negative ions. Damage enhancements in the energy ranges typical of all scattering resonances are expected, with the damage probability dominated by the low-energy shape resonances.

Formation of CO2 from formic acid through catalytic electron channel.

Low energy electrons can initiate and control chemical reactions through resonant attachment forming an electron-molecule compound state. Recently, it has been theoretically shown that free electrons can also act as catalysts in chemical reactions. We investigate this novel concept for the case of conversion of formic acid into CO2. Resonant production of CO2 from cold formic acid films by low energy electron impact is observed using Fourier transform infrared spectroscopy. The resonant peak observed at 6 eV is identified as the catalytic electron channel. The experimental results are augmented with the ab initio quantum chemical calculations.

Electron induced reactions in condensed mixtures of methane and ammonia.

We demonstrate the efficient formation of carbon-nitrogen bonds starting from CH4 and NH3 on a metal surface at cryogenic temperatures. Electrons in the energy range of 1-90 eV are used to initiate chemical reactions in mixed molecular films of CH4 and NH3 at ∼15 K, and the products are detected by performing temperature programmed desorption (TPD). Extensive dehydrogenation occurs at all energies giving the products CH2NH and HCN in preference to CH3NH2. This is likely to do with the energetics of the reactions and the subsequent stability of these species in the condensed film. Thermal processing of the irradiated mixture favours dehydrogenation as indicated by the results of using different desorption rates. Electron impact excitation and subsequent dissociation into radicals is the reaction-initiating step rather than ionization of CH4 and NH3, as inferred from the yield of products as a function of electron energy. This could give insight into the important catalytic process of the industrial scale synthesis of HCN from CH4 and NH3 over Pt. This may also be a relevant pathway in the astrochemical environment where CN and HCN are abundant and low-energy electrons are found ubiquitously.

O(-) from amorphous and crystalline CO2 ices.

O(-) desorbed from amorphous and crystalline films of CO2 at 18 K under low energy electron impact is studied using time of flight mass spectrometry. The nature of the CO2 film is characterized by Fourier transform infrared spectrometry as a function of film thickness. It is found that the desorption rate from amorphous films is considerably larger than that from crystalline films. The desorption signal from the 4 eV resonance is found to be the dominant one as compared to that from the higher energy resonances, notably the one at 8 eV observed in the gas phase. This is explained in terms of the large enhancement in the dissociative electron attachment cross section for the 4 eV resonance in the condensed phase reported earlier using the charge trapping method.