Dynamics of dissociative electron attachment to aliphatic thiols.

Dissociative electron attachment (DEA) shows functional group-dependent site selectivity in H- ion channels. In this context, thiol functional groups have yet to be studied in great detail, although they carry importance in radiation damage studies where low-energy secondary electrons are known to induce damage through the DEA process. In this context, we report detailed measurements of absolute cross-sections and momentum images of various anion fragments formed in the DEA process in simple aliphatic thiols. We also compare the observed dynamics with that reported earlier in hydrogen sulphide, the precursor molecule for this functional group, and with that in aliphatic alcohols. Our findings show substantial resemblance in the underlying dynamics in these compounds and point to a possible generalisation of these features in the DEA to thiols. In addition, we identify various pathways that contribute to the S- and SH- channels.

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.

DEA dynamics of chlorine dioxide probed by velocity slice imaging.

We report, for the first time, the detailed dynamics of dissociative electron attachment to the atmospherically important chlorine dioxide (OClO) molecule exploring all the product anion channels. Below 2 eV, the production of vibrationally excited OCl- dominates the DEA process whereas at electron energies greater than 2 eV, three-body dissociation is found to result in O- and Cl- production. We find that the internal energy of OCl- and the kinetic energy of Cl- are large enough for them to be relevant in the ozone-depleting catalytic cycle and more investigations on the reaction of these anions with ozone are necessary to completely understand the role of DEA to OClO in ozone depletion. These results also point to an urgent need for comprehensive theoretical calculations of the DEA process to this atmospherically important molecule.

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.

Observation of Quantum Interferences via Light-Induced Conical Intersections in Diatomic Molecules.

We observe energy-dependent angle-resolved diffraction patterns in protons from strong-field dissociation of the molecular hydrogen ion H_{2}^{+}. The interference is a characteristic of dissociation around a laser-induced conical intersection (LICI), which is a point of contact between two surfaces in the dressed two-dimensional Born-Oppenheimer potential energy landscape of a diatomic molecule in a strong laser field. The interference magnitude and angular period depend strongly on the energy difference between the initial state and the LICI, consistent with coherent diffraction around a cone-shaped potential barrier whose width and thickness depend on the relative energy of the initial state and the cone apex. These findings are supported by numerical solutions of the time-dependent Schrödinger equation for similar experimental conditions.

Dissociative electron attachment to N2O using velocity slice imaging.

The structure and dynamics of the negative ion resonances leading to dissociative electron attachment in N2O are studied using the velocity slice imaging technique. Distinct momentum distributions are observed in the O(-) channel for the dominant resonances below 4 eV which are considerably different than those reported so far. Also the relatively weak but distinct resonances at 8.1 eV and 13.2 eV are studied for their dynamics for the first time. For each of these resonances two different channels of dissociation are observed with differing angular distributions.

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.

Dissociative electron attachment studies on acetone.

Dissociative electron attachment (DEA) to acetone is studied in terms of the absolute cross section for various fragment channels in the electron energy range of 0-20 eV. H(-) is found to be the most dominant fragment followed by O(-) and OH(-) with only one resonance peak between 8 and 9 eV. The DEA dynamics is studied by measuring the angular distribution and kinetic energy distribution of fragment anions using Velocity Slice Imaging technique. The kinetic energy and angular distribution of H(-) and O(-) fragments suggest a many body break-up for the lone resonance observed. The ab initio calculations show that electron is captured in the multi-centered anti-bonding molecular orbital which would lead to a many body break-up of the resonance.

Dissociative electron attachment to acetaldehyde, CH3CHO. A laboratory study using the velocity map imaging technique.

A detailed experimental investigation of the dissociative electron attachment (DEA) process to acetaldehyde, CH(3)CHO is presented. To investigate this process we use a time of flight spectrometer coupled with the velocity slice imaging technique. DEA in CH(3)CHO is found to lead to the formation of CH(3)(-), O(-), OH(-), C(2)H(-), C(2)HO(-) and CH(3)CO(-) anionic products produced through scattering resonances in the electron energy range of 6 to 13 eV. Of these product ions only O(-) is formed with any measurable kinetic energy distribution indicating a two-body dissociation process. CH(3)CO(-), although formed with very low kinetic energy, shows anisotropy in the velocity slice image, indicating ejection of the H atom in the 180° direction with respect to the electron beam. The low kinetic energy distributions and absence of any anisotropy in the angular distributions of the other product ions indicate that they are formed through multiple fragmentation of the transient molecular negative ion. The angular distribution of O(-) is analysed in terms of the various partial waves.

Inelastic electron scattering induced quantum coherence in molecular dynamics.

Quantum coherence is pivotal in various applications ranging from chemical control to quantum computing. An example of its manifestation in molecular dynamics is inversion symmetry breaking in the photodissociation of homonuclear diatomic molecules. On the other hand, the dissociative attachment of an incoherent electron also induces such coherent dynamics. However, these processes are resonant and occur for projectiles with a specific energy. Here we present the most general scenario of non-resonant inelastic electron scattering inducing such a quantum coherence in molecular dynamics. The ion-pair formation (H+ + H─) that proceeds after the electron impact excitation of H2 shows a forward-backward asymmetry about the incoming electron beam. Simultaneous transfer of multiple angular momentum quanta during the electron collision induces the underlying coherence in the system. The non-resonant nature of this process makes this effect generic and points to its possible prevalent role in particle collision processes, including electron-induced chemistry.

Dissociative electron attachment to NO probed by velocity map imaging.

An experimental and theoretical investigation of the dissociative electron attachment process in nitric oxide is presented. Measurements using the recently developed ion momentum imaging conclusively show the presence of two resonance features in the O(-) channel. These are found to dissociate to give N atoms in the (2)D and (2)P excited states respectively, thus settling the controversies regarding the possible dissociation limits of this process. Though the angular distribution of O(-) shows the resonances contributing to these dissociations are of Π symmetry and a mixture of Π and Σ or Δ symmetry respectively, our calculations using R-matrix theory show no direct electron attachment channel leading to O(-) through these resonances, as all the allowed resonances below 10 eV decay to either O + N(-) or O(-) + N((4)S) channels. We propose that indirect mechanisms through curve crossings lead to the experimentally observed results.

Functional group dependent dissociative electron attachment to simple organic molecules.

Dissociative electron attachment (DEA) cross sections for simple organic molecules, namely, acetic acid, propanoic acid, methanol, ethanol, and n-propyl amine are measured in a crossed beam experiment. We find that the H(-) ion formation is the dominant channel of DEA for these molecules and takes place at relatively higher energies (>4 eV) through the core excited resonances. Comparison of the cross sections of the H(-) channel from these molecules with those from NH(3), H(2)O, and CH(4) shows the presence of functional group dependence in the DEA process. We analyze this new phenomenon in the context of the results reported on other organic molecules. This discovery of functional group dependence has important implications such as control in electron induced chemistry and understanding radiation induced damage in biological systems.

Functional group dependent site specific fragmentation of molecules by low energy electrons.

Functional group dependence is observed in the dissociative electron attachment (DEA) to various organic molecules in which the DEA features seen in the precursor molecules of the groups are retained in the bigger molecules. This functional group dependence is seen to lead to site-selective fragmentation of these molecules at the hydrogen sites. The results are explained in terms of the formation of core-excited Feshbach resonances. The results point to a simple way of controlling chemical reactions as well as interpreting the DEA data from bigger biological molecules.