140 research outputs found
Dissociation dynamics of transient anion formed via electron attachment to sulfur dioxide
We report the molecular dynamics of dissociative electron attachment to sulfur dioxide (SO2) by measuring the momentum distribution of fragment anions using the velocity slice imaging technique in the electron energy range of 2β10 eV. The S- channel results from symmetric dissociation which exhibits competition between the stretch mode and bending mode of vibration in the excited parent anion. The asymmetric dissociation of parent anions leads to the production of O- and SO- channels where the corresponding neutral fragments are formed in their ground as well as excited electronic states. We also identify that internal excitation of SO- is responsible for its low yield at higher electron energies
Dynamics of Dissociative Electron Attachment to Aliphatic Thiols
Dissociative electron attachment (DEA) shows functional group-dependent site
selectivity in the ion channel. In this context, the thiol functional
group has yet to be studied in great detail, although this functional group
carries importance in radiation damage studies where the low-energy secondary
electrons are known to induce damage through the DEA process. 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 also with 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. We also identify various pathways that contribute to the
and channels
Dynamics of the dissociative electron attachment to Ethanol
We report the detailed dynamics of the site selectivity observed in the
dissociative electron attachment (DEA) process in ethanol based on the momentum
images obtained using the velocity slice imaging technique. The H- dissociation
channel shows the site selectivity where the anion signal from the O-H site
peaks at 6.5 eV and 8 eV, and that from the C-H site peaks at 9.5 eV. The
momentum images also show the two-body dissociation dynamics for the O-H site
break-up. This dissociation channel shows a substantial effect of the torsion
mode of vibrations on the electron attachment process. In contrast, the C-H
site dissociation results from the many-body break-up consistent with the
earlier reports of DEA dynamics from organic molecules. We have also found that
the OH- channel has a resonance at 9.3eV and is produced with very little
kinetic energy. Using the isotope substitution, we show the role of H atom
scrambling in the C-O bond dissociation leading to the OH- channel. This
channel shows a substantial deviation from the corresponding photodissociation
dynamics
Electron attachment and quantum coherence in molecular hydrogen
Single electron attachment to a molecule may invoke quantum coherence in different angular momentum transfer channels. This has been observed in the 14 eV dissociative electron attachment resonance in molecular hydrogen where a coherent superposition of two negative ion resonant states of opposite parity is created, with the s and p partial waves of the electron contributing to the attachment process. Interference between the two partial wave contributions leads to a forward β backward asymmetry in the angular distribution of the product negative ions. Since these two resonant states dissociate to the same n = 2 state of H and Hβ, this asymmetry is further modified due to interference between the two paths of the dissociating molecular negative ion along different potential energy curves. This interference manifests as a function of the electron energy as well as isotopic composition. This case is akin to the quantum interference observed in photodissociation by one-photon vs two-photon absorption
Velocity slice imaging for dissociative electron attachment
A velocity slice imaging method is developed for measuring the angular distribution of fragment negative ions arising from dissociative electron attachment (DEA) to molecules. A low energy pulsed electron gun, a pulsed field ion extraction, and a two-dimensional position sensitive detector consisting of microchannel plates and a wedge-and-strip anode are used for this purpose. Detection and storage of each ion separately for its position and flight time allows analysis of the data offline for any given time slice, without resorting to pulsing the detector bias. The performance of the system is evaluated by measuring the angular distribution of O− from O2 and comparing it with existing data obtained using conventional technique. The capability of this technique in obtaining forward and backward angular distribution data is shown to have helped in resolving one of the existing problems in the electron scattering on O2
Effect of Background Signal on Momentum Imaging
The velocity Slice Imaging technique has revolutionised electron molecule
interaction studies. Multiple electrostatic lens assemblies are often used in
spectrometers for resolving low kinetic energy fragments. However, in a
crossed-beam experiment with an effusive molecular beam, the extended source of
ion generation due to the presence of the background gas creates artefacts on
the momentum images as we try to magnify them beyond a certain size. Here, we
present a systematic study of this effect on momentum imaging and the solutions
to address this issue by background subtraction with suitable magnification.
Additionally, we demonstrated that a supersonic molecular beam target helps
minimise these artefacts in the image magnification by reducing the background
signal. These systematic findings may bring valuable insight into the
investigation of low kinetic energy release processes involving electron
impact, ion impact, and merge beam experiments with large interaction volumes
where high magnification is needed
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
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