24 research outputs found
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 – 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 H−, 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
Quantum Control of Photodissociation via Manipulation of Bond Softening
We present a method to control photodissociation by manipulating the bond
softening mechanism occurring in strong shaped laser fields, by varying the
chirp sign and magnitude of an ultra-short laser pulse. Manipulation of
bond-softening is experimentally demonstrated for strong field (795 nm, 10^12 -
10^13 W/cm^2) photodissociation of H2+, exhibiting substantial increase of
dissociation by positively chirped pulses with respect to both negatively
chirped and transform limited pulses. The measured kinetic energy release and
angular distributions are used to quantify the degree of control of
dissociation. The control mechanism is attributed to the interplay of dynamic
alignment and chirped light induced potential curves.Comment: 4 pages, 4 figure
Electric-field-induced change of alkali-metal vapor density in paraffin-coated cells
Alkali vapor cells with antirelaxation coating (especially paraffin-coated
cells) have been a central tool in optical pumping and atomic spectroscopy
experiments for 50 years. We have discovered a dramatic change of the alkali
vapor density in a paraffin-coated cell upon application of an electric field
to the cell. A systematic experimental characterization of the phenomenon is
carried out for electric fields ranging in strength from 0-8 kV/cm for
paraffin-coated cells containing rubidium and cells containing cesium. The
typical response of the vapor density to a rapid (duration < 100 ms) change in
electric field of sufficient magnitude includes (a) a rapid (duration of < 100
ms) and significant increase in alkali vapor density followed by (b) a less
rapid (duration of ~ 1 s) and significant decrease in vapor density (below the
equilibrium vapor density), and then (c) a slow (duration of ~ 100 s) recovery
of the vapor density to its equilibrium value. Measurements conducted after the
alkali vapor density has returned to its equilibrium value indicate minimal
change (at the level of < 10%) in the relaxation rate of atomic polarization.
Experiments suggest that the phenomenon is related to an electric-field-induced
modification of the paraffin coating.Comment: 15 pages, 15 figure
Dissociative electron attachment to acetaldehyde, CH<sub>3</sub>CHO. A laboratory study using the velocity map imaging technique
A detailed experimental investigation of the dissociative electron attachment (DEA) process to acetaldehyde, CH3CHO is presented. To investigate this process we use a time of flight spectrometer coupled with the velocity slice imaging technique. DEA in CH3CHO is found to lead to the formation of CH3−, O−, OH−, C2H−, C2HO− and CH3CO− 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. CH3CO−, 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
Velocity imaging of H
Dissociative electron attachment (DEA) to formic acid is investigated using velocity slice imaging of H− ions. From the momentum distributions, we infer that the broad peak observed in absolute cross section of H− between 6 and 12 eV has contribution from three resonances. The resonance below 7 eV shows angular distribution fairly close to that seen from the B1 resonance in water and the first peak of the two peaks from the hydroxyl site of acetic acid. The resonance observed around 9 eV appears to have definite contribution from the C-site of the molecule, as identified from the momentum distribution and similarity with other organic molecules including methane. The analysis of the kinetic energy distribution shows that the two resonances below 8 eV are dominated by the contribution from hydroxyl site of the molecule. The results indicate that formic acid tends to behave like other small carboxylic acids and alcohols and displays functional group dependence and corresponding site specificity in the DEA process, to a large extent