Absolute Dissociative Electron Attachment Cross Section Measurements of Sulphur Dioxide Molecule

Sandeep, . (2017) Absolute Dissociative Electron Attachment Cross Section Measurements of Sulphur Dioxide Molecule. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

The aim of this Master thesis is to study the absolute dissociative electron attachment (DEA) cross section of the gaseous molecules in crossed beam electron molecule collision experiment using relative ow technique (RFT). When a low energy electron (≤ 15eV ) collides with a molecule, the electron may get attached temporarily to the molecule thus forming a temporary negative ion (TNI) state. This being a resonance, gives sharp structure in the cross section curve. The TNI may decay via two processes depending on it's lifetime. The TNI may either autodetach giving an electron and the neutral molecule which may be in it's ground state or in some excited state. But if the life time of the TNI is sufficiently high compared to it's vibrational time period, it may dissociate forming a negative ion and a neutral fragment; this process is DEA. In the cross beam experiments, the absolute cross section measurement is not a straightforward procedure since it requires a very accurate measurement of the target density and the electron current density in the interaction volume, the overlap volume of the electron molecule beam and the ion count rate of each fragment. The ion count rate is strongly dependent on the various parameters like ion detection and transportation effciency and the voltage conditions of the spectrometer (electric field), so we use the RFT, in this process the experiment is performed with two gases without changing experimental conditions, among these two gases DEA cross section of one gas is well established (by others method). By taking ratio of unknown to known (standard) gases all the the geometric factor will cancel out, like in the electron flux density (fele) is current per unit area. Therefore the ratio of two different measurements (unknown and standard) are taken. So the ratio of currents remains and other geometric factors get cancelled. Similarly in the ratio of molecular ow rate survives and other terms get cancelled. RFT is a very important technique for the measurement of absolute dissociative electron attachment (DEA) cross section. DEA to sulphur dioxide to produced O⁻, SO⁻ and S⁻ fragment ions below ≤ 10 eV. We have studied the DEA to sulphur dioxide it gives the three fragment ions, O⁻, SO⁻ and S⁻ at different energies and the absolute DEA cross section section of O⁻ and SO⁻ is measured using RFT. The Cross section curve obtained for O⁻/SO₂ showed two peaks at 4.7 eV and 7.6 eV and for SO⁻/SO₂ also has two peaks one at 4.6 eV and other a broad peak at 7.5 eV. The cross section values of sulphur dioxide is 3.91 (10⁻¹⁸ cm²) in the case of O

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