Low energy electron collisions with effusive and jet-cooled molecular beams

Jana, Irina (2020) Low energy electron collisions with effusive and jet-cooled molecular beams. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Irina Jana (14RS047)) 14RS047.pdf - Submitted Version Restricted to Repository staff only Download (8MB)

Abstract

Low-energy electron interaction with neutral molecules plays an exciting and vital role in different fundamental fields starting from plasma processing, atmospheric chemistry, radiation damage to biologically relevant molecules. When the incoming electron has low energy (below 20 eV), electron- molecule interaction is characterized by electron attachment and hence the formation of a temporary negative ion (TNI). In this resonance process, the TNI is generally formed in an excited state and is thus bound to relax either through re-emission of the electron, i.e., auto-detachment (AD) or through dissociation of a chemical bond, i.e., dissociative electron attachment (DEA). All the experimental work reported in this thesis for electron collision studies with effusive and jet-cooled molecular beams is performed using a time-of-flight (ToF) based mass spectrometer with a pulsed electron and molecule crossed-beam setup satisfying the VMI principle. A pulsed electron beam is produced from a home-made electron gun with a tungsten filament which is heated using a 2.1 A constant current source emitting electrons by the process of thermionic emission. The electron gun can produce electrons in the energy range 0 to 60 eV with a resolution of about 0.8 eV. The electron beam has been collimated using a pair of Helmholtz coils with a magnetic field of about 40 Gauss and pulsed with a frequency of 10 kHz (for effusive beam) and 1.5 kHz (for supersonic beam) to ensure that no electrons reach the detector. The current of the electron gun is constantly monitored using a Faraday cup placed at the bottom of the chamber co-axially with the electron gun and having a voltage of +40 V. The target is either an effusive beam of molecules coming out of a 1 mm diameter capillary needle. Or, for electron attachment studies with a supersonic jet of molecules a high-pressure gas is passed through a 200 mm nozzle into a low-pressure environment to undergo a quasi-static and adiabatic expansion, forming a supersonic jet. A 390 mm skimmer is placed inside the isentropic region to extract the centerline beam consisting of the coldest jet of molecules. The electron beam and the molecular beam (both effusive and jet-cooled) are made to collide in a crossed-beam configuration under single-collision condition. The base pressure of the chamber is maintained at ~ 10⁻⁹ mbar, while the pressure in the chamber is ~ 10⁻⁷ mbar with the flow of the molecular beam. The thesis is organized as follows. In Chapter 1, we introduce the concepts of electron-molecule collisions. In Chapter 2, we introduce the experimental setup used to study low energy electron collision processes with an effusive beam of molecules using the well-established VMI technique. In Chapter 3 of this Ph.D. thesis, the dynamics of DEA processes involving single and multiple bond ruptures, bond formation, and rearrangement are addressed through relative electron attachment cross-section and kinetic as well as angular distribution data. In Chapter 4 of this thesis, the structures of SO₂, SO⁻₂, CO₂ and CO⁻₂ as well as the adiabatic electron affinities of the corresponding SO₂ and CO₂ neutral parents are computed using the infinite-order coupled-cluster method with all singles and doubles and non-iterative inclusion of triple excitations (CCSD(T)) andMfller-Plesset perturbation theory up to second order (MP2). We then shift our focus to electron attachment processes in jet-cooled molecules and weakly bound van der Waals clusters in Chapter 5. In Chapter 6, we finally describe the main findings of the thesis and future prospect of research in the domain of DEA to an effusive and jet-cooled beam of molecules and weakly bound van derWaals clusters for some simple diatomic and tri-atomic molecules and the potential of the VMI spectrometer in electron-attachment studies. The research outputs presented in this thesis thus bring new insight on electron-molecule collision processes and indicate considerable promise for developing DEA to polyatomic and jet-cooled molecules and clusters for potential applications. It is hoped that the research will stimulate further research in the exciting fields of dissociative electron attachment and velocity map imaging which still has many unknown avenues to explore.

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