@phdthesis{EbanezarJohn2014,
  author    = {Angelin Ebanezar John},
  title     = {Negative Ion formation in Ion-Molecule and Ion-Surface collisions},
  journal    = {Negativ-Ionen-Bildung in Ion-Molek{\"u}l und Ion-Oberfl{\"a}chen Kollisionen},
  url       = {https://nbn-resolving.org/urn:nbn:de:gbv:9-002073-6},
  year      = {2014},
  abstract  = {Energetic ions are made to collide with atmospheric molecules. Positively charged ions of argon (Ar^+), helium (He^+), hydrogen (H\_2^+ ), and protons (H^+) with energies of 50 keV to 350 keV are used as the bombarding ion. The ion beam of desired energy is produced using a linear ion accelerator at the University of Greifswald. The mass and energy distribution of sputtered particles were analysed using an Electrostatic Quadrupole SIMS (EQS) analyser. The target gases used are oxygen (O\_2), sulfur hexafluoride (SF\_6), and nitrogen (N\_2). The ionized and fragmented particles due to collisions have been investigated. We have discovered a new process for negative ion formation in energetic ion collision with O\_2 and SF\_6 molecules. The process is a two body reaction between the projectile and the molecule without the need for a third particle (such as an external electron). It requires a direct charge transfer from the projectile to the molecule leaving it intact as O\_2^- or SF\_6^- . The process is experimentally confirmed by using a proton as projectile which does not have an electron to transfer. In comparison with positive ion fractions (O\_2^+ , SF\_5^+ ), the negative ions fraction is smaller by 2 orders of magnitude. This shows that the two body charge exchange process is weak due to the larger energy transfer required compared to the positive ion forming mechanisms. The two body charge exchange mechanism is not observed for ion collisions with N\_2 molecule. No stable negative ion exist for N\_2 molecule. The collision cross section for the ion formation during energetic ion – O\_2 collision has been determined within the investigated impact energy. For SF\_6 molecule the partial ion fraction of the secondary ions are determined for different projectiles involved. This kind of investigation is of great importance mainly in atmospheric physics. Energetic ions are constantly emitted from mass of the energy sources in the universe (e.g. sun). They interact with planetary objects or atmosphere on their way. A deep knowledge about the interaction processes is necessary to understand the ionospheric physics and space exploration. As second part of my thesis, a GaAs(100) surface is bombarded with 150 keV Ar^+ ion beam. From etching the surface to thin film coating, ion bombardment on solid surface found great role in the fabrication process of modern electronic and optical devices. In order to increase the knowledge on sputtering materials and because of profound importance in modern electronics, we choose GaAs(100) as our target. Among the sputtered atoms and ions, small sized cluster ions having more than 6 atoms have been identified. GaAs is a heteroatomic semiconductor containing gallium and arsenic in equal ratio. A preferential phenomenon of ’abundant sputtering’ of gallium compared to little arsenic (GaAs) has been investigated from their mass intensity. The experimental ion counts are compared with theoretically predicted relative abundance. This phenomenon of preferential sputtering is known for atomic species of sputtered GaAs but not for the sputtered cluster ions. The main reasons for this abundant sputtering of one element is attributed to the difference in ion formation energies and surface compositional change taking place during the sputtering process. Another notable characteristics is the preference in charge state among the sputtered ions. For instance, among sputtered atomic ions the ion counts of Ga^+ is 3 orders larger than As^+ ion and As^- is 2 orders larger than Ga^- ion. To get a clue for this behavior, we have investigated the energy distribution of both negatively and positively charged clusters. Different ion formation mechanisms were discussed. The energy distribution of atomic ion is partially explained by using a modified theory given by M. W. Thompson.},
  language  = {en}
}