Electron-capture and -loss Measurements

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Introduction The processes studied involve electron capture or electron loss by a neutral projectile in collision with a gaseous target. Examples include: O + H2 --> O+ + H2 + e-   - Electron loss (or stripping)    Data O + H2 --> O- + H2+   - Electron capture    Data For charge transfer measurements, a single detector is adequate because the primary ions can be deflected allowing the product neutrals to be measured. However, for the capture and loss measurements, two detectors are needed: one to measure the primary neutral beam and a second to measure the charged products. The measurements are more complex than those for charge transfer or direct scattering because the two detectors must be calibrated relative to each other.   Electron-capture and -loss Apparatus The schematic below represents the apparatus used to measures capture and loss cross sections. Schematic representation of the apparatus Ions are extracted from a low-pressure plasma-type ion source, accelerated to the desired energy (0.5- 5keV) and focused by an electrostatic lens. The ion beam is then mass-selected by a pair of 60 degree sector magnets and passes through a charge transfer cell where some of the ions are converted to neutrals. The neutral beam is then collimated by a pair of apertures before traversing the target cell. A position-sensitive detector (PSD) on the beam axis approximately 68 cm beyond the target cell is used to monitor the primary neutral beam. Positive or negative product ions are deflected through an angle of approximately 5 degrees and detected with a second PSD. The pressure in the target cell (typically 10-30 mtorr) is chosen to ensure that single collision conditions obtain. The relatively short target cell length, approximately 1 mm, ensures that the collisions occur within a very well defined region and as the PSD records the position of each incident ion the scattering angle is easily obtained. The number density of the target gas is obtained from a measurement of the target gas pressure using a capacitance diaphragm gauge. Knowledge of the target cell length, the target number density, the primary beam flux, and the flux and position of the scattered ions allows us to determine the absolute differential and integral electron transfer cross sections. For further details see Lindsay et al., Phys. Rev. A 70, 042701 (2004).   Physics and Astronomy | Rice Quantum Institute| Rice Space Institute Rice University   Updated May 5, 2005