Atomic and Molecular Collisions Group

 Heavy Particle Collisions


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Heavy Particle Collisions

Electron-Impact Ionization



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Charge Transfer

Electron Capture and Loss

Direct Scattering

Electron-Impact Ionization



Processes Studied

 Examples include:
O + H2 --> O + H2   - Elastic/direct scattering  


H+ + O2 --> H + O2+   - Charge transfer


O + H2 --> O+ + H2 + e-   - Electron loss (or stripping)  


O + H2 --> O- + H2+   - Electron capture  


For a particular process, the cross section and the differential cross section (DCS) are measured at 3-5 projectile energies in the 0.5-5 keV range. The charge transfer apparatus is described below. The same basic apparatus is used to measure direct scattering cross sections, although the analysis differs somewhat.

The electron-capture and -loss apparatus, is significantly different from that used for charge transfer and direct scattering and is described separately .


Charge Transfer Apparatus

The schematic below represents the charge transfer scattering apparatus. For stable targets, such as N2, a simple target cell is adequate.

Charge transfer apparatus

Schematic of the charge transfer 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 pair of collimating apertures before traversing the target cell.
  • A position-sensitive detector (PSD) on the beam axis 26 cm beyond the target cell is used to monitor both the primary ion beam and the fast neutral atoms resulting from charge transfer collisions.
  • The pressure in the target cell (typically 10 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 neutral particle 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 neutrals allows us to determine the absolute differential and integral charge transfer cross sections.
  • A brief outline of the data analysis is given with the typical charge transfer results. For further details see Lindsay et al., Phys. Rev. A 53, 212 (1996).



 Atomic Oxyen Target Apparatus

In order to conduct scattering experiments with atomic oxygen it is necessary to have some sort of well defined atomic oxygen target. However, it is very difficult to produce a well characterized atomic oxygen target because oxygen atoms recombine very quickly to form molecules. We have overcome this problem by having a constant flow of partially dissociated oxygen gas pass through a target cell. The oxygen gas is initially dissociated by a microwave discharge and then flows through Teflon conduit (which inhibits recombination) to the target region. The atomic oxygen pressure in the target cell is determined using a calibrated mass spectrometer which was specially designed for the purpose.

The picture below shows the interior of the atomic oxygen target apparatus. Only the target region, mass spectrometer, and detector housing are shown. The ion source and the magnets (just to the left of the picture) are not shown. During scattering measurements the ion beam enters the chamber from the left passes through the target cell and impacts the detector on the right. The spectrometer acceleration stage (segmented structure) must be rotated off the ion beam axis during these measurements.When scattering measurements are not being performed this stage is manually rotated into the position shown and the mass spectrometer can then be used to sample the gas effusing from the target cell exit aperture. For further details see Lindsay et al., Phys. Rev. A 53, 212 (1996).

Atomic oxygen apparatus

Atomic oxygen target scattering apparatus

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Updated May 3, 2005