Atomic and Molecular Collisions Group

 Electron-Impact Ionization



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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



Examples of processes studied include:

e- + Ar --> Ar+ + 2e-  
e- + SO2 --> SO2+ + 2e-


e- + NO2 --> NO+ + O + 2e-

Electron-Impact Ionization Apparatus

Apparatus schematic

Apparatus schematic

The electron-impact ionization apparatus is shown schematically above. It comprises an electron gun, a time-of-flight mass spectrometer with position-sensitive detector (PSD) and incorporates an absolute pressure gauge (not shown). During a cross section measurement the entire vacuum chamber is filled with the target gas at a pressure of approximately 3 x 10-6 Torr. The electron gun produces a series of 20-ns long pulses at a repetition rate of 2.5 kHz. These electrons travel through the interaction region which is located between two plates maintained at ground potential, and are collected in the Faraday cup. Approximately 200 ns after each electron pulse, a 480-V/cm electric field is applied across the interaction region driving positive ions formed by electron impact towards the bottom plate. Some ions pass through the grid-covered aperture, are accelerated and impact the position-sensitive detector (PSD). The detector records their arrival times and positions. The arrival times are used to identify the mass-to-charge ratios of the ions and the arrival positions provide information on the effectiveness of ion collection. From knowledge of the measured quantities we obtain the electron-impact ionization cross sections for production of both parent and fragment ions. For further information see Straub et al. (1995) and Straub et al. (1996).

Apparatus photos

View of the electron-impact ionization apparatus


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Updated May 5, 2005