Linear electron molecule collision spectrometer for partial and total ionization cross-section measurements

A new spectrometer consisting of a trochoidal electron monochromator (TEM) and a time-of-flight (TOF) mass analyzer was built for measuring electron ionization cross sections of neutral gas atoms and molecules. A well-defined electron beam, produced by the monochromator, is crossed at right angles with a supersonic gas jet introduced to the collision region. Positive ions, arising from electron-atom or electron-molecule collisions, are extracted from the collision region along the external magnetic field direction needed for the TEM operation. The extracted ions are then collimated, focused, detected, and counted by the TOF mass spectrometer set in linear configuration with the monochromator. The device operates in a pulsed electron beam mode and a synchronized pulsed ion extraction mode. After passing the collision region, the electron beam is deflected aside to leave the spectrometer by a short voltage pulse in order to avoid unwanted ionization outside of the collision region. Beside its compact design, this linear spectrometer is expected to provide better extraction and collection efficiency than conventionally used devices that have the TOF axis oriented perpendicularly to the TEM magnetic field direction. This device has been successfully tested for electron ionization of N2. Preliminary results are presented in this paper.