Pressing to the extremes

In the last decade the study of ion-neutral reactions has been dramatically advanced by the development and energetic exploitation of the steady-state flowing afterglow method by Eldon Ferguson and his colleagues at the National Oceanic and Atmospheric Administration (NOAA), Boulder, Colorado. This new technique has provided a very large number of ion–molecule rate coefficients for reactions at thermal energy.

In this method a fast flow of carrier gas (usually helium) is established in a long flow tube, the reactant ions (A⁺) are generated near the gas input end of the tube (the excitation region) and these ions are carried along the tube in the carrier gas flow. The neutral reactant (B) is introduced into the flowing gas stream near the mid-point of the flow tube so that the reaction A⁺+ B →products, takes place along the remainder of the tube length (the reaction region). The reactant ion A⁺ and any ions produced in the reaction are monitored by a mass spectrometer located at the gas exit end of the tube. The variation of the A⁺ mass spectrometer signal as a function of neutral reactant injection rate B yields the rate coefficient k for the reaction if gas flow rates and tube dimensions are known.

The great advantage and versatility of the steady-state flowing afterglow system lies primarily in the separate control that can be exercised over the ions and the neutrals prior to the reaction and also to the fact that the atomic processes occurring in the afterglow are susceptible to many diverse forms of investigation such as emission and absorption spectroscopy, laser spectroscopy, microwave interferometry and Langmuir probes, as well as mass spectrometry. Furthermore, chemically unstable neutral particles can be readily studied in these systems.