The Pulse Experiments of Warneck

Warneck has carried out an extensive series of experiments in which he used a pulsed nitrogen-spark light source to determine drift velocities, diffusion coefficients, and kinetic temperatures as well as reaction rates of ions in gases. These have been the only photoionization experiments in which a pulse technique has been effectively used in the study of ion-molecule reactions. The reactant ions were formed by repetitive light pulses of about 0.5-//sec average duration, which (in all cases except hydrogen) was sufficiently short to justify the approximation of instanta- 

The distinctive features of Warneck s photoionization technique are the pressure range covered (up to 0.2 Torr) and the direct measurement of ion residence times. The capability of working at high pressures makes possible the study of reactions with low rates, even termolecular association reactions. The residence time may be varied considerably and well-defined ion temperatures and drift velocities established at the higher pressures. The direct measurement of residence time eliminates certain errors which can occur in the calculation of this quantity—e.g., the electric field may be affected to an unknown extent by surface charges, space charge, contact potentials, and electric field penetration. The rate constant is directly determined from measured values of the ion residence time and of the initial and final concentrations of reactants or of products or of both. 

The use of photoionization rather than electron impact by Warneck resulted in better control of reaction conditions. While no systematic variation of internal energy was attempted, the ionizing wavelengths chosen produced the desired reactant ion in its electronic ground state and with at most only a few quanta of vibrational energy. The typical photon energy spread was about 0.1 eV. 

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