Electron swarm coefficients in THF and THF-H₂O mixtures. Negative-ion drift and clustering in THF

The flux electron drift velocity and longitudinal diffusion coefficients, the effective ionisation coefficients and the reaction rates have been measured for Tetrahydrofuran (THF, C 4 H 8 O) and its mixtures with H 2 O in the gaseous phase, using a Pulsed Townsend Apparatus. A wide range of gas pressures up to 90 Torr, and of the density-normalized electric field intensity, E/N, from 1 to 260 Td (1 Townsend=10 -17 V cm 2 ) were covered. The electron drift velocities and effective ionisation coefficients for pure THF have been fully re-measured, particularly those belonging to the electron attachment region. The measured electron drift velocity curves of the THF-H 2 O mixtures show a peculiar crossover at around E/N=65 Td, for which a thorough explanation is offered, using the two-term simulation code Bolsig+, and a selected group of electron-THF and electron-H 2 O cross sections. The electron drift velocities for THF, H 2 O and their mixtures were calculated successfully with mean deviations from the measured values with a correlation coefficient above 0.99. The electron attachment coefficient in THF was measured from 1 to 70 Td. We found no clear indication of a negative-ion dissociation region and provide an explanation to this finding. The longitudinal diffusion coefficient and the effective ionisation rates for the THF-H 2 O mixtures containing 10% to 90% H 2 O were also measured. The drift and reactions of negative ions in pure THF were measured over the low E/N range 8-14 Td. The clear pressure dependence of the measured negative-ion drift velocities and reduced mobilities, led us to formulate a simple theoretical model to calculate the measurable current, thereby rendering an apparent clustering coefficient. No mass identification was possible.