Electron motion in the gases CF4, C2F6, C3F8, and n-C4F10.

Abstract

The drift velocity w of electrons has been measured in the perfluoroalkanes n-${\mathrm{C}}_{\mathrm{N}}$${\mathrm{F}}_{2\mathrm{N}+2}$ (N=1--4) over the density-reduced electric field (E/N) range 0.03\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}17}$ V ${\mathrm{cm}}^{2}$\ensuremath{\le}E/N\ensuremath{\le}500\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}17}$ V ${\mathrm{cm}}^{2}$ using a pulsed Townsend experimental method. The present measurements of w are the first to be obtained for ${\mathrm{C}}_{2}$${\mathrm{F}}_{6}$, ${\mathrm{C}}_{3}$${\mathrm{F}}_{8}$, and n-${\mathrm{C}}_{4}$${\mathrm{F}}_{10}$ at low E/N values. The electron-drift-velocity measurements in ${\mathrm{C}}_{3}$${\mathrm{F}}_{8}$ and n-${\mathrm{C}}_{4}$${\mathrm{F}}_{10}$ are dependent on gas pressure at high E/N values, even after allowing for nonequilibrium and boundary corrections to the measured electron swarm transit time. This is the first observation of a pressure dependence in the electron drift velocity in these gases and is believed to be due to changes in the electron energy distribution function f(\ensuremath{\varepsilon},E/N) with gas pressure resulting from increases in the density-normalized electron attachment coefficient \ensuremath{\eta}/N with gas pressure. The perfluoroalkanes ${\mathrm{CF}}_{4}$, ${\mathrm{C}}_{2}$${\mathrm{F}}_{6}$, and ${\mathrm{C}}_{3}$${\mathrm{F}}_{8}$ exhibit regions of pronounced negative differential conductivity (NDC) similar to but smaller in magnitude than that in ${\mathrm{CH}}_{4}$. Possible mechanisms leading to the observation of NDC effects in these molecular gases are discussed.