Electron thermalization ranges and free-ion yields in dielectric fluids: Effects of density and molecular shape

Abstract

Secondary-electron thermalization ranges $b_{\mathrm{GP}}$ and free-ion yields $G_{\mathrm{fi}}^E$ have been measured in the coexistence vapor and liquid of spherelike (methane) and nonspherelike (ethane) molecules at densities $0.14\lesssim \frac{d}{d_c}\lesssim 2.8$, where $d_c$ is the density of the critical fluid. The density-normalized range $b_{\mathrm{GP}}d$ at low $d$ is 5.5×${10}^{-6\mathrm{}}$ kg/${\mathrm{m}}^2$ in both compounds. In the dense gas at $\frac{d}{d_c}\gtrsim 0.3$, $b_{\mathrm{GP}}d$ increases slightly; the effective cross section for epithermal electron scattering is lessened. The large density fluctuations at these conditions, which cause quasilocalization of thermal electrons, appear to affect epithermal electrons in an opposite fashion. At $\frac{d}{d_c}>1.0\mathrm{}$, conduction-band formation lifts $b_{\mathrm{GP}}d$ in liquid methane to ∼4 times the value in liquid ethane. The density effect is in the same direction as, but less marked than, that for thermal electrons. At $\frac{d}{d_c}=2.5\mathrm{}$, $b_{\mathrm{GP}}$ in methane is 1.2 times larger than at $\frac{d}{d_c}=1\mathrm{}$, but the mobility $\mu$ of thermal electrons is 15 times larger than at $\frac{d}{d_c}=1\mathrm{}$. In ethane at $\frac{d}{d_c}=2.5\mathrm{}$, $b_{\mathrm{GP}}$ is 3.4 times and $\mu$ is 33 times smaller than at $\frac{d}{d_c}=1\mathrm{}$. The degree of sphericity of the molecules greatly affects electron energy loss and localization interactions.