Migration of excess electrons in high density supercritical ammonia

Abstract

Excess electron mobility measurements in supercritical ammonia, which show a transition from the quasifree to the localized electron state in the density range from about 1×1020 to 3×1021 molecules/cm3 have been extended to densities comparable to that of liquid ammonia (n≲1.5×1022 molecules/cm3; 410⩽T⩽460 K). The drift mobility reaches a minimum of μ = 3.2×10−2 cm2 V−1 s−1 at about n≃4.7×1021 molecules/cm3. By further increasing the fluid density, the mobility increases again by a factor of ∼ 5 approaching a plateau (maximum) with μ≃1.65×10−1 cm2 V−1 s−1 at n≃1.5×1022 molecules/cm3 (T = 410 K). The temperature dependence of the electron mobility is strongest at the density where the minimum occurs. At higher densities, the temperature dependence is drastically reduced. From these facts and other available experimental results, it is concluded that a ’’trap-controlled hopping’’ transport of the electron dominates at ammonia vapor densities n<nc (nc = 8.23×1021 molecules/cm3 is the critical density of ammonia). At higher densities (n≳nc), the observed mobilities can be interpreted to be due to a weakly activated small-polaron hopping which is influenced by the molecular dynamics in supercritical ammonia. An ionlike motion of the localized (solvated) electron as often supposed can be excluded to be an essential transport channel in supercritical ammonia.