Electron mobility measurements have been performed in subcritical and supercritical ammonia vapor in the density range from 6x lo 18 to 3x io 21 molecules/cm 3 and in the temperautre range of 210 to 440 K. In the density range from 2x io 20 to about 3x io 21 molecules/cm 3 an anomalous decrease of electron mobility by more than two powers of ten is observed with increasing vapor density, indicating a "transition" from the delocalized to the localized electron state.Excess electrons in dense helium gas at low temperatures undergo a sharp "transition" from the quasifree state to a localized bubble state as the gas density is increased. 1 This system provides a prototype for the study of electronic states in a disordered nonpolar material. First attempts to get some information on electron localization in polar fluids have involved the study of radiation-chemical electron yields, monitoring the time-resolved absorption spectra of solvated electrons in polar vapors. Of most interest in the polar fluids are H 2 0 or NH 3 , since neither has a stable negative ion. For instance, the system (NH 3 )~ is unstable relative to (NH 3 + e~).Absorption spectra of solvated electrons in supercritical ammonia vapor at 420 K were measured down to a density of about 0.1 g/cm 3 by Schindewolf and co-workers. 2 Because of the strong decrease of the electron yield below 0.2 g/cm 3 at this temperature, it was concluded that below this density solvation of electrons ceases to occur. Additional experimental work conducted by Gaathon and Jortner 3 showed that solvated electrons can be detected near the liquid-vapor coexistence curve down to a density of 5xl0~3 g/ cm 3 (T = 293 K). At temperatures high above the liquid-vapor coexistence curve, however, the yield of solvated electrons decreases sharply to zero. These experiments support the concept that there is a temperature-dependent "critical" density for the transition between localized (solvated) and quasifree excess electron states in a polar fluid. It seemed to us that drift-mobility measurements in ammonia vapor would give unambiguous experimental information relevant in this context in contrast to the determination of the critical density by light-absorption measurements on solvated electrons which are somewhat restricted by the sensitivity of the optical detection equipment and the kinetic instability of the localized state. 3 The mobility of the electrons was measured by a time-of-flight method. Electrons were photoinjected from a stainless steel photocathode into ammonia vapor by a short laser pulse [second harmonic of a Nd:YAlG (Nd-doped yttrium aluminum garnet) pulse laser: wavelength 265 nm, pulse duration about 15 ns, number of light quanta per pulse ^10 16 ] and collected by the anode. The anode has been made transparent to the laser light by employing a fine mesh of stainless steel (about forty lines per centimeter), because in our concept of the "photocell" the electric field direction and the laser beam coincide. The transient photocurrent pulses were amplified and displa...
To get additional information about excess electron states in polar fluids we have measured the mobility of electrons in ammonia vapor at various densities. Mobilities of electrons, photoinjected from a photocathode by a short laser flash into ammonia vapor, were obtained by a time-of-flight method. Measurements were performed in the density range 6 X 1018 < pNHs < 4 X 1021 cm"3 and in the temperature range 210 < T < 420 K. The mobility of the electrons changes from about 900 cm2 V"1 s"1 at low densities to about 4 X 10"2 cm2 V"1 s"1 at the high density limit. In the density range 1 X 1020 < pNHs < 3.6 X 1021 cm"3 a drop of the mobility by a factor of 108 is observed indicating a "transition" from the quasi-free electron state to the localized state.The results are qualitatively discussed according to the concept that density fluctuations, i.e., preexisting clusters in the vapor, determine the probability of electron localization.
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