Free-Carrier Drift-Velocity Studies in Rare-Gas Liquids and Solids

Free-carrier drift-velocity studies were made in liquid and solid Ne, Ar, Kr, and Xe samples, using a crystal counter technique. Electron-ion or electron-hole pairs were generated in the samples by means of a ${\mathrm{Po}}^{210}$ $\alpha$-particle source which was electrochemically deposited on one electrode of the parallel electrode arrangement. Transit times of carriers drifting across the electrode spacing were determined from pulses displayed on an oscilloscope and recorded photographically. The number of ion pairs escaping from each $\alpha$-particle track was found to be electric-field-dependent, and the values obtained were less than half the corresponding number observed using $\alpha$-particle excitation in the gaseous phase of the same materials. In fields greater than about 10 kV/cm, saturated electron drift velocities were observed. Values of the saturated electron drift velocities in solid Ne, Ar, Kr, and Xe are 1.8, 1.36, 0.95, and 0.6×${10}^6$ cm/sec, respectively, while in liquid Ar and Kr, the respective values were 0.6 and 0.35×${10}^6$ cm/sec. Charge transport by holes as well as electrons were observed only in solid Xe. An argument is given which suggests that the results observed can be qualitatively explained in terms of a hot-electron model, with inelastic scattering by molecular impurities proposed as the velocity-limiting interaction.