Charge Transport in the Diatomic Molecular Solids and Liquids:N2,O2, and CO

The paper reports an investigation of the drift mobility of electronic charge carriers in several phases of the diatomic solids ${\mathrm{N}}_2$, ${\mathrm{O}}_2$, and CO. It also includes similar measurements on positive and negative ions in the corresponding liquids. Thin crystal specimens (20-400 μm thick) were grown from the liquid between parallel electrodes in a chamber attached to a miniature cryostat after careful purification of the starting gas. As in previous work on the rare-gas solids and liquids, an electron-beam technique was used for the generation of excess carriers near one of the electrodes. Measurement of the transit time led directly to the drift mobility $\mu$. In $\alpha$- and $\beta$-${\mathrm{N}}_2$, $\beta$-${\mathrm{O}}_2$, and $\beta$-CO electron mobilities between ${10}^{-3\mathrm{}}$ and ${10}^{-2\mathrm{}}$ ${\mathrm{cm}}^2$ ${\sec }^{-1\mathrm{}}$ ${\mathrm{V}}^{-1\mathrm{}}$ were obtained; a similar value was found for the hole mobility in $\gamma$-${\mathrm{O}}_2$. The temperature dependence of $\mu$ has been analyzed in terms of the nonadiabatic small-polaron theory. It is found that low-energy phonons (<10 meV) are involved in the hopping transport. The implications of these results are discussed. In the liquids, Walden's rule is obeyed. With small electrode spacings, the observed transit signals suggest the existence of two mobile species.