Electron Capture by Rotational Excitation of Polar Molecules

The problem of electron capture by a polar molecule with simultaneous rotational excitation of the molecule is analyzed. Capture cross sections and lifetimes of temporary negative-ion states so formed are calculated on the assumption that the electron interacts primarily at large distances from the molecule by means of the dipole field and forms only loosely bound ionic states. The capture probability is proportional to the ratio ${\left(\frac{D}{I}\right)}^2$, where $D$ is the dipole moment and $I$ the moment of inertia of the molecule about a perpendicular axis passing through the center of the dipole. Electron capture by rotational excitation is most probable for polar molecules that have both a relatively large dipole moment and a small moment of inertia. In order for this process to make effective contributions to momentum-transfer cross sections measured, e. g., in electron-swarm experiments, the spacing of the rotational levels of the molecule must be of the order of thermal energies, so that a relatively large number of electrons can be captured and released. These circumstances, viz., the relatively large value of $\frac{D}{I}$ and the right spacing of rotational levels, appear to offer an explanation of the particular behavior of ${\mathrm{H}}_2$O, ${\mathrm{D}}_2$O, and ${\mathrm{H}}_2$S among a number of polar molecules with which recent swarm experiments have been made. According to the theory, some other molecules (e. g., N${\mathrm{H}}_3$, HF, HCl, and ${\mathrm{H}}_2$ ${\mathrm{O}}_2$) should exhibit a similar behavior. Lifetimes of negative ions formed by this process are estimated to be of the order of ${10}^{-13\mathrm{}}$ sec.