Show that at very low energy permanent dipole effect should dominate, and polarization interaction takes over at higher energy. Show that the energy at which dipole interaction changes to polarization is about 0.01 eV (less than 0.025 eV – thermal energy at 300 K) for CO and NO, and 2.3 eV for water molecules. (Thus, only for water molecules with very high dipole moments, ion scattering by a dipole is relevant at ‘normal’ and higher temperatures.)