ENERGY-LOSS BY SLOW-ELECTRONS IN POLAR GASES

The mean energy loss by slow electrons moving in a gas and rotationally exciting the molecules is proportional to the rotational stopping cross section (RSCS) defined by multiplying the cross section for a rotational transition by the transition energy and by summing the product over. The RSCS for linear and symmetric-top polar molecules are studied in the adiabatic-rotation approximation at scattering angles larger than a critical value c and in the first-Born approximation for <c. Formulas independent of c are derived for the RSCS for any as the sum of the RSCS for the ground rotational state =0 and a term that depends on in a simple analytic form and is proportional to the dipole moment D squared. For a vanishing D, this term vanishes and the formulas reduce to a previously proven theorem for nonpolar molecules that the RSCS is independent of. Useful expressions are also derived for the average of the RSCS over the Boltzmann distribution of. © 1990 The American Physical Society.