Rotational excitation of molecules by electron collisions

The anisotropic charge distribution of a molecule can easily induce a rotational transition in the molecule during an electron collision. Further, since the level spacing of the rotational states is very small, the transition can take place over a wide range of electron energies. The rotational excitation is the dominant energy-loss process for an electron in a molecular gas, when the electron energy lies below the vibrational threshold of the molecule. In the case of polar molecules, the rotationally excited molecule promptly emits microwave (or far infrared) radiation. In this way, the rotational excitation effectively cools electrons. The present paper reviews theoretical and experimental studies of the electron-impact rotational excitation of molecules. After a general introduction of the relevant theory and experiment, case studies of five different molecular species (H2, N2, CH4, HCl, and H2O) are presented to show the characteristics of rotational cross sections. From those studies, common features of the cross sections are discussed.