Vibrational and rotational cooling of electrons by water vapor

The cooling of electrons by vibrational and rotational excitation of water molecules plays an important role in the thermal balance of electrons in cometary ionospheres. The energy loss function for rotational excitation and de-excitation of H₂O by electron impact is calculated theoretically. The rotational cooling rate is calculated using this loss function for a wide range of electron and neutral temperatures. The vibrational cooling rate is calculated using measured values of electron impact vibrational excitation cross sections. Analytical formulae are provided for some of the cooling rates. The interaction of ions with H₂O molecules is also discussed and a formula is suggested for the momentum transfer collision frequency.