Measurements of elastic electron scattering by water vapour extended to backward angles

Absolute differential cross sections for elastic electron scattering by water vapour have been measured at nine incident electron energies between 4 to 50 eV and over scattering angles between 10° and 180°, using a crossed-beam electron spectrometer. A magnetic angle-changing device based on the design of Read and co-workers has been used to extend the measurements to backward angles (125° to 180°). The elastic integral and momentum transfer cross sections are derived from these elastic differential cross sections. Previous experimental results at backward angles, available only up to 156°, are consistently higher than the present results. We also compare, where possible, with results from recent theoretical calculations.