Electron-atom superelastic scattering in magnesium at millielectron volt energies

The energy dependence of superelastic scattering is measured for electrons on Mg(3¹P) from threshold to 270 meV with a novel technique. The method uses photoelectrons produced by a narrow bandwidth laser as an approximately monoenergetic incident electron source to collide with excited atoms. Measurements are made at energies as low as 1.5 meV with a resolution of 1 meV near threshold. An efficient magnetic-bottle time-of-flight electron spectrometer allows for the simultaneous measurement of multiple scattering channels. Above-threshold ionization is also observed. The measured energy dependence for transitions from the VP state to both the 3³P and 3¹S are found to be inversely proportional to the energy from 10 to 270 meV. Below 10 meV, the dependencies are different, with the 3³P transition having an E^-1/2 dependence and the 3¹S channel retaining an E^-1 dependence.