First steps towards two-electron momentum density measurements.

Five-fold differential cross sections (5DCS) for the electron-impact double ionization of the 3s electrons of magnesium have been measured in a high-impact-energy, high-momentum-transfer regime. These measurements fulfill two conditions necessary for the measurement of two-electron momentum densities, those being impulsive ionization and identification of the double-ionization mechanisms. The form of the measured cross sections provides strong evidence for two different impulsive two-step ionization mechanisms. Second Born calculations are in agreement with the experimental data. The implications for the measurement of two-electron momentum densities are discussed. © 2003 American Institute of Physics [ABSTRACT FROM AUTHOR]