The Development of Electron Momentum Spectroscopy

The study of the valence electronic structure of atoms and molecules by (e,2e) spectroscopy, or EMS as it is now known, began in the early 1970s with a series of measurements at Flinders University. The first measurements were on argon, and they showed the importance of correlation effects in the inner valence 3s shell. The first molecular experiments were on methane, and they showed the sensitivity of the momentum distributions to details of the orbital wavefunctions. Until recently all EMS measurements were made on ground state targets with random orientations. We have, however, now made successful EMS measurements on excited states and oriented targets. Sodium atoms in the 32S1/2(F=2) ground state are optically pumped by right-handed circularly polarised light to the excited 32P3/2(F'=3, 1'111"=3) state. Thus the excited atoms are all in the I .e=l, me=l} state. These measurements are discussed in some detail.