AVS 47th International Symposium
    Surface Science Monday Sessions
       Session SS2+EL-MoA

Paper SS2+EL-MoA6
Plasmon Resonance Spectroscopy of Plutonium Metal Allotropes

Monday, October 2, 2000, 3:40 pm, Room 209

Session: Electronic Structure and Excitations
Presenter: R.K. Schulze, Los Alamos National Laboratory
Authors: R.K. Schulze, Los Alamos National Laboratory
J.D. Farr, Los Alamos National Laboratory
Correspondent: Click to Email
We have measured the plasmon resonance response of the cleaned and well characterized surfaces of the six plutonium thermal allotropes (@alpha@, @beta@, @gamma@, @delta@, @delta@', @epsilon@) using backscatter electron energy loss spectroscopy (EELS). The energy of the plasmon resonance is highly dependent upon the electronic structure of the solid surface, and in particular, the density of electrons available for conduction. Using this method we probe directly, for the first time, the number of free elec trons, and the changes in the electronic structure of the plutonium. EELS spectra were acquired on a high purity sample of plutonium with primary electron beam energies of 150, 200, 500, 700, and 1000 eV, and with the sample at seven different temperatur es: -125 (@alpha@), 45 (@alpha@), 156 (@beta@), 250 (@gamma@), 410 (@delta@), 465 (@delta@'), and 525°C (@epsilon@). Changes in the plasmon resonance spectra over the 150 to 500 eV primary beam energy range indicate that a surface reconstruction e xists for each of the Pu allotropes. The 700 and 1000 eV EELS measurements are alike, indicating that bulk characteristics are being probed at these energies. The bulk plasmon resonance energies for the allotropic series (@alpha@, @beta@, @gamma@, @delta@, @delta@', @epsilon@) are 12.23, 11.18, 11.16, 10.81, 10.94, and 10.92 eV. From these measurements, we extract the volume density of free electrons in each allotrope. In electrons per nm@super 3@ these are, for the series, 108.5, 90.7, 90.3, 84.8, 86.8, and 86.5. These results correlate directly with measurements of resistivity and magnetic susceptibility for the series of allotropes, indicating that the changes in free electron density are identically responsible for the changes in these physical properties. The implications of these results in terms of the electronic properties of Pu and in the changes of electronic structure between the allotropes will be discussed.