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approximation (42) is used to calculate the statistical weights and excitation energies of the uranium ions. This approximation is based on the observation that atoms or ions having the same number of orbital electrons have similar electron configurations. The gi,j values are calculated from (4.36), using the Li'j and Si,j values of the neutral atom having i orbital electrons. The Xi'j are obtained from spectroscopic measurements (43) of energy levels of the neutral atoms.


Lowering of Ionization Potentials

        The ionization potential Ei in equation (4.2)

represents the energy required for the ionization of an isolated atom or ion. At high pressures the plasma density increases until the atoms and ions can no longer be considered as isolated. Each particle then moves in an electric field associated with adjacent charged particles. The ionization reactions are influenced by plasma microfields associated with electrostatic polarization (Debye effect). The resulting effect is a lowering of the energy required for ionization of an atom or ion, the effect increasing with increasing charge density. To account for the lowering of the ionization potentials, an effective ionization potential, Ei eff is defined as


                    E.eff = E. - AE.                (4.37)
                    1        1     1