The excited electronic states of UF6 between 3–10 eV are investigated by ab initio configuration interaction (CI) calculations using a relativistic effective core potential (RECP) on the uranium and nonrelativistic ECP’s on the fluorines. Spin‐orbit coupling effects are incorporated by diagonalizing an effective one‐electron operator over the CI wave functions. This represents one of the first studies of polyatomic molecules to treat both spin‐orbit effects and multiplet effects arising from electron–electron interactions. A detailed analysis of the electronic states of UF6 is given both with and without spin‐orbit coupling effects. Assignments for the major peaks in the 3–10 eV energy range are given, as aided by recent electron impact studies. Comparisons are made between the present many‐electron states and previous one‐electron theoretical treatments.

Abinitio studies of excited states of polyatomic molecules including spin‐orbit and multiplet effects: The electronic states of UF6

The A–X band of gaseous UF6 has been studied using the laser‐induced fluorescence technique. Excitation and fluorescence spectra are presented for various pressures of UH6 at room temperature. The fluorescence spectra show little change over the pressure range 0.2–20 torr, with a peak wavelength at 421 nm. In contrast, the peak wavelength of the excitation spectra shows a noticeable shift from 391 to 385 nm when the UF6 pressure is increased from 2 to 20 torr. A Stern–Volmer plot of UF6 self‐quenching data shows a pronounced nonlinear behavior over this same pressure region. From an analysis of the laser‐induced fluorescence spectra, we develop a simple kinetic model for UF6 excited‐ state dynamics. A standard kinetic treatment of this scheme yields a prediction for the dependence of the fluorescence decay lifetime on total pressure that is in good agreement with experimental results.

Laser‐induced fluorescence of gaseous UF6 in the A–X̃ band

Near UV photophysics of gaseous UF6

The process of UF6 photodissociation as function of pressure and excitation wavelength has been investigated through measurements of the light scattered by UF5 particles formed after excitation of UF6 in the range 365–425 nm. The influence of N2 as collisional partner has been studied.

Study of UF6 photodissociation through light scattering of UF5 particles

Fluorescence excitation spectra of the lowest charge transfer state of UF6 isolated in solid argon and krypton at 12 K have been obtained using a pulsed, N2‐laser‐pumped dye laser. The complex, multiline spectrum from 360 to 406 nm is much better resolved than previous absorption spectra and is assigned in terms of the ν1, ν3, ν4, ν5, and ν6 normal modes and their overtones and combinations. The role of Jahn–Teller distortion in the upper state is discussed, and the matrix, neat crystal, and gas spectra are compared. The lifetime of the electric dipole forbidden emission is measured at 600 μs in solid argon.

C.A. Sacchi

Papers

Fluorescence from gaseous UF6 excited by a near-UV dye laser