MULTIPHOTON IONIZATION OF URANIUM HEXAFLUORIDE

Multiphoton ionization (MPI) time-of-flight mass spectroscopy (TOFMS) and photoelectron spectroscopy (PES) studies of UF6 are reported using focused light from the Nd:YAG laser fundamental (lambda = 1064 nm) and its harmonics (lambda = 532, 355, or 266 nm), as well as other wavelengths provided by a tunable dye laser. The MPI mass spectra are dominated by the singly and multiply charged uranium ions rather than by the UF(x)+ fragment ions, even at the lowest laser power densities at which signal could be detected. In general, the doubly charged uranium ion (U2+) intensity is much greater than that of the singly charged uranium ion (U+). For the case of the tunable dye laser experiments, the U(n+) (n = 1-4) wavelength dependence is relatively unstructured and does not show observable resonance enhancement at known atomic uranium excitation wavelengths. The MPI-PES studies reveal only very slow electrons (less-than-or-equal-to 0.5 eV) for all wavelengths investigated. The dominance of the U2, ion, the absence or very small intensities of UF(x)+ (x = 1-3) fragments, the unstructured wavelength dependence, and the preponderance of slow electrons all indicate that mechanisms may exist other than ionization of bare U atoms following the stepwise photodissociation of F atoms from the parent molecule. The data also argue against stepwise photodissociation of UF(x)+ (x = 5,6) ions. Neither of the traditional MPI mechanisms (''neutral ladder'' or the ''ionic ladder'') are believed to adequately describe the ionization phenomena observed. We propose that the multiphoton excitation of UF6 under these experimental conditions results in a highly excited molecule, superexcited UF6**. The excitation of highly excited UF6** is proposed to be facilitated by the well known ''giant resonance,'' whose energy level lies in the range of 12-14 eV above that of ground state UF6. The highly excited molecule then primarily dissociates, via multiple channels, into U(n+), UF(x)+, fluorine atoms, and ''slow'' electrons, although dissociation into F- ions is not ruled out.