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) an d its harmonics (lambda = 532, 355, or 266 nm), as well as other wavel engths provided by a tunable dye laser. The MPI mass spectra are domin ated by the singly and multiply charged uranium ions rather than by th e UF(x)+ fragment ions, even at the lowest laser power densities at wh ich signal could be detected. In general, the doubly charged uranium i on (U2+) intensity is much greater than that of the singly charged ura nium ion (U+). For the case of the tunable dye laser experiments, the U(n+) (n = 1-4) wavelength dependence is relatively unstructured and d oes not show observable resonance enhancement at known atomic uranium excitation wavelengths. The MPI-PES studies reveal only very slow elec trons (less-than-or-equal-to 0.5 eV) for all wavelengths investigated. The dominance of the U2, ion, the absence or very small intensities o f UF(x)+ (x = 1-3) fragments, the unstructured wavelength dependence, and the preponderance of slow electrons all indicate that mechanisms m ay exist other than ionization of bare U atoms following the stepwise photodissociation of F atoms from the parent molecule. The data also a rgue against stepwise photodissociation of UF(x)+ (x = 5,6) ions. Neit her of the traditional MPI mechanisms (''neutral ladder'' or the ''ion ic ladder'') are believed to adequately describe the ionization phenom ena observed. We propose that the multiphoton excitation of UF6 under these experimental conditions results in a highly excited molecule, su perexcited UF6*. The excitation of highly excited UF6** is proposed t o be facilitated by the well known ''giant resonance,'' whose energy l evel lies in the range of 12-14 eV above that of ground state UF6. The highly excited molecule then primarily dissociates, via multiple chan nels, into U(n+), UF(x)+, fluorine atoms, and ''slow'' electrons, alth ough dissociation into F- ions is not ruled out.