The multiphoton ionization of uranium hexafluoride

Multiphoton ionization (MPI) time-of-flight mass spectroscopy and photoelectron spectroscopy studies of UF₆ have been conducted using focused light from the Nd:YAG laser fundamental (λ=1064 nm) and its harmonics (λ=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. The laser power dependence of ion signals indicates that saturation can occur for many of the steps required for their ionization. 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 Uⁿ⁺ (n = 1-4) wavelength dependence is relatively unstructured and does not show observable resonance enhancement at known atomic uranium excitation wavelengths. The dominance of the U²⁺ ion and the absence or very small intensities of UF_x⁺ fragments, along with the unstructured wavelength dependence, indicate that mechanisms may exist other than ionization of bare U atoms after the stepwise photodissociation of F atoms from the parent molecule. The data also argue against step-wise photodissociation of UF_x⁺ (x=5,6) ions. Neither of the traditional photoinduced mechanisms, the "neutral ladder" or the "ion ladder", are believed to adequately describe the ionization phenomena observed. These results suggest an alternate mechanism which better explains the multiphoton excitation and dissociative ionization of UF_6-. It is likely that the multiphoton excitation of UF₆ under these experimental conditions results in a superexcited molecule, UF₆**, which primarily dissociates into Uⁿ⁺ (through multiple channels), fluorine atoms, and "slow" electrons. The excitation of such superexcited molecules may be facilitated by the existence of a previously reported "giant resonance" at 12-14 eV.