Knowledge Bank

\maketitle The decay dynamics of threshold ionization of UO$2$ havebeen examined using a 1+1 REMPI technique. Ionization is accomplished via resonant excitation of the 2g $\leftarrow$ X $3\Phi 2u$ transition of UO$2$ at 314.09 nm, followed by ionization just above threshold using a second tunable dye laser operating at $ \lambda \leq$ 573 nm. While the peak widths for U$+$ and UO$+$ in the time of flight mass-spectrum are limited by the experimental resolution, the peak for UO$\mathrm{2+}$ is significantly broader, showing a characteristic decay towards longer flight times, which is the result of delayed ionization. The lifetime of this process is approximately 100 nanoseconds (at threshold) and decreases exponentially as the ionizing laser is tuned higher in energy. \ The ionization potential (IP) of UO$2$ has been measured previously, namely 6.128(3) eV, using photoionization efficiency curves. Refinement of the IP using mass analyzed threshold ionization (MATI) spectra was unsuccessful, as the delayed ionization may have interfered with this detection technique [J. Han \emph{et al.} J. Chem. Phys. \textbf{120}, 5155 (2004)]. Here we report high-resolution pulsed-field ionization zero kinetic energy photoelectron (PFI-ZEKE) spectra which have allowed a more accurate determination of the IP of UO$2$, as well as providing a spectroscopic probe of UO$\mathrm{2+}$. Accurate values for the electronic term energies and vibrational frequencies of UO$\mathrm{2+}$ have been obtained. A comparison of the current experimental results with theoretical calculations will be presented.