Two-photon laser isotope separation of atomic uranium: Spectroscopic studies, excited-state lifetimes, and photoionization cross sections

Experimental studies are described wherein an atomic beam of uranium vapor produced by electron-beam evaporation is selectively excited and ionized by light from two short-pulse narrow-bandwidth, tuned dye lasers, and detected and analyzed by a mass spectrometer. The total number of ions per pulse produced is small; however, the time of production is known precisely. By counting single ions, using digital logic, and multiplexing the mass spectrometer between U
²³⁸
and U
²³⁵
, it is possible to measure isotope ratios as a function of exciter wavelength and to correct for background effects and spurious ions. These results demonstrated 50 percent enrichment of U
²³⁵
/U
²³⁸
. Excited-state lifetimes were measured by observing yields as a function of the delay between the two laser pulses. In addition, for an excitation wavelength of 4266.325 Å, the variation of two-step photoionization efficiency was measured as a function of the wavelength of the ionizing laser. The maximum yield at an ionizing wavelength of 3609 Å corresponds to a cross section of
2 \times 10^{-17}
cm
²
for matched linewidths. Also the ionization potential of uranium was determined to be 6.187 ± 0.002 eV.