Triple-resonance autoionization of uranium optimized for diode laser excitation

The photoionization of uranium via three-step excitation has been optimized for isotope selective trace analysis. A search found 13 new J = 6, 7, and 8 odd-parity states in the 36,850–37,200 cm− 1 region that allow blue–red–red three-photon excitation at wavelengths favorable for commercial diode laser systems. From each of these 13 states, near-threshold autoionization spectra were recorded at a resolution of 3 × 10− 4 cm− 1. Some 30 even-parity autoionizing levels with J = 5 to 9 have also been observed and characterized. Comparison of the spectra allows J assignment for the new intermediate levels as well as selection of an optimized path for trace analysis. We show that, for a chosen scheme using a narrow J = 9 autoionizing resonance at 49,972 cm− 1, all of the excitation steps can be saturated with powers available from single-mode cw diode laser systems when exciting an atomic beam in perpendicular geometry.