PNNL

Abstract

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 36850-37200 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 49972 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.