Optical Properties of Tetravalent Neptunium in Lead Molybdate

Abstract

The optical properties of neptunium in single-crystal lead molybdate have been measured to determine the suitability of Np4+ ions for laser applications. It was found that neptunium enters the crystals predominantly in the tetravalent state, with Np4+ ions occupying Pb2+ lattice sites. Another ionization state, probably Np6+, also appears, but is eliminated by the presence of Na+ in the crystals. Optical absorption at wavelengths longer than 0.6 μ is due to transitions from the 4I9/2 ground state to the other members of the 4I multiplet. Fluorescence, arising from 4I11/2→4I9/2 transitions, is excited by pumping in the 0.6-to-1.3-μ region. The emission fills a band between 1.6 and 2.2 μ at room temperature, but at 77°K it is confined largely to five lines, the narrowest of which has a width of about 13 cm−1. The quantum efficiency at 77°K approaches unity. Laser threshold calculations for PbMoO4 containing 0.005 mole % Np4+ indicate a rather high pulsed threshold at 77°K. However, crystals containing 0.1 mole % at Np4+ would have thresholds low enough for cw operation. Concentrations of this magnitude are not realizable in PbMoO4, but this is not necessarily the case for the other scheelite-structured tungstates and molybdates.