L Journal of Alloys and Compounds 341 (2002) 134–138 www.elsevier.com / locate / jallcom 3 1 New paths of excitation of up-conversion emissions in LaCl :U 3 a, b c d e * ´ P.J. Deren , M.-F. Joubert , J.-C. Krupa , R. Mahiou , M. Yin a ´ ´ Instytut Niskich Temperatur i Badan Strukturalnych PAN, ul. Okolna 2, Skr. Poczt. 1410, 50950 Wrocl «aw 2, Poland b ´ ´ ˆ Laboratoire de Physico-Chimie des Materiaux Luminescents, URA-442 du CNRS, Universite Lyon 1, Bat. 205, F-69622, Villeurbanne, France c ´ Institut de Physique Nucleaire, CNRS-IN2P3, 91406 Orsay Cedex, France d ´ ´ ` Laboratoire des Materiaux Inorganiques, ESA-6002, Universite Blaise Pascal and EBSCCF, F-63177 Aubiere Cedex, France e Department of Physics, University of Science and Technology of China, 230026 Hefei, China Abstract 31 Green and red up-conversion emissions centred at 555 and 681 nm, respectively were investigated in LaCl :U . They were observed 3 2 4 after IR excitation in resonance with transitions between excited states. The green K → I emission is excited via two different 15/2 9/2 4 4 processes and the red F → I one via one excited state process only. Excitation paths of these emissions as well as possibilities of a 9/2 9/2 two-photon absorption and an avalanche up-conversion in this material are discussed.  2002 Elsevier Science B.V. All rights reserved. Keywords: Actinides; Up-conversion emission; Avalanche 1. Introduction excited in the IR by photon energies in resonance with electronic transitions between excited levels. Such an As far as spectroscopic properties are concerned, ac- excitation was chosen to investigate a possible photon tinide ions are very interesting for optical investigation. avalanche up-conversion process not reported yet for The 5f-electrons are not so well shielded by outer electrons actinides. After cw excitation at 975.2, 977.7 and 979.3 as in rare earth ions and consequently they are more nm, strong green and red emissions were observed at 10 K. sensitive to the crystal field interaction responsible in Examination of the excitation spectra shows their depen- particular of the mixing of the wave functions describing dence on monitored emission: some lines disappeared states with the same J-value. In addition, the 5f-orbitals in when the monitor was tuned from the red to the green 31 light actinides are more extended and accordingly, the emission of U :LaCl . Possible mechanisms for the 3 absorption and emission transitions within 5f-states are observed up-conversion emissions as well as a possible broadened due to a stronger coupling to the crystal lattice photon avalanche up-conversion process are discussed. vibrations. Therefore, the oscillator strengths of 5f–5f transitions are higher than those determined for the same type of transitions in iso-electronic rare earth ions [1]. Anti-Stokes emission could be obtained via different 2. Experimental mechanisms, i.e. Two Photon Absorption (with and with- out an intermediate level), Energy Transfer Up-conversion LaCl crystal doped with uranium ions (0.1 wt.%) was 3 and Photon Avalanche Up-conversion. Up-conversion grown by the Bridgman method and mounted in a quartz 31 brings information about levels not accessible in one- tube filled with a partial pressure of He gas. U ions 31 photon absorption spectroscopy. However, there are only a occupy the C symmetry site of La in LaCl lattice. 3h 3 few reports on anti-Stokes emission in actinides [2–4,6,7]. Visible continuous wave excitation focused on the sample In this paper, we report on anti-Stokes emissions with an 80-mm glass lens was performed with a Spectra 31 31 obtained in LaCl doped with U ions. U ions were Physics Ti-Sapphire laser pumped by an argon ion laser. 3 The sample was set in a circulating He cryostat and the sample luminescence was analysed through a Jobin-Yvon HRS1 monochromator equipped with a Hamamatsu photo- multiplier. The built up emission curves were recorded *Corresponding author. ´ E-mail address: deren@int.pan.wroc.pl (P.J. Deren). using a Lecroy 9410 digital oscilloscope. 0925-8388 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388(02)00097-X