VUV spectroscopy of lanthanides: Extending the horizon

The use of vacuum ultraviolet radiation (VUV; <200 nm) in lanthanide spectroscopy has been triggered by the need for new luminescent materials for VUV excitation. In spite of this 'applied' incentive, the VUV studies have resulted in a wealth of new fundamental knowledge on the luminescence of lanthanides. Here, an overview of our recent results is presented. For the intraconfigurational 4f (n) transitions many new VUV energy levels for various lanthanides ions have been observed, resulting in an extension of the 30 year old Dieke diagram from 40,000 to 70,000 cm(-1). New emitting 4f levels in the VUV are identified as well. For interconfigurational 4f (n) --> 4f (n-1)5d (fd) transitions the observation of weak spin-forbidden fd bands for the heavy lanthanides is reported. For Er3+ and Tm3+ both spin-allowed and spin- forbidden fd emission is observed. A decrease of the matrix element for the fully allowed fd transitions through the lanthanide series is observed and explained by lanthanide contraction. Finally it is shown how the new information on the VUV levels of lanthanides can be used to obtain luminescent materials with a quantum efficiency of more than 100 % via downconversion. This gives new hope for the applications which triggered this research.