Journal of Luminescence 94–95 (2001) 79–83 4f n 24f n1 5d transitions of the trivalent lanthanides: experiment and theory L. van Pieterson a, *, M.F. Reid b , R.T. Wegh a , A. Meijerink a a Debye Institute, Physics and Chemistry of Condensed Matter, Utrecht University, P.O. Box 80 000, 3508 TA Utrecht, The Netherlands b Department of Physics and Astronomy, University of Canterbury, Christchurch, New Zealand Abstract Emission and excitation spectra of 4f n 24f n1 5d transitions of lanthanide ions in LiYF 4 are reported and are succesfully reproduced by energy level calculations using a theoretical model that extends established models for the 4f n configuration to include fd states. Some interesting trends are observed. Lifetime measurements for the spin-allowed fd emission show that the electric dipole matrix element for the fd transition decreases through the lanthanide series. Also the splitting between the high spin and low spin fd states decreases through the lanthanide series. Both effects are reproduced by our model. r 2001 Elsevier Science B.V. All rights reserved. Keywords: 4f5d transitions; Lanthanides; VUV 1. Introduction In the past few years, research on the VUV spectroscopy of the lanthanide ions has increased as part of the effort to design new phosphors for lamps and displays. For the 4f n -4f n (4f) transi- tions, new energy levels were identified in the VUV and were reproduced by energy level calculations. Also the 4f n -4f n1 5d (fd) excitation spectra have been measured for various lanthanides in the VUV, but contrary to the large understanding of the 4f transitions, the knowledge on the fd transitions is incomplete. In this paper, a summary of our recent results on fd transitions is given [1– 4,6]. We present a theory which successfully reproduces the structure in the fd excitation spectra of the lanthanide ions. Furthermore, emission spectra and lifetimes can be calculated using this theory and trends that we observe in the lifetimes and the splitting between the low spin (LS) and high spin (HS) fd states of the lanthanides are reproduced by theoretical calculations. 2. Theory To calculate the 4f n1 5d energy levels, the established theoretical models for 4f n levels are extended to include the splitting of the 5d state (by the crystal field and the spin–orbit coupling) and the interaction between the 4f n1 and 5d config- urations. A detailed theoretical description and the parameters used in the calculations are given in Refs. [1–3]. The transitions from 4f n to 4f n1 5d and vice versa are electric dipole allowed and it is straightforward to calculate the matrix elements of *Corresponding author. Fax: +31-30-253-2403. E-mail address: l.vanpieterson@phys.uu.nl (L. van Pieterson). 0022-2313/01/$-see front matter r 2001 Elsevier Science B.V. All rights reserved. PII:S0022-2313(01)00251-4