Relaxation of the 4f ^n–1 5d ¹ electronic states of rare earth ions in YPO ₄ and YBO ₃

Abstract

Large band-gap materials doped with rare earth-ions are currently of great interest as new vacuum UV phosphors for lighting and displays. In this report, the optical properties of YPO 4 and YBO 3 doped with Pr, Tm, Er, and Eu are described. The emission resulting from the VUV excitation of the parity allowed 4f n-1 5d 1 states and their quantum efficiencies are reported. Relaxation between the 4f n-1 5d 1 and nearby 4f n excited states is observed for some of these ions, and the dynamics of these excited states is reported. In doubly-doped samples, the prospects for quantum cutting by using cross relaxation energy transfer in which some of the energy of the initially excited 4fn-1 5d 1 state is transferred to an acceptor ion so that both donor and acceptor ions are left in an excited state from which they each can emit a photon are examined.The high-lying excited states of rare-earth ions are of considerable importance for a wide range of applications including scintillators, vacuum ultraviolet (VUV) phosphors, and UV lasers. In this work, the problem of designing efficient VUV phosphors will be considered. The potential utility of a scheme for converting the exciting VUV photon into two visible photons by using cross relaxation energy transfer involving a parity allowed 4f n-1 5d → 4f n transition on one ion and a transition within the 4f n configuration of a second ion will be tested in the hosts YPO 4 and YBO 3 . These materials have large band gaps that fall well into the VUV.So called "quantum cutting" or "multiphoton" phosphors offer a significant challenge for their realization. While the idea has been demonstrated in real systems, these systems exhibit serious drawbacks preventing their commercialization. Cascade luminescence from the 1 S 0 state of Pr 3+ in YF 3 was shown to occur with a quantum yield of about 1.4 [1]. Unfortunately, most of the energy in the first step in the stepwise relaxation back to the ground state occurs at about 400 nm, which is at a wavelength for which the human eye is insensitive. Cross relaxation in [3]. The demonstration of the latter scheme suggests that the use of parity allowed transitions on one of the ions in the ion-pair should be considered further, and this provides motivation for the present work.

VUV Optical Properties of Rare Earth Doped YPO4 Prepared by Different Routes

“…host em host abs = η t · act em act abs [2] In this notation, the subscript denotes absorbed or emitted fluxes while the superscript denotes if absorption or excitation is via the host or directly via the activator (i.e. [3] The first ratio in Equation 3 is taken from excitation spectra.…”

Luminescence properties of Sm3+ doped YPO4: Effect of solvent, heat-treatment, Ca2+/W6+-co-doping and its hyperthermia application

“…15 The origin of luminescent properties is from the intra 4f and 4f-5d transitions of different kinds of rare earth ions, especially the intra 4f transitions which are less influenced by the surrounding ligands owing to the shielding of the 4f orbital by the outer 5s and 5p orbitals. Though [16][17][18][19][20][21][22][23][24] The lowest excited level of Sm 3+ is 4 G 5/2 and it has three main emitting levels 6 H j (j = 5/2, 7/2 and 9/2) located at 564, 601 and 645 nm, respectively, which gives reddish-orange emission. 23,24 However, when wet a Author to whom correspondence should be addressed.…”

A Quantum‐Splitting Phosphor Exploiting the Energy Transfer from Anion Excitons to Tb3+ in CaSO4:Tb,Na

“…[4] Among inorganic phosphors, CaSO 4 is uniquely placed between fluorides and other hosts presently used in PDP. Luminescence studies of CaSO 4 under VUV excitation are, however, limited to a few dopants only.…”

“…[5][6][7] Similar to fluorides, CaSO 4 has a wide bandgap (> 10 eV) with a low crystal field. Localized energy transfer through charge-transfer state (CTS)-mediated anion excitons that are created within SO 4 2-complexes bring about its unique properties. However, such self-trapped exciton (STE)-mediated energy transfer in various hosts has so far been regarded as a highly unwanted process for two-photon cascade emission since impurity-mediated absorption of VUV radiation without any interference from the host lattice has been considered as the efficient mode for two-photon emission.…”

“…The VUV excitation spectra of CaSO 4 :Tb 3+ with Na + as a charge compensator show two prominent excitation bands at 147 and 216 nm. The former band is attributed to the charge-transfer excitations within SO 4 2-complexes while the latter was assigned to the 4f 8 → 4f 7 5d transitions on Tb 3+ . The energy-transfer mechanism from anion excitons to Tb 3+ strongly raises the possibility of two-photon emission via a second-order down-conversion under the VUV excitation, which is basically a new approach in the goal of achieving a quantum-splitting phosphor.…”