title: Upconversion Between 4f-5d Excited States in Tm2+-Doped CsCaCl3, CsCaBr3, and CsCaI3.
abstract: Near-infrared to visible upconversion luminescence in CsCaCl3:Tm2+, CsCaBr3:Tm2+ and CsCaI3:Tm2+ is presented and analysed. The upconversion process involves exclusively the 4f-5d excited states of Tm2+, which is a novelty among upconversion materials. The presence of more than one long-lived 4f-5d excited state is the prerequisite for this. Multiple emissions from Tm2+ are observed in the title compounds. This is made possible by the favourable energy structure within the 4f-5d states and the low phonon energies of the materials. The energy positions of the relevant 4f-5d states, and thus the photophysical and light emission properties, are affected by the chemical variation along the series. The upconversion efficiency increases from chloride to iodide and the mechanism is found to be a combination of absorption and energy-transfer steps.
authors.affiliation: Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland.
authors.forename: J
authors.initials: J
authors.lastname: Grimm
authors.forename: E
authors.initials: E
authors.lastname: Beurer
authors.forename: P
authors.initials: P
authors.lastname: Gerner
authors.forename: H U
authors.initials: HU
authors.lastname: Güdel