Mechanism of ultraviolet upconversion luminescence of Gd³⁺ ions sensitized by Yb³⁺-clusters in CaF₂:Yb³⁺, Gd³⁺

Ultraviolet (UV) luminescence under the near infrared (NIR) excitation is very important for the photodynamic therapy and NIR photocatalysis. In this paper, UV emission of Gd³⁺ ions in CaF₂:Yb³⁺, Gd³⁺ excited by 980 nm laser is studied theoretically on the basis of the cooperative energy transfer from excited lanthanide tetramers. The main mechanism for the cooperative energy transfer from an excited Yb³⁺-tetramer to a Gd³⁺ ion is dq-dq-qq (d means an electric dipole and q means an electric quadrupole) and the energy transfer rate is calculated to be 1.7 × 10⁻⁶s⁻¹, which is well comparable with the previous calculated cooperative luminescence rate of 1.8 × 10⁻⁶s⁻¹ from an Yb³⁺-trimer. This is reasonably agreed with our previous experimental result that showed UV luminescence of Gd³⁺ ions and Yb³⁺-trimers within the same order of magnitude in CaF₂:Yb³⁺, Gd³⁺ under the excitation of 980 nm laser. The present paper suggests that CaF₂:Yb³⁺, Gd³⁺ is a potential up-conversion luminescent material for UV, and stimulates the design and development of new UV luminescent phosphors sensitized by lanthanide ion clusters.