Application of photoconductivity measurements to photodynamic processes investigation in LiYF4:Ce and LiLuF4:Ce crystals

Photoconductivity measurements are applied to the studies of photodynamic processes in LiYF₄:Ce³⁺ and LiLuF₄:Ce³⁺ crystals undergoing ultraviolet irradiation. Photoconductivity spectra were registered by means of microwave technique under irradiation at 220-320 nm at the room temperature. Photoconductivity signal appeared at ∼300 nm and monotonically increased with the shortening of wavelength. Pumping energy dependencies of photoconductivity within 225-305 nm spectral range were registered and revealed the change of the dependence degree from quadratic at longer wavelengths through linear to saturation-like at shorter ones. Numerical simulation based on four-level model of photodynamic processes was performed. Values and spectral distributions of probabilities of different types of photodynamic processes for 225-295 nm were estimated. Ce³⁺ ions excited-state photoionization cross-section spectra in both investigated materials revealed a band with a peak around 270 nm most probably corresponding to 5d-6s transition of Ce³⁺. Recombination cross-section in Ce:LiLuF₄ appeared to be two orders of magnitude higher than in Ce:LiYF₄. A complete energy level diagram of “Ce³⁺ ion-LiYF₄ crystal” system has been proposed.