Application of photoconductivity measurements to photodynamic processes1investigation in LiYF_4:Ce~(3+) and LiLuF_4:Ce~(3+) crystals

摘要

Photoconductivity measurements are applied to the studies of photodynamic processes in LiYF_4:Ce~(3+) and LiLuF_4:Ce~(3+) 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. Ce~(3+) 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 Ce~(3+). Recombination cross-section in Ce:LiLuF_4 appeared to be two orders of magnitude higher than in Ce:LiYF_4. A complete energy level diagram of "Ce~(3+) ion-LiYF_4 crystal" system has been proposed.