Rare earth photoionization study by the resonant microwave cavity technique

Photoionization processes of rare-earth dopant ions in ionic crystals

“…As the number, depth and density of such traps may vary from one sample to another, the shape of the ''electronic signal'' at low temperature should vary as well. Using the microwave resonator method, photoionization measurements performed at 300 K under nanosecond laser excitation in BaF 2 :Eu 2+ exhibit ''electronic signals'' either in absorption mode or dispersion mode [63]. Scanning the laser wavelength between 275 and 400 nm reveals the photoionization threshold at 310 nm (4 eV) in agreement with previous photocurrent measurements [51].…”

“…This weak photocurrent occurring in the energy region of the lowest 5d absorption band is interpreted by the competition between photoionization and relaxation to the Eu-trapped exciton state (see below). Results obtained by use of the resonant microwave cavity method show clearly a threshold at 4.0 eV and a weak photoconductivity signal at lower energy [63].…”

“…Also, the observation of the absorption mode (record of the real part of the reflected microwaves) and dispersion mode (record of the imaginary part of the reflected microwaves) signals gives insight of the route leading to recombination. [60][61][62][63]. In some of these materials, beside the microwave modulation coming from rare-earth electrons released in the conduction band following absorption of light into the 4f nÀ1 5d electronic states of the involved RE ions, another type of signal is clearly identified as due to thermal effect.…”

What may be expected from the “microwave resonant cavity technique” applied to rare‐earth‐doped insulating materials?

“…2 in Ref. [3]). It agrees very well with the photoconductivity spectrum previously obtained by the conventional method of photocurrent measurement [8].…”

“…As a matter of fact, the energy level diagrams of RE ions usually reported do not take this positioning into account and, consequently, lack the required information to help in understanding the complex processes associated with the absorption of high energy photons by RE-doped materials used as scintillators, ultraviolet lasing media or phosphors for fluorescent tubes and plasma display panels. MRCT was shown to be also helpful in gaining significant information on the photoionization process, photoconductivity dynamics, nature of the photoexcited electrons and trapping effects [2][3][4]. MRCT consists in detecting the dielectric response to a pulsed laser irradiation of a sample placed inside a microwave resonator.…”