Kinetics of the cathodoluminescence flash in crystals with competing trapping centres

The present work aims to bring proof of possibility of the existence of the process to increase the number of luminescence centers in the radiative state. That apparently has the effect of the buildup of luminescence excited by a pulsed electron beam. To demonstrate the possible influence of competing centers on the decay kinetics of luminescence materials were selected on the basis of crystal YLiF4 and YLiF4: Nd3+. Found that the kinetics of flash luminescence when excited LiYF4: Nd crystals pulse electron beam at room temperature in the initial stage of a flare- up of up to 300 ns was observed. The model of the exchange of electronic excitations between the centers of the emission and capture is discussed.


Introduction
Exposure of wide-band gap materials to short time pulses radiation initiates a luminescence flash which then decays in time. In the kinetics of some materials under certain experimental conditions the luminescence intensity is found to increase at the initial period after excitation in time exceeding the duration of the excitation pulse. The effect of the luminescence buildup under excitation by a pulsed electron beam was observed in [1][2][3]. The luminescence buildup at the initial period after exposure may be caused by increase in the amount of luminescence centers in a radiative state due to transfer of energy from the trapping centers excited by radiation pulse. The present research sets out to prove the possibility of this process and to develop the model of the luminescence buildup in the crystal with competing luminescence and trapping centers.

Experimental results
The structure of the PCL spectra of pure YLiF 4 crystals, and those doped with impurities is complex [4,5]. Two groups of bands are distinct in the PCL spectrum: in the ranges of 3.7 ... 5.5 and 2.0 ... 3.7 eV. The characteristics of the two groups of bands differ due to different luminescence centers. The longwave region of the spectrum is caused by uncontrolled impurities which occur in the crystal in its growing. Intentional doping with impurities suppresses the luminescence of the uncontrollable impurities and changes the spectrum. Figure 1 shows the PCL spectra in the region of 2.0 ... 3.7 eV measured after 10 and 200 ns after the end of the excitation pulse for YLiF 4 crystal doped with 0.7 mol% Nd at 300 K. The spectrum measured instantaneously after the end of the excitation pulse is found to be a superposition of the spectra caused by the uncontrollably entered impurity and intentionally doped neodymium. In time after the excitation, the radiative spectrum caused by the uncontrolled impurity disappears and after 200 ns its contribution is negligible. The luminescence caused by the doped neodymium with the bands characteristic of this chemical element becomes dominant. The shape of the spectrum changes and the intensity of the neodymium luminescence increases in time.  Figure 2 shows the kinetic curves of the change in the luminescence intensity at 2.3 and 2.5 eV. One of the main lines of the neodymium luminescence is found at 2.3 eV, and the greatestspectrum notching accounts for 2.5 eV. In this region, intense luminescence after 10 ns is caused by the luminescence of the uncontrolled impurity. The research results show a substantial difference in the kinetic curves.

Model of the luminescence flash buildup kinetics
Consider the model of the process shown in Figure 3. Assume that the crystal contains two competing centers of excitation energy trapping. In the energy scheme of the process, E 1, E 2, I 1 and I 2 are the energies and intensities of transitions in C 1 and C 2 centers, and and are the energy barriers for the exchange of the electron excitation between the centers. Assume that the characteristics of the luminescence centers (trapping centers) are as follows: 1. The radiation efficiency of the luminescence centers C 1 is high (high light yield). The radiation efficiency of the trapping (luminescence) centers C 2 is low or transition to the ground state is nonradiative. 2. The luminescence centers C 1 appear to be deeper electron excitation traps than C 2 centers. However, the cross section of electron excitation trapping by C 2 centers is larger than that by C 1 centers. 3. Exchange of electron excitations between the luminescence centers is considered to be possible.
Since, according to the model , the activation energy of the transition of electron excitation from C 2 to C 1 is less than , the activation energy of the transition of C 1 to C 2 , the transfer of the electron excitation energy will occur mainly from C 2 to C 1 . 4. The lifetime τ 2 of C 2 luminescence centers in an excited state is much shorter than the lifetime τ 1 of C 1 centers. Recombination in the radiative center C 1 is followed by generation of radiation with the intensity of I 1 . The radiation intensity and the power of radiation from the unit of the crystal volume are determined by the recombination rate: where k 11 is the radiation energy yield, N 1 * is the concentration of the excited centers per time t disappearing within time Δt. The concentration of C 1 centers in an excited state is determined by the concentration of the luminescence centers entered in the crystal and the density of the absorbed excitation flux energy. The concentration of the luminescence centers in phosphor crystals is typically more than 10 18 cm 3 . The density of the absorbed energy of the excitation pulse is typically less than 1 J. At high pulse energies, mechanical fracture of the crystal occurs. In case the probability of the electron excitation energy transfer to the luminescence center is k 21 , the concentration of the luminescence centers excited by instantaneous energy pulse will be: where P is the density of the absorbed energy of the excitation pulse, N 1 is the concentration of the entered luminescence centers.
The luminescence centers in an excited state spontaneously transfer to the ground state with the lifetime of τ 1 : (3) In this case the radiation intensity caused by the luminescence centers is: The number of the excited states which can jump the barrier at time t is determined by the expression: The total concentration of the excited C 1 centers at time t with account of recombination in these centers in time τ 1 (with τ 1 > τ 2 ) becomes equal to: Hence, the kinetics of the luminescence flash in centers C 1 is to be described by the expression: ) (

Conclusion
In the kinetics of the luminescence flash in LiYF 4 : Nd crystals excited by nanosecond electron beam at room temperature, the buildup is observed at the initial stage up to 300 ns. The buildup of PCL can be attributed to the transfer of the excitation energy stored in the trapping centers to the luminescence centers. A model of the exchange of electron excitations between the luminescence and trapping centers is suggested. According to the model, the buildup stage becomes possible if a number of conditions have been fulfilled. The buildup is possible only for certain ratios of the characteristic times of excitation relaxation in competing centers, concentration of these centers, the sample temperature and excitation power. Typically, the excitation pulse energy is not instantaneous in relation to the time resolution of the measuring system. The temporal resolution of the device is to be adjusted to the chosen pulse duration to obtain the maximum relevant measurement. In case the pulse shape is known and takes the form of p(t), substitution of the function p(t) in (12) makes it possible to take into account the impact of the pulse shape on the luminescence flash kinetics. Moreover, it allows us to obtain information with a high temporal resolution by means of mathematical processing.