用光子级联发射测量SrAl_(12)O_(19)中Pr^(3+)~3P_0能级的量子效率

Measuring Quantum Efficiency of the ~3P_0 Level of Pr^(3+) in SrAl_(12)O_(19) by Photon Cascade Emission

在SrAl12 O19中 ,Pr3 + 的1S0 处于 4f5d之下 ,可以观察到光子级联发射。研究了SrAl12 O19∶Pr3 + 的光谱性质。把第一步发射的谱线强度作为第二步发射起始能级上电子数的度量 ,测量了SrAl12 O19∶Pr,Mg中Pr3 +3 P0 能级的量子效率 ,测量结果反映了浓度猝灭的影响。低浓度样品中可见区发射的量子效率约为 70 % ,它显著小于

Pr ions in SrAl 12 O 19 occupy high coordination sites, hence experience a weaker crystal field environment. The 4f 2 1S 0 level lies below the 4f5d configuration, photon cascade emission has been reported. Photon cascade emission process also provides a unique method for directly measuring the quantum efficiency (QE) of 3P 0 in this material. Since the separations between the Stark levels of 1I 6 , 3P 1 and subsequently 3P 0 are small, it is reasonable to assume that the depopulation of 1I 6 is dominated by multiphonon relaxation. All the electrons from 1S 0 down to 1I 6 would relax to 3P 0 afterwards. The 1S 0 → 1I 6 fluorescence intensity can be taken as a measure of the initial 3P 0 population. Thus the QE of 3P 0 can be written as η ( 3P 0)=photon number of photons emitted from 3P 0 number of electrons arrived at 3P 0 =intensity of all the emission lines from 3P 0 intensity of the emission 1S 0 → 1I 6 = I ( 3P 0 - 3H 4 ) βI ( 1S 0- 1I 6 )= Rβ , where β is the branching ratio of the 3P 0 → 3H 4 transition. The emission spectrum within 370～520nm of the 1% sample is shown in Fig 4. The excitation wavelength being used is 205nm. The ratio of the 3P 0 → 3H 4 and the 1S 0 → 1I 6 emission intensities, R , can be obtained from the spectrum. QE can then be derived from η=R/β. In Table 1 we listed the measured QE of some samples with different Pr concentrations or grown from different raw materials. For comparison, we also listed the result measured in CaAl 12 O 19 ∶Pr,Mg. As it is shown, QE decreases with increasing of Pr concentration. This reflects the effect of concentration quenching. It should also be noted that even in dilute sample, QE is still remarkably less than 100%. This might be the result of thermal excitation of the 1I 6 and 3P 1 states. In fact, in the spectrum measured at room temperature, we do observe 3P 1 → 3H 5 emission peaked at 525nm (B in Fig 1). Besides, there are also other lines (b, c in Fig 5) appear as the temperature rises from 77K to room temperature, which may be attributed to the emissions from 3P 1 or 1I 6 . According to the Stark levels given in Ref., at room temperature, approximately 32% of the electrons would be at 1I 6 , while 7% at 3P 1 . And it is estimated by Judd Ofelt analysis, comparably great part of the 1I 6 emission intensities falls into infrared.