Dependence of the 5D o → 5D 1 fluorescence dynamics versus the optical pumping frequency in Sm 2+:BaClF

Abstract

Very recently, the role of the 5D o → 5D 1 upwards radiationless transition in the de-excitation mechanisms of the Sm 2+ ion in BaClF at temperatures ranging from 295 to 650 K was clearly established [1]. Using selective pulse laser excitation into the first metastable excited state 5D o (14533 cm −1), the fluorescence of the 5D 1 multiplet located at 1339 cm −1 above 5D o was detected. Both 5D 1 and 5D o fluorescence at long time after the laser pulse were observed to decay exponentially with the same time constant (∼1.5 ms at room temperature) characteristic of the 5D o lifetime in this materials, as shown in the Figs. 1a) and b). This experiment confirms that a thermalization process between levels 5D o and 5D 1 takes place in this system, as observed 16 years ago using an ultraviolet excitation promoting the Sm 2+ ion up to 4f 5-5d states [2,3]. Surprisingly enough the 5D o and 5D 1 fluorescence decays under excitation into the 5D 1 level do not bear evidence of such a thermalization process, as shown in Figs. 1c) and d), the 5D 1 fluorescence being observed to decay exponentially with a time constant equal to the intrinsic 5D 1 lifetime (∼150 μs at room temperature). Moreover, all the measurements we performed in previous works pumping the system either in the 5D 1 or 5D 2 level were found to be consistent with the assumption that the 5D o → 5D 1 radiationless transition did not contribute significantly ▪ Fig. 1. Fitted decay-curves of the 5D o (b,c) and 5D 1(a,d) fluorescences under selective excitation into the 5D o(a,b) and 5D 1 (c,d) levels. ▪ Fig. 2. Comparison between the temperature dependences of the ratio S oo/S 11 of the integrated intensities of the 5D 1 → 7F o emission to the 5D 1 → 7D 1 one in the spectra recorded under pulsed selective excitation into the 5D 1 level (o) and continuous ultraviolet excitation (+). to the optical properties of the Sm 2+: BaClF system at temperatures up to 300 K [4-6]. The physical explanation for these discrepancies lies in the fact that the feeding levels 5D o and 5D 1 during the excitation pulse depends strongly on the pumping conditions [1]. Obviously, the role of the 5D o → 5D 1 radiationless transition will be enhanced if the population of the 5D o state at the end of the pulse is significant, as is the case for the 5D o pumping. On the other hand, the contribution of this transition will be reduced if the 5D o population remains equal to zero during the excitation pulse, as is the case for the 5D 1 or 5D 2 pumping. Under ultraviolet excitation, the feeding of the 5D o state during the pulse is ensured by fast non radiative processes connecting the 4f 5-5d states directly to the 5D o state at temperatures above 200 K as it clearly appears from the comparison between the temperature dependences of the ratio S oo.S 11 of the integrated intensities of the 5D o/ 7F o emission to the 5D 1 → 7F 1one in the spectra recorded under selective excitation into the 5D 1 level on one hand, and ultraviolet excitation on the other hand (Fig. 2). Thus, the thermalization near room temperature between two rare-earth levels separated by an energy gap of several times the maximum phonon frequency in the crystal (294 cm −1 in BaClF) [6] may be observed or not, depending on the optical pumping frequency. Similar investigations in other rare-earth systems such as Eu + doped materials, with larger 5D 1 - 5D o gaps and higher 4f 5-5d state energies should be of interest to complement this observation.