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Physical Review Letters

Evidence of Phonon Annihilation Processes in the Excited States of BaClF: Sm2+

A. S. M. Mahbub'ul Alam, Baldassare Di Bartolo

Published Oct 30, 1967• 16 citations
Publisher

Abstract

We have found direct evidence of upward, phonon annihilation, processes in the decay patterns of two metastable states $^{5}D_{0}(14542 {\mathrm{cm}}^{\ensuremath{-}1})$ and $^{5}D_{1}(15878 {\mathrm{cm}}^{\ensuremath{-}1})$ of BaClF: ${\mathrm{Sm}}^{2+}$. Under pulse excitation, the decay curve of the $^{5}D_{1}$ fluorescence includes a long tail, with a time constant equal to the lifetime of $^{5}D_{0}$, which appears at \ensuremath{\sim}280\ifmmode^\circ\else\textdegree\fi{}K and grows in intensity with temperature; finally at \ensuremath{\sim}400\ifmmode^\circ\else\textdegree\fi{}K both states decay with the same time constant. This finding is consistent with the thermal dependence of the ratio of the intensities of the $^{5}D_{1}$ to the $^{5}D_{0}$ flourescence.

DOI
10.1103/physrevlett.19.1030

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Physical review. B, Condensed matter•Mar 15, 1986•26 citations
We have found direct evidence of upward, phonon annihilation, processes in the decay patterns of two metastable states 5D0(14542cm\ensuremath−1)^{5}D_{0}(14542 {\mathrm{cm}}^{\ensuremath{-}1})5D0​(14542cm\ensuremath−1) and 5D1(15878cm\ensuremath−1)^{5}D_{1}(15878 {\mathrm{cm}}^{\ensuremath{-}1})5D1​(15878cm\ensuremath−1) of BaClF: Sm2+{\mathrm{Sm}}^{2+}Sm2+. Under pulse excitation, the decay curve of the 5D1^{5}D_{1}5D1​ fluorescence includes a long tail, with a time constant equal to the lifetime of 5D0^{5}D_{0}5D0​, which appears at \ensuremath{\sim}280\ifmmode^\circ\else\textdegree\fi{}K and grows in intensity with temperature; finally at \ensuremath{\sim}400\ifmmode^\circ\else\textdegree\fi{}K both states decay with the same time constant. This finding is consistent with the thermal dependence of the ratio of the intensities of the 5D1^{5}D_{1}5D1​ to the 5D0^{5}D_{0}5D0​ flourescence.
A crystal of SrF2{\mathrm{F}}_{2}F2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+ was operated as a pulsed optical maser at temperatures close to 4.2\ifmmode^\circ\else\textdegree\fi{}K. The 6969 \AA{} output corresponds to the wavelength of an exceedingly sharp line ($\ensuremath{\Delta}\ensuremath{\nu}<0.6$ ${\mathrm{cm}}^{\ensuremath{-}1}$) in which almost all of the low-temperature fluorescence is concentrated. In contrast to what prevails in CaF2{\mathrm{F}}_{2}F2​: Sm+2{\mathrm{Sm}}^{+2}Sm+2 masers, the output is characterized by strong relaxation oscillations. This may be explained by the large difference in the fluorescent lifetimes in the two systems. While in CaF2{\mathrm{F}}_{2}F2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+ the value of $\ensuremath{\tau}$ is approximately 1.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ sec, in SrF2{\mathrm{F}}_{2}F2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+ it has a value of 15 msec. The reason for the large difference in $\ensuremath{\tau}$ values, in turn, depends upon the relative proximity of the long-wavelength edge of the red band to the position of the metastable 5D0^{5}D_{0}5D0​ level from which fluorescence occurs. Absorption measurements taken at helium temperatures on SrF2{\mathrm{F}}_{2}F2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+ show that the lowest vibrational level of the lowest lying $4f\ensuremath{-}5d$ band lies \ensuremath{\sim}600 ${\mathrm{cm}}^{\ensuremath{-}1}$ above the position of the 5D0^{5}D_{0}5D0​ level while in CaF2{\mathrm{F}}_{2}F2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+ there is near coincidence of the corresponding levels and consequently strong configurational mixing. Crystals of SrCl2{\mathrm{Cl}}_{2}Cl2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+ were also studied in this regard and display an expected close similarity in spectral characteristics to SrF2{\mathrm{F}}_{2}F2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+. The vibrational structure on the $4f\ensuremath{-}5d$ bands is much more pronounced, however, than in CaF2{\mathrm{F}}_{2}F2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+ and SrF2{\mathrm{F}}_{2}F2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+, indicating weaker coupling to the lattice in the excited 5d5d5d state. A tentative interpretation for the splitting of the red band seen in the absorption spectra of Sm2+{\mathrm{Sm}}^{2+}Sm2+ ions is given.
The two-photon transitions between the Stark levels of the 7F0^{7}\mathrm{F}_{0}7F0​ and 5D2^{5}\mathrm{D}_{2}5D2​ multiplets of the Sm2+{\mathrm{Sm}}^{2+}Sm2+ ion in BaClF are investigated both from an experimental and theoretical viewpoint. Direct two-photon excitation spectra of the 5D2^{5}\mathrm{D}_{2}5D2​ fluorescence resulting from an optical excitation with an infrared laser beam are presented and the polarization dependence of the observed transitions is analyzed. These new data are interpreted along the lines of a simple model which originates in the second-order theory developed by Axe [Phys. Rev. 136, A42 (1964)] for two-photon processes in complex atoms. The extension of this theory to the case of rare-earth ions embedded in crystalline fields is carried out by use of symmetry-adaptation techniques based on the use of an SU(2)\ensuremath{\supset}G∗{\mathrm{G}}^{\mathrm{*}}G∗ chain of groups. It is shown that the resulting model (limited up to second-order terms) describes correctly the relative intensities of the two-photon transitions between the Stark levels of the 7F0^{7}\mathrm{F}_{0}7F0​ and 5D2^{5}\mathrm{D}_{2}5D2​ multiplets of Sm2+{\mathrm{Sm}}^{2+}Sm2+ in BaClF.
One-color two-photon excitation spectra of the 7^{7}7F0{\mathit{F}}_{0}F0​${\ensuremath{\rightarrow}}^{5}$D0{\mathit{D}}_{0}D0​ and 7^{7}7F0{\mathit{F}}_{0}F0​${\ensuremath{\rightarrow}}^{5}$D2{\mathit{D}}_{2}D2​ transitions of Sm2+{\mathrm{Sm}}^{2+}Sm2+ in SrF2{\mathrm{SrF}}_{2}SrF2​ have been measured at low temperature and their dependences upon photon polarization analyzed. Linewidths on the order of 0.2, 0.6, and 1.0 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ are obtained for the 7^{7}7F0{\mathit{F}}_{0}F0​${\ensuremath{\rightarrow}}^{5}$D0{\mathit{D}}_{0}D0​(A1g{\mathit{A}}_{1\mathit{g}}A1g​), 7^{7}7F0{\mathit{F}}_{0}F0​${\ensuremath{\rightarrow}}^{5}$D2{\mathit{D}}_{2}D2​(Eg{\mathit{E}}_{\mathit{g}}Eg​), and 7^{7}7F0{\mathit{F}}_{0}F0​${\ensuremath{\rightarrow}}^{5}$D2{\mathit{D}}_{2}D2​(T2g{\mathit{T}}_{2\mathit{g}}T2g​) Stark components observed at 15 K, respectively. The 7^{7}7F0{\mathit{F}}_{0}F0​${\ensuremath{\rightarrow}}^{5}$D0{\mathit{D}}_{0}D0​(A1g{\mathit{A}}_{1\mathit{g}}A1g​) transition is observed to be isotropic within experimental error, while the Eg{\mathit{E}}_{\mathit{g}}Eg​ and T2g{\mathit{T}}_{2\mathit{g}}T2g​ components of the 7^{7}7F0{\mathit{F}}_{0}F0​${\ensuremath{\rightarrow}}^{5}$D2{\mathit{D}}_{2}D2​ transition exhibit the anisotropic patterns expected in Oh{\mathrm{O}}_{\mathit{h}}Oh​ symmetry. Calculations of two-photon transition intensities are conducted within the framework of the phenomenological model previously used for Sm2+{\mathrm{Sm}}^{2+}Sm2+ in MClF (M=Ba,Sr) and Eu3+{\mathrm{Eu}}^{3+}Eu3+ in LuPO4{\mathrm{LuPO}}_{4}LuPO4​, where the rare-earth ion occupies sites of C4v{\mathit{C}}_{4\mathit{v}}C4v​ and D2d{\mathit{D}}_{2\mathit{d}}D2d​ symmetries, respectively. The present results are compared and contrasted with those obtained in these previous cases.
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