On the Experimental Determination of 4f–4f Intensity Parameters from the Emission Spectra of Europium (III) Compounds

Abstract

Eu 3+ complexes and specially β-diketonate compounds are well known and studied in several areas due to their luminescence properties, such as sensors and lightning devices. A unique feature of the Eu 3+ ion is the experimental determination of the 4f-4f intensity parameters Ωλ directly from the emission spectrum. The equations for determining Ωλ from the emission spectra are different for the detection of emitted power compared to modern equipment that detects photons per second. It is shown that the differences between Ωλ determined by misusing the equations are sizable for Ω4 (ca. 15.5%) for several Eu 3+ β-diketonate complexes and leads to differences of ca. 5% in the intrinsic quantum yields Q Ln Ln . Due to the unique features of trivalent lanthanide ions, such as the shielding of 4f-electrons, which lead to small covalency and crystal field effects, a linear correlation was observed between Ωλ obtained using the emitted power and photon counting equations. We stress that care should be exercised with the type of detection should be taken and provide the correction factors for the intensity parameters.In addition, we suggest that the integrated intensity (proportional to the areas of the emission band) and the centroid (or barycenter) of the transition for obtaining Ωλ should be determined in the properly Jacobian-transformed spectrum in wavenumbers (or energy). Due to the small widths of the emission bands of typical 4f-4f transitions, the areas and centroids of the bands do not depend on the transformation within the experimental uncertainties. These assessments are relevant because they validate previously determined Ωλ without the proper spectral transformation.

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“…Therefore, experimental Ω λ (λ = 2 and 4) were determined from the emission spectra of both the Eu 3+ complexes, measured at room temperature (298 K), using the 5 D 0 → 7 F J transitions ( J = 1, 2, and 4) of the Eu 3+ ion, where the FED and DC mechanisms are considered simultaneously. The relative emission intensities ( I ) of the bands under normal excitation conditions may be given by eq : , I 0 → J = A 0 → J N 0 …”

“…The special character of the MD allowed 5 D 0 → 7 F 1 transition is exploited for the experimental determination of the A 0→ J emission coefficients from the emission spectra, which is formally insensitive to the chemical environment around the Eu 3+ ion, and can be used as a reference . The Ω 2,4 values are obtained from eq : , A 0 → J = 4 e 2 ω 3 3 ℏ c 3 1 false( 2 J + 1 false) χ ∑ λ = 2 , 4 , 6 normalΩ λ ⟨ D 0 5 | | U ( λ ) | | F J 7 ⟩ 2 where χ = n 0 false( n 0 2 + 2 false) 2 9 is the Lorentz local field correction and n 0 is the refractive index of the medium ( n 0 is assumed to be equal to 1.5) . The squared reduced matrix elements ⟨ D <...…”

“…The squared reduced matrix elements ⟨ D 0 5 | | U ( λ ) | | F J 7 ⟩ 2 have values of 0.0032 and 0.0023 for J = 2 and 4, respectively. The spontaneous emission coefficients ( A 0→ J ) are then obtained from eq : , A 0 → J = ( S 0 → J S 0 → 1 ) A 0 → 1 where the S 0→ J factors correspond to the integrated area under the curve related to the 5 D 0 → 7 F J transition. The A 0 → 1 parameter is calculated using eq : , A 0 → 1 = 8 π 3 e 2 3 ( m e c ) 2 ⟨ F 1 7 …”

“…The spontaneous emission coefficients ( A 0→ J ) are then obtained from eq : , A 0 → J = ( S 0 → J S 0 → 1 ) A 0 → 1 where the S 0→ J factors correspond to the integrated area under the curve related to the 5 D 0 → 7 F J transition. The A 0 → 1 parameter is calculated using eq : , A 0 → 1 = 8 π 3 e 2 3 ( m e c ) 2 ⟨ F 1 7 ∥ L + 2 S ∥ D 0 5 ⟩ 2 n 0 3 σ 0 → 1 3 = 3.13 × 10 − 12 n 0 3 σ <...…”

“…The spontaneous emission coefficients ( A 0→ J ) are then obtained from eq : , where the S 0→ J factors correspond to the integrated area under the curve related to the 5 D 0 → 7 F J transition. The A 0 → 1 parameter is calculated using eq : , where m e is the mass of the electron and σ 0 → 1 is the transition barycenter in cm –1 .…”