Some misconceptions concerning the electronic spectra of tri-positive europium and cerium

Abstract

Tripositive europium attracts wide interest in diverse fields such as phosphors, sensors, and time-gated bioimaging agents based upon its optical emission spectra. Some inaccurate descriptions of these spectra are being amplified throughout the literature. In this tutorial review the background of electronic states, energy levels and transition intensities is provided as a pre-requisite for clarifying these misconceptions. The topics discussed encompass the electric dipole nature of intraconfigurational 4f electronic transitions, the use of europium as site symmetry and centrosymmetry probe, as well as an indicator of nephelauxetic effects. The frequently mis-used term Stokes shift is also clarified and alternative terms are given for the situations where it is incorrectly applied.

Resolving the ambiguity in the relation between Stokes shift and Huang–Rhys parameter

“…Fig. 10 displays the spectra from different initial vibrational states m, with line intensities of transitions to the different final states n according to eqn (1). Contrary to the spectrum for m = 0, spectra from vibrationally excited states (m 4 0) do not have an envelope with the shape of one broad band (compare gray lines for m = 0 and m = 1).…”

“…1). Anharmonicity leads to a deviation from the fixed intervals of ho between lines in the vibronic progression, and to line strength distributions that deviate from eqn (1). 34 In the situation of harmonic but unequal vibrational energies in the ground and excited state, the absorption and emission spectra have different shapes and are not mirror images of each other.…”

“…The concept 'Stokes shift' should not be used for the energy difference between absorption by one electronic transition and emission from another. 1 In some systems the vibronic progressions of two or more electronic transitions overlap, making it impossible to accurately resolve the shape and peak position of individual transitions. This problem may be solved by recording spectra at cryogenic temperatures, where transition lines are sharper and less likely to overlap.…”

“…S, in turn, is directly related to changes in chemical bonding upon electronic excitation, as described in Section 2. The relation E Stokes = (2S À 1) ho is used in many reports, 1,[12][13][14][15][16][17][18][19] while other works use the alternative relation E Stokes = 2S ho (ref. [20][21][22][23][24], or are ambiguous about it.…”

“…The Stokes shift is defined as the energy difference between the maxima of these two bands belonging to the same electronic transition. 1,2 The theory explaining the shape of absorption and emission bands was first developed for electronic transitions on F-centers, [3][4][5] with major contributions from Huang and Rhys. 6 The theory was first applied to a dopant impurity in a crystalline solid when Williams and Hebb 7 used it for Tl + -doped KCl, and was worked out in more detail by Keil.…”