Luminescence line shapes

The identification of a Stokes shift is based on a comparison of the absorption and luminescence line shapes (Street 1978) (also see Section... 

The opposite point of view is that the luminescence line shape and energy are determined by the band tail shapes and the convolution of... 

The absorption edge shifts to the blue (Fig. 1). The photoluminescence has a broad band peaking at 640 nm. The luminescence line shape is not Lorentzian and has a strong Stokes shift. Photoluminescence excitation (PLE) spectra have revealed a fine substructure of the band at its short-wave wing whose origin is attributed to the intrinsic luminescence contribution and to radiative recombination on defects. 

Although the general theory outlined provides a satisfying unified interpretation of the many relaxation processes mentioned, at present reliable numerical predictions are not possible. This must be considered the most serious technical limitation of the analysis we have reviewed. Because of the technical difficulties encountered in the a priori calculation of matrix elements, densities of states, etc., it is tempting to reverse the analysis to obtain information about the relevant intramolecular matrix elements, densities of states, etc., from line shape data and the several luminescence decay times. For example, it seems likely that the complex spectrum of a molecule such as NOa could be analyzed in this fashion, and thereby provide information not now available from any other source. 

Fig. 8.4. Predicted line shapes for luminescence and absorption in a material with strong phonon coupling when the excited state is (a) discrete or b) part of a continuum.

Figure 3 A typical zero phonon luminescence line from a bulk sample would appear as in (A) with a typical inhomogeneous width (r) of 1-10 cm- As the number of emitting impurities decreases, the smooth line shape typical of a large distribution is lost (B) and eventually the contributions of individual homogeneously broadened contributions appear in the wings (C). The homogenous width is exaggerated by a factor of about 10 for temperatures of less than about 4 K.

Figure 5.9 Two examples of dynamic induced band-shape effects, (a) Weak coupling an absorption single line of the Yb + ion in LiNbOs (denoted by an arrow) is accompanied by the appearance of phonon side bands (reproduced with permission from Montoya et al., 2001) (b) Strong coupling the broadband luminescence of the Cr + ion in LiNbOs (reproduced with permission from Camarillo et al., 1992).

The situation with Mn + center distribution between Ca(I) and Ca(II) positions in the apatite lattice is the opposite to this for REE + in artificially activated apatite the Mn(I) center clearly dominates the fluorescence spectra (Ryan et al. 1970), while in the natural one only Mn(ll) centers have been detected (Tarashchan 1978). In order to clarify the distribution in different Ca positions, luminescence spectra have been measured with different polarizations from one section or from prismatic and basal sections with the same analytical conditions. As was foimd earher (Barbarand and Pagel 2001) the shapes of the spectra are usually the same for both crystallographic orientations, while the major difference in the spectra is their intensity, with the mean intensity for the basal section lower than for the prismatic face. Nevertheless, in certain cases polarization changes lead to different spectra (Fig. 5.46). In this case spectra are composed mainly of the Nd and Mn with relatively weak Eu lines. The polarization change results in an inversion of the relative intensities of the liuninescence bands Mn +(I) emission dominates in one orientation and REE emission is practically not seen, but Mn " (II) in other orientations is much weaker compared to Mn +(I), while the... 

The method developped by Struck and Fonger" offers the possibility for a quantitative description of the temperature quenching of broad band and narrow line emissions. The parameters which are used in this method to calculate the non-radiative rate can be obtained from the band shapes and the peak positions of the features observed in the luminescence spectra. The influence of small variations of the... 

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