Bound-Electron-Lattice Coupling and Vibronic Spectra

Abstract

The radial extent of the coupling between the bound electronic states of a defect ion in a crystal and the vibrations of the lattice ions is investigated. An electrostatic-coupling model is used to evaluate the electronlattice interaction. It is found that those electronic transitions which involve a change in configuration are coupled primarily to the vibrations of ions in the immediate vicinity of the defect. Consequently, if there exist local or pseudolocalized vibrations, the resultant vibronic spectra are dominated by transitions to vibrational modes which have high local amplitude. In contrast, it is found that in general, electronic transitions within a configuration are coupled to nonlocalized vibrations as well as to the localized vibrations. In the special case of the transition 5 JD 0 -> 7 FQ of Sm 2+ , the coupling is primarily to nonlocalized vibrations, so that the vibrations resemble those of ions of the undisturbed lattice. The observed vibronic structure accompanying this transition of Sm 2+ in the alkali halides is presented. A comparison is made for Sm 2+ :KBr between the singularities in the phonon density of states as observed in the vibronic structure, and those predicted from the published dispersion curves of KBr obtained from neutron-scattering data. The agreement is found to be very good. The vibronic structure accompanying the same transition of Sm 2+ in the alkaline-earth halides is also discussed. 1 W. E. Bron and M. Wagner, Phys. Rev. 139, A233 (1965).

Zero‐Phonon Transitions of Eu++ ‐ and Sm++ ‐Doped Alkali Halides

“…The optical properties of rare earth ions in alkali halides have been found to agree fairly well with the predictions of MNT [3] and may be summarized as follows.…”

“…Therefore the main difference between the two types of transitions consists of short-range interaction (quadrupole) for the interconfigurational transitions and long-range interaction (dipole) for the intraconfigurational transitions. It has been recognized [3] that a remarkable consequence of these forces is a) the coupling of the electron transition with local (perturbed by the impurity) phonons in the case of short-range forces (interconfigurational transitions) and b) the coupling with crystal phonons (not perturbed) in the case of long-range forces (intraconfigurational transitions). Furthermore, the electron-phonon interaction is stronger in the interconfigurational case since the f $d change of orbital causes a set of forces stronger than those accompanying the f f transition.…”

“…I n the present analysis we shall confine ourselves to the rare earth ions, namely Eu++ and Sm++, in alkali halides and will discuss only those elements of the theory which are essential in order to understand the experimental results. The electron-phonon interaction has been discussed by Wagner and Bron [2], by Bron [3], and recently also by MNT [4], who have given a comprehensive treatment for impurities that are not loosely bound. This certainly applies to the tightly bound rare earth ions in alkali halides and therefore we shall follow the lines of the MNT work.…”

“…Generally the experimental results have been interpreted in terms of electron-phonon coupling theories which involve a Debye spectrum. Recently, however, the experiments have shown that the sidebands are directly connected with the actual density of phonons; therefore efforts have been made in order to describe the electronphonon interaction using the true density of states as shown by Bron and Wagner [2], Bron [3], and very recently by Mulazzi, Nardelli, and Terzi (MNT) Our investigation has been devoted mainly to the behaviour of the zerophonon line at low temperatures in order to derive the parameters involved in the electron-phonon interaction of Eu++ and Sm++ in alkali halides. The data have been compared with the theoretical analysis of Nardelli and co-workers [4] which we review and discuss in Section 2.…”

Vibrational Infrared and Raman Spectra of Non-Metals

“…Experimental vibronic sidebands accompanying electronic transitions of Sm + + impurities in alkali halides and the theory of vibronics have been presented in a series of papers by BRON, WAGNER and coworkers (BRON and HELLER, 1964;BRON, 1965BRON, , 1969BRON and WAGNER, 1965WAGNER, 1964bWAGNER, , 1965WAGNER, , 1967bWAGNER, , 1968bWAGNER and BRON, 1965;KOHNER andWAGNER, 1970, 1972;KOHNER et aI., 1972;BRON, 1975), previously measured by KAPL YANSKII andFEOFILOV (1962, 1964) with later contributions by the McMASTER research group (TIMUSK and BUCHANAN, 1967;TIMUSK et aI., 1968;BUCHANAN and WOLL, 1969) and, e.g., IGNATEV and OVSYANSKIN (1977). A spectrum of CaF 2 :Eu 3 + has been observed by Ward and Whippey (1974).…”