An analysis of low temperature lineshapes for various absorption lines of several RE3+ ions from the beginning Nd3+ and end of lanthanide series Er3+, Tm3+ embedded in the same crystal - YAG known as important laser systems for 1-3 micrometers emission is presented. New aspects of the dependence of electron-phonon coupling on RE3+ ions and matrix are observed from the analysis of many transitions of these ions at low temperature. These data show that eh electron-phonon coupling presents a symmetric behavior in the lanthanide series, larger toward beginning and end. With only very few exceptions, less than 2 percent, the broadenings or splittings of zero-phonon liens could be connected to optical Raman phonons of undoped lattice. That means that the parity of 4fn states could still be considered a good parent quantum number, even if the local symmetry has no inversion, and the near resonant coupling involves the even part of the electron-phonon coupling operator. Besides, the one phono relaxation involve the undoped lattice phonons. For ions with small spin-orbit coupling, the vibronic contribution to the line intensity given by ir phonons through a Van Vleck mechanism could be important for spin forbidden transitions.
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