Vibronic intensities for Er3+ in Cs2 NaErCl6

In this current study, we have undertaken vibronic intensity calculations for the absorptions ((4I15/2) Γk) → ((4I13/2) Γl) of the Er3+ in the Cs2NaErCl6 elpasolite type system. This system is extremely complicated to handle from both a theoretical and an experimental viewpoints. This theoretical work shows that over an energy range of about 400 cm-1, a substantial amount of transitions are likely to take place (about 100 transitions; twenty five of them are magnetic dipole allowed and seventy five are vibronically allowed). It is then a formidable task to identify and assign all of these transitions in a non-ambiguous way. Also the experimental evidence available for these absorptions is related to a total of about twenty lines in the luminescence spectrum of this system. The spectrum itself is very challenging and the superposition of spectral features is most likely to occur. A careful analysis of the calculated vibronic intensities and overall oscillator strengths for the various transitions indicates that the current model used is both flexible and appropriate to deal with this kind of systems. In a forthcoming paper, we will examine the rather unusual high intensity associated with the ((4I15/2) Γk) → ((4S3/2) Γl) excitations. (Author)