Vibronic transitions in rare earth spectroscopy

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Abstract
An overview of the studies of vibronic transitions in the rare earth ions is given with special attention to the intraconfigurational transitions. First the general theory is shortly reviewed, later that of the rare-earth vibronic transitions. The vibronic spectroscopy of Gd3+ is extensively dealt with, because it is an ideal ion for this type of study due to its non-degenerate ground state. Vibronic coupling with several vibrational modes has been observed, also with modes in which the central ion is not directly involved. Attention has also been given to Eu3+ and Pr3+and some other specific cases of interest. The use of Gd3+ as a probe ion is pointed out. A comparison with relating physical phenomena is made. The experimental results are compared with the theoretical expressions derived up till now. The agreement is satisfactory, although of a qualitative nature. The concentration dependence of the vibronic intensity is not well understood. The role of opposite-parity configurations is probably of more importance than generally accepted and the admixture into the 4fn configuration not equal for all its levels.