We present a new method implemented in our new package \textit{FitAik}, to perform least-squares fitting of calculated and experimental atomic transition probabilities, by using the mono-electronic transition integrals
⟨nℓ∣r∣n′ℓ′⟩ (with
r the electronic radial coordinate) as adjustable quantities. \textit{FitAik} is interfaced to the Cowan suite of codes, for which it automatically writes input files and reads output files. We illustrate our procedure with the example of Er
+ ion, for which the agreement between calculated and experimental Einstein coefficients is found to be very good. The source code of \emph{FitAik} can be found on GitLab, and the calculated Einstein coefficients are stored in our new database CaDDiACs. They are also used to calculate the dynamic dipole polarizability of Er
+.
This Letter presents results of analysis of the absorption spectra of AlN:Er 3+ bulk crystals. In the spectral of 370–700 nm, absorption lines responsible for intraconfiguration electron transition from the ground state 4 I 15/2 to the excited states of Er 3+ ions are found. Transitions to the levels of the 4 F 9/2 , 2 H 11/2 , and 4 G 11/2 states at 2 K are studied in detail. The number of observed lines for these transitions fully agrees with that theoretically possible for f – f electron transitions in Er 3+ ions found in a noncubic crystal field. The small width of the observed lines and their number indicate that erbium ions displace mostly one regular crystal position. Most probably, Er 3+ occupies the position of Al. Energy positions of excited states for the considered transitions are determined.