“…The occurrence of this transition is a well-known example of the breakdown of the selection rules of the Judd-Ofelt theory (a 0-0 transition is forbidden by the J selection rule of the Judd-Ofelt theory). Several authors have tried to theoretically explain why this transition is observed [132][133][134][135][136][137][138][139][140][141][142]. Theoretical models include the breakdown of the closure approximation in the Judd-Ofelt theory and third order perturbation theory.…”