Temperature dependence of the polarisation of the emission of the Ga+centre in alkali halides

The temperature dependence of the linear polarisation of the A T emission of a Ga + centre is studied in NaCl, KCl, KBr and KI. A strong reduction of the polarisation in KCl and KBr between 1.6 and 10K is attributed to relaxation inside the Jahn-Teller wells and to transitions between the orthorhombic wells associated with a main Jahn-Teller tetragonal distortion with an activation energy of 30-40K. The existence of a plateau at higher temperature shows that transitions to trigonal Jahn-Teller wells take place before the transitions between the main tetragonal Jahn-Teller wells become effective.