THERMAL-PROPERTIES OF (F-2(-2(+))(AH) CENTERS IN ALKALI-HALIDES())(H)AND (F)

Several thermal properties of (F-2(+))(H) and (F-2(+))(AH) centres are studied. (i) Positions and widths of the infrared absorption band hav e been measured as functions of temperature in seven cases; the result s are discussed in the frame of a simple configuration coordinate mode l. (ii) The thermal stability of the (a) variety at room temperature i s reported for a great number of (F-2(+))(H) and (F-2(+))(AH) centres. (iii) The spontaneous (b) --> (a) transformation in the 150 to 250 K domain is briefly studied for eight (F-2(+))(H) or (F-2(+))(AH) centre s and in more details in four cases. (iv) It is demonstrated that the so-called ''(b)'' variety of (F-2(+))(H) centres in NaCl: O2- is reall y a mixture of two different species, probably the (a) variety and a ( be) one, with relative proportions depending on the temperature of the ''F-light'' irradiation which has converted (a) into ''(b)''. An ener gy scheme is proposed to explain the behaviour of the (a) <-> ''(b)'' intertransformations, both thermal and radiative.