Formation ofF2+Centers in KI

When undoped KI is x rayed at 130 °K and then irradiated with $F$ light at 240 or 270 °K, the $M$ and $R$ centers are formed. Subsequent irradiation at 78 °K with $M_L$ light results in the formation of an optical-absorption band at 1650 nm at the expense of the $M$ band. Studies suggest that it is associated with $F_2^{+}$ centers (formerly called $M^{+}$, which are formed at 78 °K by (i) $V+F_2+\left(M_L \mathrm{light}\right)\to V^{-}{F}_2^{+}$ (where $M_L$ light represents light absorbed by the $M_L$ bands) and destroyed by x rays at 78 °K, by (ii) $V^{-}+F_2^{+}+\left(\mathrm{x}\mathrm{rays}\right)\to V+F_2$ [where $V^{-}$ is an electron center containing a halogen ($V$) center core]. $F_2^{+}$ centers may also be destroyed at 78 °K by $F$ light, by (iii) $V^{-}+F_2^{+}+F+h{\nu }_F\to V^{-}+F_2+\alpha$. Studies show that the $F_2^{+}$ center is not formed optically as a result of $V_K$ excitation, but only by direct ionization. The formation of $F_2^{+}$ centers by $M_L$-light irradiation in x-rayed colored crystals seems to depend on the existence of $V$ centers which act as special electron traps.