Unrelaxed2SState of theFCenter in Alkali Halides Studied by the Stark Effect

The Stark effect in the $F$-center absorption has been observed in KCl, KBr, KI, RbCl, and NaCl. Changes of the absorption coefficient due to the electric field have been measured as a function of photon energy. From these data the energy difference between the $2S$ and $2P$ levels, and the dipole matrix element $〈2S\mathrm{}\left|z\right|\mathrm{}2P〉$, have been evaluated.