Polaron Model of Electron-Excess Color Centers. II. Application to the $U_1$ Center

A method for calculating the low-lying states of a color center having two trapped electrons is developed by using the basic idea of the polaron model of an electron-excess color center proposed in a recent paper. An application of this method to the interstitial ${\mathrm{H}}^{-}$ ion in KCl is given. It turns out that the $U_1$ absorption band may be due to the optical transition from the ground state of an ${\mathrm{H}}^{-}$, which is located at an interstitial site tetrahedrally surrounded by four alkali ions as well as by four halide ions in the crystal, to its $1s2p$-like singlet excited state modified by the crystal field.