Electronic and vibrational absorption and emission properties ofF-center–OH−defect pairs in cesium halides

Association of F centers and ${\mathrm{OH}}^{\mathrm{-}}$ substitutional ions on 〈100〉 next-nearest-neighbor positions in CsCl, CsBr, and CsI forms a new stable defect pair [ $F_H$(${\mathrm{OH}}^{\mathrm{-}}$) center] with strong interaction and coupling between both partners. The neighboring ${\mathrm{OH}}^{\mathrm{-}}$ ion splits the F absorption band into widely (∼0.7 eV) separated $F_H$(1) and $F_H$(2) electronic absorptions polarized parallel and perpendicular to the pair axis, while the neighboring F center shifts the 2.8-μm ${\mathrm{OH}}^{\mathrm{-}}$-stretch band to lower energy by 30–60 ${\mathrm{cm}}^{\mathrm{-}1}$. Optical excitation of the low-energy electronic $F_H$(1) transition in CsBr and CsI produces a weakly Stokes-shifted 〈100〉-polarized electronic luminescence of short lifetime, indicating a relaxation with small electron-phonon coupling into a low-lying and compact relaxed excited state. In CsI an additional weak 〈100〉-polarized ${\mathrm{OH}}^{\mathrm{-}}$ vibrational luminescence was observed (for the first time in any alkali halide) under $F_H$(1) excitation, indicating electronic-vibrational (E-V) energy transfer inside the $F_H$(${\mathrm{OH}}^{\mathrm{-}}$) center and 〈100〉 rotational alignment of the ${\mathrm{OH}}^{\mathrm{-}}$ molecule by the attached F center.