Infrared Absorption Spectrum of the Relaxed Excited State of theFCenter in Potassium Halides

The transient-absorption spectra associated with the excited $F$ center in KCl, KBr, and KI are observed in the infrared. The absorption components observed can be ascribed to transitions from the relaxed excited state to the $3p$, $3d$, and $4p$ states of the $F$ center. The spectra show a series of multiphonon structures with a coupling constant less than unity, suggesting weak electron-phonon coupling of the transition in striking contrast to the $F$-band absorption.