First-order Raman off- and on-resonance scattering induced byFcenters in CsCl and CsBr

Experimental first-order Raman spectra for CsBr and CsCl doped with $F$ centers are described. All spectra were studied as a function of the wavelength ${\lambda }_L$ of the laser line allowing excitation off and on resonance with the $F$ band. Off-resonance spectra were calculated using phonons derived from the eleven-shell model parameters which fit with neutron data, and assuming that the electron-phonon interaction is linear in the displacements of the first- and second-shell ions surrounding the $F$ center. Softening of the longitudinal force constants between the $F$ center and its first nearest-neighbor (nn's) (55% in CsBr, 50% in CsCl) and between its first nn's and its second nn's (8% in CsBr, 13% in CsCl) accounts for the main peaks. The relative contributions o the ${\Gamma }_1^{+}$, ${\Gamma }_3^{+}$, ${\Gamma }_5^{+}$ modes to the 2nd moment of the $F$ band deduced from Raman-scattering cross sections agree with previous magnetic-circular-dichroism measurements. It is shown that in CsBr, $F$-electron coupling with the first-shell ions is preponderant while in CsCl, the coupling to the ${\Gamma }_3^{+}$ symmetrized displacements of the second-shell ions appears relatively large.