Width of the Narrow "No-Phonon" Lines in the Optical Spectra of Impurities in Solids

Abstract

Phonon-impurity interactions responsible for part of the width of the no-phonon components of the sharp-line optical spectra of impurities in crystal lattices can be interpreted physically in terms of the secular-perturbation model proposed for magnetic resonance by Bloembergen, Purcell, and Pound. The low-frequency secular excursions of the impurity energy levels produced by the lattice vibrations appear as a finite linewidth to an experimenter making line-shape measurements over macroscopic time intervals. The total linewidth will of course include additional contributions from the finite impurity-state lifetime implicit in interlevel transitions induced by photons and nonsecular phonons. The theory of the no-phonon components of impurity optical spectra is closely related to the theory of line narrowing in magnetic-resonance spectra.