F-aggregate optical absorption in LiF at 3.0°K

Abstract

In addition to the broad band absorption of F-aggregate color centers observable at room temperature, twenty-six discrete peaks appear at low temperature in the region between 3500 angstroms and 5234 angstroms. All of these absorptions are attributed to (1) zero-phonon interactions in which no energy is absorbed by the lattice during the electronic transition and (2) multiphonon interactions in which 1, 2, 3, etc., phonons of a single frequency are created during the transition. Six prominent zero-phonon lines were observed (at 3908 angstroms, 4123 angstroms, 4613 angstroms, 4747 angstroms, 4874 angstroms, and 5234 angstroms) corresponding to transitions in which the most probable internal symmetry vibrations of the R, N1 , N2 , and N3 centers are excited. The predicted 0-phonon lines for the internal vibrations were observed for the R2 (3908 angstroms) and N2 (4874 angstroms) transitions. These are called "zero-phonon vibronic transitions." The more diffuse subsidiary structure located on the high energy side of the R1 , N1 (at 5234 angstroms), N1 , and N3 (at 4747 angstroms) are attributed to phonon interactions corresponding to a weak center-lattice coupling. The N1 and N3 transitions show only single phonon interactions; this implies that the center-lattice coupling is weaker than for the R2 and N2 transitions which exhibit a multiphonon subsidiary structure. The R center is shown to be coupled with the TO phonon propagating in the [111] direction. All the N centers are shown to be weakly coupled to the complete phonon spectrum of LiF as calculated by Karo and Hardy. The polarizations of the various transitions were determined (where possible) by obtaining relative absorptions for polarized light before and after optical bleaching with linearly polarized light. The polarization of the R2 transition was found to be in the [111] direction; the N1 and N3 transitions were found to be in the (111) plane; and the N2 transition is "suggested" to be in the [111] direction. A complete theory was developed to adequately describe both "zero-phonon vibronic absorption, 11 and multiphonon absorption, as well as to determine the polarization of transitions from absorption data.