Vibrational hole burning, energy transfer and anti-hole formation in the ordered and disordered CN- sublattices of KCN and RbCN

Abstract

Using a single mode tunable color-center-laser for excitation and as a high-resolution spectrometer, hole burning studies have been performed at 4 K in the second harmonic CN- vibrational absorption of alkali-cyanides around 4000 cm−1. In antiferroelectrically ordered KCN, a large number of sharp (∼10−3cm−1) holes can be burned persistently in the ∼1 cm−1 broad absorption bands of the four CN− isotopes. Spectrally selected CN− dipoles (aligned in the ∼107 V/cm mean crystal field) disappear under excitation producing holes with an efficiency of ∼10−3, and reappear as “anti-hole absorption” at about 10 cm−1 higher frequency: now oriented opposite to the mean field. Besides this “direct” process, the primary localized excitation can diffuse spectrally and spatially by vibrational-vibrational (V-V) energy transfer through the CN- sublattice, producing “indirect” molecular reorientations on its path of migration. The formation of the (broad) hole component caused by this process could not be observed on the existing strong absorption background, while a build-up of the ∼10 cm-1 shifted anti-hole absorption on zero background can easily be detected: either in the isotope system under optical excitation, and/or in the isotopes of smaller eigenfrequencies (down to 13C15N), where the migrating excitation energy can be trapped.In antiferroelastically ordered but electrically disordered RbCN, many sharp, persistent, and 100% deep holes can be burned. Anti-hole absorption build-up, which is not spectrally separated, but spread over the whole absorption band due to the random “electric dipole glass” field in the crystal, cannot be detected.