Cooperative stretching vibration absorption in alkali cyanides

In all four alkali cyanides (NaCN, KCN, RbCN, CsCN), a weak satellite absorption band at νc is found to accompany the second-harmonic CN^- stretching absorption ν₂ (in the 4100-cm^-1 range). This satellite band (of width equal to that of the ν₂ band) shifts with temperature through the phases of various molecular order exactly parallel to ν₂(T), shares the stress-alignment dichroism of the ν₂ band, and is unaffected by variations in the crystal purity. These properties exclude a perturbed CN^- molecule as a possible origin of this band. The position of the satellite band at twice the fundamental frequency suggests strongly that it is caused by excitation of two vibrational excitons by a single photon. We believe this to be an example of a two-stage process in which the radiation field excites (virtually) a second-harmonic ($n=2$) vibration at one site, followed by a transition to a state of two vibrational excitons by virtue of a vibration-vibration interaction of two CN^- molecules. An example of a mechanism for the latter step is the electric dipole-dipole interaction. Indeed, the relative strength of the νc band varies strongly among the cyanides with CN^- spacing in a manner consistent with CN^--CN^- dipole-dipole coupling. Moreover, an experiment with ¹³C-enriched KCN provides a critical test of the validity of our assignment. A theoretical model is presented which is able to account for the overall features of the observations, though the use of the bare electric dipole-dipole coupling Hamiltonian leads to a predicted intensity lower than observed by {~} 6.8. We suggest that vibration-vibration modulation of the Born-Mayer repulsive interaction may be responsible for the difference. The experiments, and our preliminary theoretical analysis, give us confidence in the claim that this is the first conclusive experimental observation of a cooperative vibrational absorption effect.