Knowledge Bank

Small molecular cations may be substitutionally incorporated at halide ion vacancies in the lattices of the most of the alkali halides. The relatively high symmetry of the crystal field permits a variety of crystallographically equivalent orientations of the ion within the vacancy site. Additionally, for ions with less than four atoms, the potential barrier for conversion between crystallographically equivalent sites is often less than 0.5 eV. For example, the nitrite ion ($NO2-$) is essentially a free rotor about one momental axis in potassium chloride (1), while the barrier for the isoelectronic anion $PO2-$ is found to be 0.14 eV. Several diatomic ions ($O2-$ and $S2-$) have barriers intermediate between those of $NO2-$ and $PO2-$. When excitation to low lying electronic excited states produces a large change in molecular geometry, as in the cases of $NO2-$ and $PO2-$, subsequent electronic relaxation to the ground state may proceed with a re-orientation of the ion in the multi-minimum crystal potential. At temperatures sufficiently low that thermal re-orientation of the ions is quenched, it is possible to prepare completely photo-oriented arrays of small ions in the anion vacancies by irradiation with plane polarized light at special angles. Photo-orientation evidently results from an extraordinarily strong linear electron-phonon coupling and may be used for a detailed study of this type of interaction. Additionally, the observed kinetics may be used to test calculations of the crystal potential hypersurface using various electrostatic models. The dynamics of the photo-orientation process will be discussed.