A spectroscopic and optically detected magnetic resonance study of PO-2 in potassium chloride. III. Photo-orientation

Abstract

The small molecular anion PO-2 has been incorporated at chloride ion vacancies in the potassium chloride lattice, where the relatively high symmetry of the crystal field permits a variety of crystallographically equivalent orientations of the ion within the vacancy site. Electronic excitation to the first excited singlet state of PO-2 (1B2) is followed by rapid radiationless relaxation and intersystem crossing to the phosphorescent 3B1 state. Subsequent electronic relaxation to the ground state is found to proceed with a reorientation of the ion in the multi-minimum crystal potential. At temperatures sufficiently low that competitive thermal reorientation of the ions is quenched, it is possible to prepare photo-oriented arrays of PO-2 ions in the anion vacancies by irradiation with plane polarized light at special angles. The temperature dependence of the rate of photo-orientation has been determined and it has been found possible to describe the observed kinetics in terms of a simple phenomenological rate equation. 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. The potential barrier for conversion between crystallographically equivalent sites is found to be 783 cm-1.