Journal of Luminescence 31 & 32(1984)851-854 851 North-Holland, Amsterdam LASER EXCITATION AND SPIN COHERENCE OF PHOSPHORESCENT ~ CENTERS IN CaO M. Glasbeek, M. Casalboni, and R. Vreeker Laboratory for Physical Chemistry, University of Amsterdam, Nieuwe Achter— gracht 127, 1018 WS Amsterdam, The Netherlands Spin coherence in the laser—excited triplet state of F 2+ centers in CaO is studied. Selective probiq~of the F~—centerlocal prop~rtiesis provided by the measurement of the F~ —center spin—echo decay at different excitation wavelengths. In addition, spin—echo beat phenomena arising at high-power pulsed—laser excitation are reported. In recent years, extensive spectroscopic studies concerning F-type point defects in additively colored alkaline earth oxides have been reported. In par- ticular, oxygen vacancies containing two electrons are of interest because they can be photo—excited into phosphorescent triplet states. Several experimental techniques have been applied for studying such triplet states including optical— microwave double resonance and spin coherence techniques1. In this paper, we report on a spin coherence study of the phosphorescent triplet state of the F~ center in CaO, the triplet state being prepared by excitation with cw- and pulsed—dye laser light. In CaO, the F~ center consists of an oxygen divacancy (along a <110> direc- tion of the fcc crystal) containing two electrons. Upon photo-excitation between 400 nm and 650 nm, the ~ defects, which are randomly dispersed in the host lattice, become excited into the emissive 3B 1 state. Using a method of optical detection of spin echoes 2, irreversible spin dephasing was studied (at 1.2K and zero magnetic field) for the DI-~E~ zero-field resonance within the 3B 1 state (at 1.870 GHz), as a function of the wavelength of the cw exciting dye—laser light. Fig. 1 shows some representative results. The displayed decays clearly illustrate the changes that occur in the shape of the decay curves when the excitation wavelength is varied. Homogeneous spin dephasing of centers (in CeO) in the 3B 1 state has pre- viously been shown 2 to be due to a niagrietic dipolar interaction between the probed triplet spins and the S=1/2 spins of abundant F~ centers (which consist of a single oxygen vacancy containing one electron). The F+_center spins, in turn, are considered to form a bath of randomly distributed dipolar coupled doublet spins with a characteristic dephasing rate, R. Expressions have been derived2 to relate the time behavior of the amplitude of the echo signal (S~(2T)) obtained for photo—excited ~ centers to the density (dB) and the de— 0022—2313/84/$03.OO© Elsevier Science Publishers By. (North-Holland Physics Publishing Division)