Phase relaxation of photoexcited triplet spins in CaO

Spin dephasing in the photoexcited ${}_{}{}^3{}_{}{}^{}B_1^{}{}_{}{}^{}$ state of the $F_2^{2+}$ center in CaO is investigated using techniques for optical detection of microwave-induced coherent transients. Measurements of the spin-echo decay were performed at different (low) temperatures and (low) magnetic field strengths. The spectral diffusion results give quantitative support for a model in which the dephasing is due to random modulation of the dipolar coupling between the excited $S=1\mathrm{}$ spins and $F^{+}$-center spins of $S=\frac{1}{2}$. A projection operator formalism is used to treat the dipolar-induced dephasing. The theory also allows for the experimental determination of the $F^{+}$-center spin-dephasing rate. It appears that between 1 and 10 K the phenomenon of one-phonon assisted exchange narrowing occurs in the $F^{+}$-center spin ensemble.