Laser selective excitation and double resonance in Jahn-Teller system:Fcenter in CaO

Reorientation processes in the relaxed excited state of the $F$ center in CaO have been studied through optically detected paramagnetic resonance; difficulties usually encountered because of the random internal stresses are largely overcome by exciting selectively—with a dye laser, through the zero-phonon line—states which are subjected to a definite internal stress. The reorientation between vibronic states is confirmed to be faster than radiative decay, and for strongly stressed sites, it is shown not to involve any spin change. Finally, spin-lattice relaxation times are estimated experimentally. The model is discussed with constant reference to previous work which had required summations over the random strains.