Photon-Induced Reorientation of Color Centers

The present experiment investigates the mechanism by which color centers reorient after the absorption of light. In the cases of $M$ and $F_A$ centers in alkali-halide crystals, the $F$-center portion of a center jumps to an adjacent lattice site after the center has absorbed a (∼2-eV) photon. From conventional optical measurements, Lüty deduced that the jumping occurs while the $F$ center is in the relaxed excited state. To verify that the reorientation is indeed not due to a hot-spot type of phenomenon, the time dependence for the reorientation is measured. That is, the behavior of a color center is analyzed in terms of submicrosecond kinetics. To carry out this measurement, $F_A$ centers are first oriented along a particular axis by illumination with polarized light in the $F_{A2\mathrm{}}$ absorption band. The centers are then excited, essentially instantaneously, by $Q$-switched ruby-laser light. From 0.1 to 10 μsec after the laser flash, the number of centers oriented along each of the cubic axes is measured by the dichroic absorption of polarized $F_{A2\mathrm{}}$ light. Thus the reorientation process is followed for times comparable to the life-time of the excited state. The dichroism of the KBr: $F_A\left(\mathrm{Li}\right)$ and RbCl: $F_A\left(\mathrm{Na}\right)$ systems is found to be time dependent. This result confirms the previous deduction that reorientation occurs by thermally activated jumping while the $F_A$ center is in the relaxed excited state. An atypically low activation energy, ∼ 0.06 eV, allows the jumping to occur rapidly even below 100°K. The present mechanism also explains the photon-induced reorientation of the $M$ center.