Photon-induced reorientation of color centers

Abstract

The present experiment investigates the mechanism by which color centers reorient after the absorption of light. In the cases of M and FA 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, FA centers are first oriented along a particular axis by illumination with polarized light in the FA2 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 FA2 light. Thus the reorientation process is followed for times comparable to the life-time of the excited state. The dichroism of the KBr: FA(Li) and RbCl: FA(Na) systems is found to be time dependent. This result confirms the previous deduction that reorientation occurs by thermally activated jumping while the FA 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. © 1970 The American Physical Society.