Der Quartett-Zustand desR-Zentrums in Kaliumchlorid-Kristallen

Optical absorption and paramagnetic resonance of KCl crystals containing preferentiallyR-centers have been studied during and after excitation with 430 mμ or 365 mμ light at 90 °K. A partly metastable, partly stable state has been detected and identified as the quartet state of theR-center, which proves to be an association of threeF-centers forming an equilateral triangle in the (111) plane of the crystal as proposed byvan Doorn andHaven andPick. — The crystals used were additively colored to 6.6·10¹⁷ F-centers/cm³ and illuminated withF-band light at room temperature until theR/M-ratio reached its maximum value. During light excitation the optical spectrum in the region of theR-bands shows characteristic small changes. A part of these changes disappears spontaneously with a time constant τ=14.5 sec when the light excitation is removed (“temporary bleaching”). The remaining part has a similar spectral distribution. It is completely stable in the dark, but can be bleached within a few seconds by illumination with 840 mμ light. — Electron spin resonance of the crystal under light excitation shows in addition to theF-center line a seven-line spectrum with strong angular dependence, if the field is rotated in a (110) plane. The maximum splitting is 674 G, the width of the individual lines 35.5 G. The angular dependence of the spectrum can be fitted to a fine-structure spin Hamiltonian

$$H = g\mu _B \vec H_0 \vec S + DS_z^2 - \tfrac{1}{3}S(S + 1)]$$