Experimental Aspects of Optically Detected EPR and ENDOR

Abstract

In this chapter some of the essential features of spectrometers for optical detection of EPR and ENDOR are described. Unless necessary, we will not distinguish between ODEPR and ODENDOR, but simply refer to ODMR. Basically, there are two kinds of ODMR spectrometers. In one, the ODMR effect is measured as a microwave- or rf-induced change of the fluorescence or phosphorescence light intensity. It is often achieved by adding optical components to an ordinary EPR spectrometer and providing it with a special cavity (emission-type spectrometer). Since such spectrometers were described elsewhere previously [9.1–3], they will be dealt with only briefly here. The other type of ODMR spectrometer is based on a spectrometer to measure the magnetic circular dichroism of the absorption (MCDA) or the magnetic circular polarization of emitted light (MCPE), be it fluorescence or phosphorescence, to which the microwave and rf components are added (MCDA-type spectrometer). This type of spectrometer was described earlier [9.4,2] for the special purpose of investigating excited states of F centers in alkali halides. It has since been developed further for a more general use to study EPR and ENDOR of ground and excited states of many types of defects, and will, therefore, be described here in more detail. Several of the more critical components of the MCDA-type spectrometer are discussed in view of experience gathered in the Paderborn group over the last ten years. The MCDA-type spectrometer requires a higher degree of precision for the optical components compared to the emission-type spectrometer. It is usually not used for conventional detection of EPR, although this would be possible by including a microwave bridge. The emission-type spectrometer is usually also operated as a conventional EPR spectrometer.