Optical detection of magnetic resonance

Abstract

Abstract Electron spin resonance (ESR) is a technique normally used to measure the ground-state magnetization of an electronic spin system. In a typical experiment the sample is placed in a resonant microwave cavity and irradiated with microwave power. The spin degeneracy of the ground state is lifted by a magnetic field, the strength of which may be varied. When the energy separation between magnetic sublevels which differ in M, value by ± I equals the energy of a microwave photon, hv, microwave power is absorbed by the spin system. Such an absorption of microwave power upsets a balanced microwave bridge circuit, and this permits the resonance condition to be detected. Most experiments are carried out at a microwave frequency near 9.5 GHz, 20 GHz, or 35 GHz at temperatures in the range 1.6-300 K. Simple aspects of such experiments were discussed in Chapter 6. More detailed accounts are given by Orton (1968) and Abragam and Bleaney (1970). ESR may also be observed in excited states of electronic centres if a sufficient population of centres can be maintained in the excited states. Clearly the thermal equilibrium population of a state (say) some 20000 cm-1 above the ground state is very small and some technique for enhancing the excitedstate population must be employed. The excess population in the excited state is achieved by irradiating the sample with optical photons of appropriate energy. Even so, the excited-state population achieved by optical techniques is often many times smaller than that detectable by conventional ESR methods. It is then necessary to observe the effects of microwave-induced ESR transitions on the optical absorption or luminescence spectrum of the centre. This technique which detects ESR transitions optically is usually referred to as Qptical Qetection of agnetic Resonance or simply ODMR. The absorption of microwave power associated with ESR in the excited states upsets the equilibrium populations of the sublevels and so changes the pattern of optical transitions between excited and ground state. Such changes in luminescence indicate that the resonance condition is met.