Interference Effects in Spin Lattice Relaxation by Two‐Phonon Processes

The usual theory of relaxation by two‐phonon processes assumes that the relaxation rate is the sum of a non‐resonant Raman contribution and resonant Orbach contributions. This neglects terms which may be regarded as interference between these contributions and which appear when several excited states contribute to the relaxation. These terms affect the temperature dependence of the relaxation rate and will affect the values of energy level splittings obtained from the Orbach terms. Further the contributions of the several excited states to the relaxation rate may themselves tend to cancel. The usual approximation of omitting phonon energies in energy denominators is then invalid, as the cancellation is sensitive to their presence. A comparison of the exact theory and the usual theory is made for MgO:Co2+. Agreement with experiment is rather better in the exact treatment, but poor in both cases. Possible reasons for the discrepancy are suggested.