Symmetry in coherence transfer

The symmetry of coherence transfer between pairs of transitions in quantum mechanical systems is investigated. Conditions for symmetric transfer under composite processes consisting of pulses and precession periods are formulated. The symmetry of two-dimensional (2D) N.M.R. spectra with respect to reflection about the diagonal serves as a concrete example. The current experimental 2D N.M.R. techniques are analysed in view of symmetry and examples of experiments are discussed that lead to asymmetric coherence transfer and asymmetric 2D spectra.