Abstract
Recent electron-spin-resonance experiments on the one-dimensional Heisenberg antiferromagnet, NENP, have observed 47-GHz transitions in magnetic fields of 1.4–5 T, depending on orientation. We argue that these should be understood as transitions between magnon states of a wave vector near π and different polarizations, whose energies are split by crystal-field anisotropy and magnetic fields. Neutron scattering measurements and/or models of the magnon spectrum thus determine the ESR resonance frequencies.